With your own solar system on the motorhome roof, you gain independence from mains power and can recharge your leisure battery even when stationary. The DIY build is not rocket science if you follow a few basic technical rules and dimension the components correctly. In this step-by-step guide you will learn how to plan your motorhome solar system, which battery – AGM, GEL or LiFePO4 – is optimal for your purpose, how to mount the modules properly and which pitfalls to avoid with wiring and the charge controller. Electronicx GmbH from Cleebronn provides you with the necessary expertise and high-quality components.
A typical self-sufficient solar power supply consists of solar modules, a charge controller, the leisure battery as well as fuses, cables and mounting material. The correct matching of power (watts), voltages (volts) and capacities (ampere-hours, Ah) determines efficiency and longevity. For the battery, AGM deep-cycle batteries have become established as a robust standard, while LiFePO4 batteries save weight and offer significantly more cycles. We show you how to design your system step by step.
1. Needs analysis and planning: how to dimension your solar system
Before you order components, determine your daily power consumption. Note down all 12 V consumers: fridge (approx. 2–4 A), LED lighting (0.1–0.5 A per light), water pump (3–8 A briefly), heating fan, smartphone charging, possibly a TV or inverter. The sum of the ampere-hours (Ah) per day multiplied by 12 V gives the required energy in watt-hours (Wh).
Example: a compressor fridge draws 3 Ah, LED interior lighting a total of 1 Ah, water pump 2 Ah (on average), smartphone charging 0.5 Ah – that is 6.5 Ah per day, i.e. 78 Wh. In practice, allow for a reserve and seasonal fluctuations; therefore set at least 30% buffer and dimension the battery for around 2–3 days of self-sufficiency without sun.
This gives you the required battery capacity. With AGM batteries, you should limit the usable capacity to 50 % so as not to drastically shorten the cycle life. If you need 60 Ah per day, for example, choose an AGM of at least 120 Ah (C20 rate). With LiFePO4, the depth of discharge can be 80–90 %, so a 100 Ah lithium battery often already delivers 80 Ah usable – considerably lighter and more compact. Our overview on the AGM vs. GEL vs. LiFePO4 page will help you decide.
2. The right battery for your solar system
Three battery technologies come into consideration for the solar supply in a motorhome: AGM, GEL and LiFePO4. Each has characteristic properties in terms of cycle stability, weight, price and charging behaviour that you should definitely take into account in your planning. The following table summarises the most important differences:
| Property | AGM battery | GEL battery | LiFePO4 battery |
|---|---|---|---|
| Cycles at 50 % depth of discharge | 300 – 600 | 500 – 800 | 2000 – 5000 |
| Usable capacity | ~50 % (at 50 % DoD) | ~50 % (at 50 % DoD) | 80 – 90 % |
| Weight per 100 Ah (12 V) | approx. 28 – 32 kg | approx. 30 – 34 kg | approx. 12 – 14 kg |
| Position-independent | yes | yes | yes |
| Price range (100 Ah, 12 V) | 150 – 250 € | 200 – 350 € | 400 – 900 € |
| Charging voltage constant voltage | 14,4 – 14,8 V | 14,1 – 14,4 V | 14.2 – 14.6 V (typical) |
| Maintenance-free | yes | yes | yes |
2.1 AGM batteries – the robust all-rounder
AGM batteries (Absorbent Glass Mat) are lead-based, valve-regulated VRLA batteries with electrolyte bound in a glass fleece. They are inexpensive, insensitive to vibration and well suited to moderate cyclic loads. Capacities around 80 Ah and 100 Ah are particularly popular. Important: AGM batteries cannot tolerate deep discharge below 50 % – a reliable voltage monitor or charge controller cut-off is essential. You will find all the details on the technology in our guide AGM battery explained.
2.2 GEL batteries – solid for continuous use
GEL batteries use gelled electrolyte and are characterised by high cycle stability under medium loads. Their charging voltage is somewhat lower, so the charge controller must support the appropriate charging programme. For solar campers travelling in moderate use all year round, a GEL battery can be a good alternative. However, the charge acceptance is lower – an MPPT controller helps here to make use of more solar energy.
