Solar panel + battery setup: off-grid camping bible

Section 1

The foundation of every self-sufficient off-grid camping setup is the exact sizing of the solar panel – neither under- nor oversizing will get you there. Anyone parked far from civilisation in a campervan, motorhome or expedition vehicle must know their daily energy in watt-hours (Wh) precisely before deciding on a 100 W, 200 W or 400 W solar panel. A wrongly calculated solar area means either limited freedom through constant recharging or unnecessary weight on the roof.

1.1 Typical energy consumers and their daily consumption in everyday camping

Before you plan a single watt of solar power, you must draw up a realistic energy balance of all consumers. An average camper with a 12.8 V on-board voltage typically runs a 45-litre compressor fridge with an average of 280 Wh per day in summer operation. Add to this LED interior lighting (15–30 Wh/day), a diesel air heater in winter with up to 150 Wh/day for the fan, and the charging of smartphones, laptops, camera batteries and drone batteries (80–200 Wh/day). In practice, the total daily draw lies between 500 Wh and 1,200 Wh, with a modern, economically equipped panel van at the lower end and an expedition vehicle with Starlink internet, gaming laptop and e-bike charging at the upper end. Use this rule of thumb: “Determine your worst-case daily consumption and multiply it by a factor of 1.3 for reserves and charging losses.”

1.2 Yield calculation: from module output to actual daily yield

A solar panel with a 100 Wp rated output delivers exactly 100 watts under ideal laboratory conditions (1,000 W/m² irradiance, 25 °C cell temperature, AM 1.5). In the real camping world – whether in southern Norway, on a Greek island or in the Moroccan desert – you must take into account seasonal sunshine hours and the angle of the panels mounted flat on the vehicle roof. In a Central European summer you can expect around 5 effective sunshine hours per day; in winter this value drops to 1–2 effective sunshine hours. This means that a 200 Wp panel generates around 1,000 Wh/day in July – enough for the frugal solo camper. In January, however, the same panel delivers only 200–400 Wh/day, which means you will have to draw heavily on battery reserves without an additional charging source. For year-round self-sufficiency, we therefore recommend a minimum of 300–400 Wp of solar area on the vehicle roof, combined with a powerful battery storage system. For flat-roof mounting without tilting, allow for a yield loss of 15–25 % compared with the optimal tilt angle.

1.3 Solar panel types compared directly: monocrystalline vs. polycrystalline vs. CIGS

The physical efficiency of the solar cells determines how much area you have to sacrifice for your desired output:

For confined roof areas, high-efficiency monocrystalline is the first choice. CIGS thin-film panels, on the other hand, score with unbeatable low-light behaviour on cloudy autumn days and lower sensitivity to partial shading – an often underestimated advantage when a roof hatch or satellite antenna casts a shadow. Modules certified to DIN EN 50342 offer mechanical resilience and electrical safety in mobile use.

Section 2

Between the solar panel and the battery bank sits the brain of the off-grid system: the charge controller. It determines charging efficiency, battery life and the possible utilisation of the precious solar area. At the same time, cable cross-sections must be dimensioned in accordance with DIN VDE 0100-712 in order to rule out dangerous voltage drops and fire risks from overheated cables.

2.1 MPPT vs. PWM: yield differences measured in sheer freedom

A PWM controller (pulse width modulation) caps the module voltage at battery level – the 18–22 V module voltage of a 12 V panel simply becomes 12.8–14.4 V charging voltage. The surplus voltage is dissipated unused as heat, and the power loss amounts to 20–35 %. An MPPT controller (Maximum Power Point Tracking), by contrast, continuously scans the optimum operating point of the solar module and, via an integrated DC/DC converter, converts the high input voltage into a higher charging current with low losses. The gain: from a 200 Wp module, an MPPT controller extracts a real charging current of 14–16 A in winter at cold cell temperatures with 22.5 V peak power voltage – the PWM controller delivers 10–11 A at best. In addition, MPPT controllers charge the battery considerably more gently thanks to multi-stage IUoU charging characteristics (bulk, absorption, float) and prevent sulphation in lead-acid batteries through regular trickle charging.