2.3 LiFePO4 – the lightweight solution with maximum performance
Lithium iron phosphate batteries offer the best power-to-weight ratio and extreme cycle stability. Despite higher initial costs, they pay for themselves for frequent users thanks to their long service life. Our comparison AGM vs. GEL vs. LiFePO4 provides a sound basis for making a decision.
3. Solar modules: technology and power calculation
Monocrystalline modules are the standard today, as they achieve the highest efficiency (18–22 %) with limited roof space. For motorhomes, modules with rated outputs between 80 and 200 watts are usually installed. A rule of thumb: per 100 Ah of battery capacity (with AGM), you should plan at least 100 to 150 watts of solar power to reliably fully charge the battery on a sunny day.
The real yield depends on irradiation, orientation and time of year. In Germany, a 100 W module achieves around 400–500 Wh per day in summer. A 200 W set therefore delivers around 800–1000 Wh, which is enough for an average motorhome household with a compressor fridge and two overnight stays off-grid. Calculate your required module power using the following formula:
Module watts (Pmodule) = (daily energy requirement in Wh) / (average sunshine hours × 0.7)
The factor 0.7 accounts for system losses (charge controller, cables, shading).
4. Charge controllers: MPPT vs. PWM and choosing the right model
The charge controller is the heart of your solar system. Simple PWM controllers (pulse width modulation) switch the module voltage directly to the battery and only make sense if the module voltage is just above the battery voltage. However, since modern 12 V solar modules have rated voltages of 18 V and more, PWM controllers waste valuable power. MPPT controllers (Maximum Power Point Tracking) convert the excess voltage into higher charging current and make up to 30 % better use of the module power.
For a system from 100 W and valuable batteries such as AGM or LiFePO4, we therefore always recommend an MPPT solar controller. Look for adjustable charging curves (AGM, Gel, Lithium), temperature compensation (for lead) and a load output with programmable deep discharge protection. Good devices for 20–30 A cost around 60–150 €.
5. Wiring, fuses and system voltage
Use only flexible, double-insulated copper cables with finely stranded conductors (e.g. H07V-K or solar cable). The cable cross-section depends on the current and cable length: at 10 A and 5 m cable length (there and back), 4 mm² is sufficient; at 20 A and 8 m it should be at least 6 mm². For the connection between panel and charge controller, 4 mm² is often sufficient; between charge controller and battery, 6–10 mm² is better to keep voltage drops low.
Use the following fuses:
- String fuse (per parallel module string) – mandatory when connecting several panels in parallel.
- Surge protection (varistor or surge arrester) on the module side – protects the controller against indirect lightning effects.
- Fuse or circuit breaker between charge controller and battery, rated for the maximum charging current + 25 %.
- Battery isolator switch to allow work to be carried out with the system de-energised during servicing.
All system components must be water-protected to IP65 if they are installed outside or in damp storage compartments. The charge controller and battery should be placed in a dry, ventilated interior space.
6. Step-by-step installation of solar panels on the motorhome roof
6.1 Preparing the roof surface
Clean the roof surface thoroughly and check its load-bearing capacity. Mark the position of the panels so that they are not shaded by roof vents, antennas or air conditioning units. Use corrosion-free aluminium mounting profiles and an aerodynamic arrangement (crosswise or lengthwise mounting).
6.2 Mounting: bonding or screwing?
On many modern motorhomes, bonding with Sikaflex roof adhesive is state of the art. Aluminium brackets are bonded over a large area, and the panels are later screwed to these brackets. Never screw directly through the roof skin – the risk of leaks is too high. If you can use an existing roof rail, you remain flexible and avoid new openings.
6.3 Cabling and roof penetration
Route the solar cables through a double cable gland (screwed-in roof cowl) into the vehicle interior. Additionally seal the penetration with butyl tape or permanently elastic sealant. Route the cables inside the vehicle to the installation location of the charge controller; avoid kinks and sharp edges.
7. Commissioning and testing
Once all components are connected, proceed in this order for initial commissioning:
- Check battery terminals and fuses – connect the battery positive terminal first.
- Start the charge controller: first connect the battery, then the solar panels. The controller automatically detects the system voltage (12 V).
- Configure the charging programme: select the correct battery type (AGM, gel, LiFePO4) according to the manufacturer's specifications. Some controllers detect this automatically.
- Short-circuit and voltage test: measure the module open-circuit voltage and charging current with a multimeter.
- Test deep discharge protection: switch on loads at the load output and observe the voltage drop; the controller must switch off when the limit voltage is undershot.
- Monitor the charging profile with a battery computer or on the controller display – the final charging voltage should be within the recommended values (e.g. AGM 14.7 V).
A working display now shows the current charging power, battery status and daily energy yield. Note the initial values as a reference for later system checks.
8. Common mistakes and troubleshooting
- Cable cross-sections that are too thin: Causes voltage drops that mislead the charge controller and drastically reduce efficiency.
- No separate earth connection: Always connect the battery earth to the charge controller first, otherwise the internal reference voltage can be lost.
- Shading of a single module: In parallel connection without bypass diodes, the output of all modules drops dramatically. Modern panels have integrated bypass diodes – make sure of this.
- Incorrect charging voltage: If the voltage is too high, lead batteries gas; if it is too low, they sulphatate. The charge controller settings are therefore critical – see our guide to charging a 12V battery.
- Ignoring temperature: In cold conditions, lead batteries require a voltage increase – a temperature sensor on the controller is advisable for AGM and GEL batteries. More on this under Protecting an AGM battery from frost.
9. Care and long-term operation
Clean the solar panels at least twice a year with clean water and a soft sponge. Resin, bird droppings or pollen reduce light yield by up to 10 %. Check that the connection terminals on the battery and controller are secure and, if there are warning signs, contact Electronicx support at [email protected] or by phone on +49 7135 7194106.
During longer periods of inactivity in winter, store the lead-acid battery fully charged and protected at room temperature; LiFePO4 batteries can be stored cool at a 50 % state of charge.
Frequently asked questions (FAQ)
What solar output do I need for my motorhome?
That depends on your daily consumption. As a rough rule of thumb: at least 100 to 150 watts of solar output per 100 Ah of AGM battery capacity. For a typical two-person household with a compressor fridge, 150–200 watts is usually enough to remain self-sufficient in good weather.
Can I connect the solar system to the starter battery?
Yes, via a split charge relay or a charge controller with a separate output. However, it is better to use a separate leisure battery (auxiliary battery), as starter batteries (AGM car battery) are not cycle-resistant and can be damaged by deep discharge.
Which battery is best for solar – AGM or LiFePO4?
AGM is cheaper, LiFePO4 offers more usable capacity and cycles. For intensive use, lithium technology is worthwhile. For occasional campers, a good AGM is often the more economical choice. You can find a detailed comparison under AGM vs. GEL vs. LiFePO4.
Do I need an MPPT charge controller or is PWM enough?
From around 100 watts of solar module output and especially with expensive batteries, we recommend MPPT, because it delivers up to 30 % more yield and extends battery life. PWM can be sufficient for very small systems (50 W) with a suitable module voltage.
Where should I install the charge controller?
As close as possible to the battery, cool, dry and well ventilated. The cable runs from the roof to the controller and from the controller to the battery should be short to keep voltage losses low.
Can I expand the solar system later?
Yes, with MPPT controllers the module voltage can be increased by connecting in series, as long as the controller's maximum input voltage is not exceeded. A second controller is also possible. Plan the controller size with a reserve from the outset (e.g. 30 A instead of 20 A).
How do I protect the leisure battery from deep discharge?
Use the load output of the charge controller with deep discharge protection or an external battery monitor. With AGM and gel batteries, you should set a cut-off at 11.8–12.0 V so as not to jeopardise the cycle life. More tips in the 12-volt battery guide.
What does a complete DIY solar system cost?
Depending on output and battery, the cost is between 300 and 1,500 €. A 150 W set with MPPT controller and wiring costs around 300–400 €, a 100 Ah AGM battery about 150–250 €. Complete with a LiFePO4 battery and 200 W modules, you should expect 800–1,200 €.
About Electronicx GmbH – your specialist retailer for mobile energy and safety technology. Address: Hindenburgstr. 37A, 74389 Cleebronn, Germany. Phone: +49 7135 7194106, email: [email protected]. Managing Director: Andreas Emschanow, VAT ID: DE300176428, HRB 752730 (Stuttgart Local Court).