2.2 Wiring and cable cross-section according to DIN VDE 0298-4

The most dangerous weak point of many DIY solar systems is wiring that is too thin between module and controller and between controller and battery. The voltage drop along a cable is calculated relentlessly according to Ohm's law: with 3 m cable length (outgoing and return conductor = 6 m), a charging current of 15 A and an insufficient cross-section of 2.5 mm², you lose a full 1.26 V – that corresponds to a power loss of almost 19 W and critical undercharging of the battery, because the controller misinterprets the actual battery voltage. The standard DIN VDE 0298-4 specifies the following continuous current capacity for flexible copper cables in the vehicle sector:

Practical tip: do not route the cables from the solar module to the controller through the nearest rubber grommet, but use waterproof cable glands with strain relief. Between controller and battery, a 30 A midi fuse must be fitted directly at the positive terminal of the battery – it protects the cable, not the device.

2.3 Parallel versus series connection: adding voltage or adding current

The decision on how you connect two or more solar modules to one another determines the input voltage at the MPPT controller and, in the case of partial shading, the yield of the entire string. With a parallel connection, the currents add up (e.g. 2 × 100 Wp in parallel give 18 V / 11.2 A) – a shaded module does not drag the second one down with it, but you need thicker cables. With a series connection, the voltages add up (36 V / 5.6 A), the current remains low, so you can route thinner cables and MPPT controllers already reach the necessary start voltage early in the morning in cool weather. Critical disadvantage of the series connection: a partially shaded module – whether from leaves, bird droppings or a roof hatch – can cause the entire string to collapse massively if the bypass diodes do not work correctly. For vehicle roofs with unavoidable sources of shade, the parallel connection with 4–6 mm² cable cross-section is the more robust, safer route.

Section 3

The heart of every self-sufficient camping power supply is the battery bank – it buffers solar energy harvested during the day for the night and compensates for spells of dull weather. The cell chemistry dictates the usable capacity, cycle life, cold-weather behaviour and safety requirements. The inverter connected to it turns 12.8 V DC into mains-equivalent 230 V AC / 50 Hz and must be precisely matched to the battery characteristics.

3.1 LiFePO4 versus AGM/lead-acid: the chemistry decision for 2026

Lithium iron phosphate batteries (LiFePO4) with a nominal voltage of 12.8 V and a usable capacity of 80–100 % (at 100 Ah, therefore, a real 80–100 Ah usable) have technically overtaken the AGM battery. A high-quality AGM battery with 100 Ah, on the other hand, may according to the manufacturer's recommendation only be cyclically discharged to 50 % if one does not want to suffer a drop in capacity within 300 instead of 800 cycles. Effectively, therefore, only 50 Ah are available. With a LiFePO4 battery that achieves 3,500–5,000 charge cycles at 80 % DoD (Depth of Discharge), the twice as high purchase price is amortised after just 3–4 years of intensive camping use. A built-in BMS (Battery Management System) permanently monitors cell voltages, temperatures and balancing and protects against deep discharge below 10.0 V as well as against overcharging above 14.6 V. Modern LiFePO4 batteries with integrated heating (e.g.

3.2 Battery bank capacity planning using a practical example

Our sample camper with a determined daily consumption of 900 Wh (fridge 300 Wh, heating 150 Wh, chargers 200 Wh, laptop 150 Wh, light 100 Wh) wants to survive 3 days without solar radiation (bad weather autonomy). Calculated, it requires 2,700 Wh of usable capacity. With AGM technology (50 % DoD), it would have to install 5,400 Wh gross – that corresponds to a monstrous bank of 4 × 100 Ah lead-acid batteries with a total weight of 120 kg. A single LiFePO4 battery with 300 Ah nominal capacity and usable 240 Ah (× 12.8 V = 3,072 Wh), by contrast, weighs only 28 kg, delivers the required energy from a compact housing and can be stored in a space-saving manner under the driver's seat or in the bench seat. An ADAC test from June 2025 confirms that modern LiFePO4 on-board batteries with a Bluetooth interface and smartphone app raise the monitoring of the state of charge, cell balance and historical energy consumption to a completely new level.

3.3 Inverter technology: pure sine wave, modified sine wave and dimensioning

For operating sensitive 230-volt consumers such as induction hobs, medical CPAP sleep devices, coffee grinders with AC motors or chargers for e-bike batteries, a pure sine wave inverter is absolutely essential. Devices with a modified sine wave generate a stepped output voltage that can cause inductive loads to hum excessively, overheat or, in the worst case, be destroyed. The inverter's nominal power must cover the simultaneous peak consumption: an 800 W hairdryer plus 60 W fridge (230 V version) plus 90 W laptop power supply requires a continuous power of at least 1,200 W. Bear in mind the efficiency of 88–94 %: at 1,000 W output power, around 1,100–1,140 W are drawn from the battery, which at 12.8 V corresponds to a hefty current of 86–89 A – here a cable cross-section of at least 16 mm² and protection with a 125 A ANL fuse are imperative. High-quality inverters with ECO mode (e.g. Victron Phoenix Inverter 1,200 VA) reduce standby consumption at idle to below 2 W and thereby protect the battery at night.

Section 4

A precise energy balance calculation is the life insurance against cold, dark nights with an empty battery. It adds up all consumption and compares it with the solar yield. Winter camping below -10 °C additionally requires a strategic frost strategy for the entire electrical installation.

4.1 The daily energy balance in detail: from watt-hours to the self-sufficiency forecast

Create a detailed table of all consumers with their electrical power in watts and the daily operating time in hours. Multiplying the two gives the daily consumption in watt-hours. A practical example for a campervan in a Central European summer looks like this:

• Compressor cool box 45 L: Ø 35 W at 30 % duty cycle → 8.4 h effective → 294 Wh/day
• LED interior lighting: 15 W for 3 h → 45 Wh/day
• Parking heater (fan): 20 W for 6 h on cold nights → 120 Wh/day
• Water pump: 30 W for 0.3 h → 9 Wh/day
• Laptop USB-C PD: 60 W for 3 h → 180 Wh/day
• Smartphone, camera, drone: cumulatively 55 Wh/day
• Starlink Mini internet: Ø 25 W for 4 h → 100 Wh/day

The sum of 803 Wh/day represents the base requirement. Adding 10 % for charging losses and inverter self-consumption gives a gross requirement of 883 Wh/day. An MPPT-controlled 200 Wp solar string delivers an average of 900–1,100 Wh/day in Central Europe in July. The system is therefore balanced in summer, but in October (only 2.5 sun hours = 500 Wh/day) there is a daily shortfall of 383 Wh, which must be covered from the battery reserve. Use this calculation to check the autonomy of your battery bank for periods of bad weather.

4.2 Winter camping and frost strategy for the solar system and battery

Winter camping under extreme conditions – for example in Scandinavia at an outside temperature of -20 °C – requires a fundamentally different energy architecture. An unheated LiFePO4 battery under the vehicle seat cools down to ambient temperature within 6–8 hours without a heat source. From a cell internal temperature of 0 °C, the built-in BMS rigorously blocks the charging current in order to prevent lithium plating. You need a battery with a heating mat and automatic frost detection that, when charging current from the MPPT controller is present, opens the heating circuit and pre-warms the cells to 5–10 °C before the main charging current flows. The energy required to heat a 200 Ah battery from -10 °C to +5 °C is around 180–250 Wh – an investment that places an additional burden on the balance when solar yield is minimal in winter. For continuous winter operation, we recommend a combination of an insulated battery compartment with a silicone heating mat (30–50 W) and a redundant charging source, such as a powerful B2B charge booster (30–50 A) from the vehicle alternator, which warms and fully charges the battery while driving, while the solar panel only makes a maintenance contribution during the day. This ensures the operation of the parking heater, lighting and communication over several days even at -25 °C.

4.3 Advanced diagnostics: understanding energy flows with a shunt and smart monitoring

Precise battery monitoring using a high-precision shunt (e.g. Victron SmartShunt 500 A/50 mV) measures the actual currents flowing to an accuracy of ±0.1 A and calculates the state of charge (SoC) via Peukert compensation – for LiFePO4 with a Peukert exponent close to 1.02 compared with 1.25–1.35 for lead batteries. Via Bluetooth or an integrated display, you can see in real time: current charging power from the solar panel (128 W), consumption by the inverter (420 W), net amperes into the battery (+7.2 A) and the remaining time until full discharge at the current consumption (16 h 42 min). Monitoring as transparent as this enables you to switch consumers on or off in a targeted way and never to run the system below 20 % SoC – a reserve threshold that massively extends the LiFePO4 service life.

Section 5

The solar industry offers a confusing variety of module types and manufacturers. Between uncompromisingly rigid glass-glass high-performance modules, lightweight flexible folding panels for trekking use and compact complete solutions, the user has to make a well-founded decision.

5.1 Rigid glass modules versus folding ETFE modules: weight, efficiency and resilience

Rigid solar modules with tempered solar glass (3.2 mm) and an anodised aluminium frame offer the highest mechanical stability and a long service life. They withstand hail with 25 mm grain size at 80 km/h. Folding ETFE-based modules (e.g. Renogy 200 W folding) weigh only just two thirds as much at 7.2 kg and can be folded down to travel bag size. However, their service life is limited by the constant mechanical stress on the flexible cell connections and the frameless exposed construction. In addition, after 300–500 folding cycles, folding modules show microcracks in the solder joints that lead to creeping power loss. For applications permanently mounted on the roof, rigid glass modules are without alternative; for mobile use without a roof rack – for example when canoe camping or tent trekking – a folding 120 Wp panel offers the perfect addition for the camp.

5.3 Long-term investment or budget solution: the total cost of ownership over 10 years

For passionate off-grid campers, this is the most sustainable and lowest-risk investment.

6. Solar module dimensioning and precise energy balance for the mobile off-grid system

6.1 From consumer profile to required rated power (100 W to 400 W)

The dimensioning of the solar module area follows the iron physical rule: energy demand in watt-hours (Wh) per day divided by the effective solar irradiation in peak sun hours (PSH) gives the minimum total module power in watts peak (Wp). In Central European summer, 4 to 5 PSH are realistic; in the winter half-year, the values fall to 0.5 to 1.5 PSH. A typical compressor cool box draws 380 Wh/day, LED lighting and mobile phone charging add 120 Wh/day. The sum of 500 Wh/day therefore already requires a module with 125 Wp (500 Wh ÷ 4 h), including a safety margin. If you want to supply a diesel parking heater with fan (160 Wh/day), you quickly end up with 200 Wp to 300 Wp.

A calculation example from camper practice: an Audi A3 8V with start-stop requires a 70 Ah AGM battery under the bonnet in standard condition. For off-grid camping by the lake, however, a separate supply battery with 120 Ah LiFePO₄ (1,536 Wh) is installed, which is charged exclusively via the solar module. With a daily consumption of 600 Wh (cool box, LED, laptop) and a summer yield of 4.2 PSH, the calculation gives a module of 143 Wp – in practice, a rigid 180 Wp module is chosen, which keeps the SoC (state of charge) of the LiFePO₄ battery stable between 30% and 95% even under light cloud cover.

6.2 System voltage and module configuration: parallel, series or combined

The choice of system voltage (12 V vs. 24 V) is determined by the maximum input voltage of the charge controller and the battery configuration. A single 12 V solar module with 22.4 V VOC (open-circuit voltage) and 18.2 V Vmpp (nominal voltage) supplies an MPPT controller ideally for 12 V batteries. With two identical modules in series, the voltage doubles to 44.8 V VOC, which minimises line losses but requires a controller with at least 60 V DC input withstand capability. Parallel connection keeps the voltage low and tolerates partial shading of individual modules better, but requires Y-connectors (MC4) and larger cable cross-sections. For most medium-sized motorhomes between 5.5 m and 7.4 m in length, the series connection of two 150 Wp modules into a 300 Wp string has proven successful, connected to the MPPT controller via a 6 mm² solar cable (tinned copper stranded wire according to DIN VDE 0283-6).

6.3 Energy balance table: daily consumption vs. solar harvest

7. Charge controller technology: MPPT vs. PWM and cross-section calculation according to DIN VDE

7.1 The fundamental difference: MPPT tracking vs. PWM direct coupling

The definition of the two controller types belongs in every solid knowledge base:

PWM (pulse width modulation)

A PWM controller switches the solar module directly onto the battery and merely modulates the duty cycle once the end-of-charge voltage (e.g. 14.4 V for AGM) is reached. The module is pulled down to the voltage level of the battery: an 18.2 V module then delivers only 13.8 V × current – the voltage surplus of 4.4 V is dissipated unused as heat. The efficiency loss with deeply discharged batteries is 20 % to 30 %. PWM controllers are suitable exclusively for 36-cell modules (VOC < 25 V) on 12 V systems.

MPPT (Maximum Power Point Tracking)

An MPPT controller electrically decouples the module from the battery and operates a DC/DC converter (buck converter topology) that continuously tracks the module's operating point to the point of maximum power (Vmpp). The voltage surplus is transformed into additional charging current: from 18.2 V / 9.9 A at the module, 13.2 V / 13.6 A at the battery – a current gain of 37 %. The galvanic isolation and the wider input voltage range (up to 150 V DC) allow serial module strings that fully exploit the MPPT principle in the first place.

7.2 Cable cross-section and voltage drop according to DIN VDE 0100-520

The permissible voltage drop between solar module and charge controller as well as between charge controller and battery must not exceed 1 % of the nominal voltage in order to avoid tracking losses and battery undercharging. In a 300 Wp system, the module side carries a maximum of 8.25 A when connected in series (VOC 44.8 V, Vmpp 36.4 V). For a cable length of 6 m (one way), the minimum cross-section is calculated using the formula A = (2 × L × I) / (κ × ΔU) with the specific conductivity of copper (κ = 56 m/Ωmm²). Result: cross-section 2.5 mm² for ΔU = 0.36 V (1 % of 36 V). On the battery side, at 12 V and 25 A MPPT charging current, it is already 10 mm² for ΔU = 0.12 V. Tinned solar cables (H1Z2Z2-K, 1.5 kV DC) are UV-resistant and halogen-free and replace the outdated PV1-F standard.

7.3 Practical recommendation: MPPT controller matrix for camping setups

The choice of charge controller must be matched to the module power and the battery type. A Victron SmartSolar MPPT 100/20 (up to 20 A charging current) is suitable for solar modules up to 290 Wp on 12 V batteries, while the 75/15 is already limited at 220 Wp. For larger arrays from 400 Wp, the SmartSolar MPPT 100/30 or the Renogy Rover 40 A is recommended.

8. Battery bank design and winter camping frost strategy for LiFePO₄ systems

8.1 Capacity sizing: depth of discharge, C-rate and parallel operation

The battery bank must cover two to three days of autonomy (bad weather reserve) without solar input. With a daily consumption of 624 Wh, this results in a required net capacity of 1,872 Wh. While AGM batteries only tolerate 50 % DoD (Depth of Discharge) – meaning they would have to provide a nominal 3,744 Wh (312 Ah/12 V) –, a LiFePO₄ battery with 90 % DoD allows a gross capacity of only 2,080 Wh (162 Ah/12.8 V). Another focus is the C-rate: a 100 Ah LiFePO₄ with 0.5C continuous current can deliver 50 A, more than enough for a 1,000 W inverter (83 A DC). If several batteries are connected in parallel, the leads must be routed symmetrically with equal cable lengths (< 30 cm difference) and 35 mm² cross-section in order to prevent circulating equalising currents (drift) caused by internal resistance differences in the milliohm range.

8.2 Frost strategy: charging window and heating mat configuration at sub-zero temperatures

The Achilles heel of the LiFePO₄ cell is the lithium plating reaction when charging below freezing point. Electrolyte conductivity drops dramatically at -5 °C; lithium-ion intercalation into the graphite anode is irreversibly displaced by metallic lithium deposition. Quality BMS systems (e.g. JBD, Daly, Victron VE.Bus) block the charging path completely from 0 °C cell temperature. Three strategies are established for winter camping:

1. Integrated battery heating: Models such as the Ective LiFePO₄ LT series have factory-fitted heating foils that, at ≤ 0 °C, divert the charging current into heating power (approx. 3 A self-consumption) until the cells reach 5 °C.
2. External tank heating mat: A 12 V / 25 W heating mat under the aluminium battery tray, controlled by a snap-disc thermostat (closing 2 °C, opening 10 °C), consumes about 60 Wh per heating phase.
3. Insulated box with phase-change material: Paraffin wax integrated into the battery box keeps the temperature at 8–12 °C for 8–10 hours through latent heat storage, even at outside temperatures of -15 °C.

8.3 Fuse concept: NH fuses, T-class fuses and fire protection to ECE R 10

Protecting the battery bank requires a slow-blow high-current fuse (ANL or NH00, 125 A–200 A) directly at the positive terminal, rated for the short-circuit current of the LiFePO₄ bank, which can be 1,500 A to 3,000 A. The cable from the charge controller to the battery is protected with a 30 A MIDI fuse; the wiring must comply with ECE Regulation 64 for bodies on motor vehicles. Lightning and surge protection (Type 2 SPD, DC 600 V, Victron SPM 100) is only mandatory for permanent outdoor installation of the camper with a solar module permanently under the open sky; in mobile use, the bypass diodes of the modules already provide basic protection against hot-spot effects.

9. Inverter sizing and module designs: folding vs. rigid

9.1 Pure sine wave vs. modified sine wave: effects on inductances

For all devices with inductive loads – compressor fridges, microwaves (800 W), induction hobs (max. 1,800 W) – a pure sine wave inverter (THD < 3 %) is essential. Modified sine waves (stepped voltage) cause increased iron loss (hysteresis) in transformers and motors, which manifests as 30 % more heat generation and audible humming. For off-grid operation of a touring vehicle with 1,500 W continuous load, an inverter with 2,500 VA peak power must be specified to handle the compressor's starting current (3–5 times the rated current for 200 ms). The self-consumption (standby) of high-quality inverters is 2 W to 8 W (e.g. Victron Phoenix Smart IP43).

9.2 Rigid glass-film modules vs. folding ETFE modules: yield and aerodynamics

The physical limits of both designs are sharply defined: rigid monocrystalline modules with tempered solar glass (3.2 mm) and anodised aluminium frame achieve module efficiencies of 20–22 % and power degradation of 0.5 % per year. Folding modules with ETFE coating (ethylene tetrafluoroethylene) and CIGS thin film (copper indium gallium selenide) weigh only 2.8 kg per 100 Wp, but their efficiency is 14–16 %. Crucial for winter camping: folding modules can be aligned at a 70° tilt angle to the low sun and secured to the vehicle with carabiners – a rigid 15 kg flat roof installation naturally does not offer this azimuthal freedom. Permanently mounted rigid modules score with theft protection and resistance to hail (25 mm grain at 23 m/s, IEC 61215), while folding modules must be protected from continuous mechanical stress in the storage compartment when not in use.

9.3 System integration: charge booster coupling with the alternator

No modern camper setup is complete without the bidirectional charge booster (e.g. Votronic VCC 1212-30 or Victron Orion-Tr Smart 12/12-30A), which charges the leisure battery with 30 A continuous current from the alternator as soon as the starter battery (e.g. the 70 Ah AGM in the Audi A3 8V with start-stop) has reached the charging voltage of 13.8 V. This coupling eliminates the risk of undersupply during winter solar deficits. The integrated IU1oU2oU3 charging characteristic automatically detects LiFePO₄ and AGM batteries and limits the current so that the alternator is not overloaded with a diode plate temperature of maximum 140 °C.

Section 11 – Solar module sizing & energy balance: the 100‑400W calculation

11.1 Daily energy demand as the foundation (Wh/day)

Every off-grid design begins with balancing daily consumption in watt-hours (Wh). Note all 12 V and 230 V consumers with their power consumption (W) and daily operating time (h). A typical camper fridge (Dometic CRX 50) draws an average of 45 W at 30 % duty cycle – that gives 324 Wh/day. A diesel parking heater (Webasto Air Top 2000 STC) requires up to 30 W during the start phase, then runs at 15 W and adds up to 180 Wh over 10 h of winter operation. LED interior lighting (10 W), smartphone charging (5 W, 2 h) and a laptop (65 W, 3 h) add a further 230 Wh. The total in this realistic scenario is 734 Wh/day. For reserves (cloud cover, charging losses) we multiply by the safety factor 1.3 – the energy to be covered is therefore 954 Wh/day.

11.2 Yield model and module power (100 W–400 W)

A solar module delivers its rated power only under standard test conditions (STC: 1000 W/m² irradiance, 25 °C cell temperature, AM 1.5). In Central Europe you achieve 4–5 sun hours in summer with optimum orientation, and only 1–2 in winter. A 100 W module therefore generates about 400–500 Wh/day in July, and 100–150 Wh in December. For the 954 Wh daily balance, a 200 W module is sufficient in summer. In winter, however, at least 400 W is required – better still 2 × 200 W in parallel, to use diffuse irradiance more efficiently. Foldable modules (CIGS thin film) often have a 10–15 % higher low-light efficiency than rigid monocrystalline modules under overcast skies, but cost 1.5 times as much.

STC (Standard Test Conditions)

Test conditions with 1000 W/m² irradiance, 25 °C cell temperature and air mass 1.5 for comparability of module power ratings in accordance with IEC 61215.

Sun hour

One hour with 1000 W/m² irradiance. In Germany approx. 4.5 h in summer, approx. 1.5 h in winter.

CIGS (copper indium gallium selenide)

Thin-film solar cell technology with high low-light efficiency and flexibility, often used in folding modules.

MPPT (Maximum Power Point Tracking)

Charge controller technology that dynamically adjusts the voltage and current of the solar module in order to always draw the maximum power (efficiency >95 %).

PWM (pulse width modulation)

Simple charge controller that reduces the module voltage to battery level in a duty cycle, but does not use the full module power in doing so.

Voltage drop according to DIN VDE 0100-520

Maximum permissible voltage loss in cables: 3 % for lighting, 1 % for charging cables (recommended).

Efficiency (η)

Ratio of output power to input power. MPPT controllers: 95–98 %, PWM: 70–85 % depending on the voltage spread.

LiFePO4 (lithium iron phosphate)

Battery chemistry with 3.2 V nominal voltage per cell, high cycle life (≥3000 cycles) and inherent safety thanks to the olivine structure.

BMS (Battery Management System)

Electronic protection circuit for monitoring cell voltages and temperatures and for balancing, preventing deep discharge and overcharging.

Peukert effect

Phenomenon in lead-acid batteries whereby the available capacity decreases as the discharge current increases. LiFePO4 is barely affected by this.

Bypass diode

Diode integrated into solar modules that, under partial shading, routes the current around the shaded cells and prevents hot-spot damage.

INSULATION RESISTANCE

Measurement according to DIN EN 62446-1 for the safety check of module insulation before the system is put into operation.

Section 12 – Charge controller technology: MPPT vs. PWM and the art of wiring

12.1 Physics of the charge controller: why MPPT makes the difference

A 36-cell solar module (60 cells at >300 W) delivers its maximum power at about 17–18 V (V_mp). A 12 V LiFePO4 battery, however, requires a charge end voltage of 14.4–14.6 V. A PWM controller clips the excess voltage by briefly connecting the module directly to the battery – the full module current flows, but not the full module power, because the voltage is limited to battery level. With a 100 W module (I_mp = 5.56 A), PWM gives a maximum of 5.56 A × 14.4 V = 80 W. An MPPT controller converts the excess voltage into additional charging current and, with the same module, delivers 100 W × 0.96 (efficiency) = 96 W. At cool temperatures and high irradiance, the module voltage rises to 19–20 V; the MPPT advantage grows to 20–25 % more yield compared with PWM.

12.2 Cable cross-section according to DIN VDE 0100-520 and voltage drop

According to the recommendation of DIN VDE 0100-520, the voltage drop between charge controller and battery must not exceed 1 % of the nominal voltage, i.e. 0.12 V at 12 V. For a 4 m long positive and negative line (2 m one-way length) at 40 A charging current, the required cross-section is given by the formula:

A = (2 · L · I) / (κ · ΔU) where κ (conductivity of copper) = 56 m/(Ω·mm²).

Calculation: A = (2 · 2 m · 40 A) / (56 · 0.12 V) = 160 / 6.72 = 23.8 mm². The next higher standard cross-section 25 mm² is selected. For module supply lines (10 A, 10 m length), the same calculation shows that 6 mm² is sufficient. Use only finely stranded copper cables with double insulation (H07RNF rubber-sheathed cable) and rely on MIDI fuses or circuit breakers (e.g. 50 A) directly at the positive terminal of the battery, in accordance with ISO 8820-5.

12.3 Combination with battery management system (BMS)

An external BMS (e.g. Victron VE.Bus BMS) communicates with the MPPT controller via Bluetooth (VE.Smart Networking) or cable in order to control the charging curve depending on temperature. With LiFePO4 cells, the BMS deactivates the charge controller at cell voltages above 3.65 V or temperatures below 0 °C (protection against lithium plating). High-quality controllers such as the Victron SmartSolar MPPT 100/30 offer an adjustable absorption and float voltage as well as automatic compensation of -2 mV/°C per cell – essential for the service life of your 100 Ah LiFePO4 battery.