# Solar for a camper: panel, battery and controller in plain terms

> How to estimate consumption in watt-hours, size panel and controller, choose between LiFePO4 and AGM, and what winter in northern Europe allows.

Source: https://camping-spirit.com/magazine/rv-solar-basics/

*By CampingSpirit Editors, 2026-09-28*

A camper solar system has four parts: panels that collect energy, a charge controller that manages it, a battery that stores it, and often an inverter that turns 12 V into 230 V. Each part is simple enough, and trouble usually comes from sizing them without knowing how much energy is actually used. The starting point is always consumption, measured in watt-hours (Wh), not the panel.

## Start with consumption

List every device, its power in watts and the hours it runs per day, then multiply. A compressor fridge may average around 300 Wh a day in warm weather, LED lighting 60 Wh, phones and a laptop 150 Wh, and a water pump and fan together 90 Wh. That totals about 600 Wh a day, a plausible figure for two people living modestly. These values are illustrative: measure your own with a battery monitor or the data sheets, because heating, a coffee machine or an induction hob change the picture completely.

## Sizing the panel

A panel’s rated watts apply under laboratory conditions of 1,000 W per square metre. On a roof, flat and sometimes warm, a rule of thumb is to count on roughly 70 to 75 per cent of that. With about five hours of strong sun in summer in southern Europe, a 200 W panel then yields around 200 × 5 × 0.75, or 750 Wh a day, enough for the 600 Wh example. These are estimates; real output varies with latitude, tilt and haze.

Shading deserves more attention than panel size. Panels wired in series lose a large share of their output when even a small area is shaded, for example by a roof vent, an aerial or a tree, so a layout away from obstacles is worth more than extra watts. If the roof is crowded, consider parallel wiring or smaller panels, and tilt-up mounts only if parking direction allows it.

## Controller: MPPT or PWM

A PWM controller essentially connects the panel to the battery and tapers the current as the battery fills; the panel voltage must therefore match the battery system. An MPPT controller converts the panel’s voltage to what the battery needs and can track the panel’s best operating point, which allows higher-voltage panels or series strings and helps in cold, cloudy conditions. On small systems the efficiency gain may not justify the price; on larger ones or with several panels in series, MPPT is the normal choice.

To size it, divide panel watts by battery voltage: 200 W at 12 V is about 17 A, so a 20 A controller leaves little margin and a 30 A model is more comfortable. Check the maximum input voltage in the data sheet, because a panel’s open-circuit voltage is higher than its working voltage and rises further in the cold.

## Battery: LiFePO4 or AGM

Capacity is not the same as usable capacity. AGM batteries are normally used to about 50 per cent depth of discharge for longevity, so a 100 Ah unit gives roughly 50 Ah. LiFePO4 can usually be cycled to 80 to 100 per cent. Cycle life is often quoted at 300 to 800 cycles for AGM and 3,000 to 5,000 or more for LiFePO4, and a 100 Ah AGM weighs about 27 to 32 kg against 10 to 14 kg for lithium; manufacturers’ figures should be read as indicative.

Lithium has one important condition: charging below 0 °C causes lithium plating, which permanently reduces capacity, and the usual recommended charging window is about 5 to 45 °C. A battery with a temperature-aware management system, or a heated model, is needed for winter use, and the battery should sit inside the heated living space where possible. For the 600 Wh example, one 100 Ah LiFePO4 battery stores about 1,280 Wh nominal and covers roughly a day and a half to two days without sun.

## Inverter and safety

An inverter wastes some energy even when idle, so switch it off when 230 V is not needed and compare the no-load figure in the data sheet. Pure sine wave models suit sensitive electronics. Every positive cable from the battery needs a fuse close to the battery terminal, rated for the cable, and cable cross-sections must match the current. Use a DC-rated breaker on the panel side, never work on live wiring, and have a competent installer check anything above basic 12 V work.

## What to expect in northern Europe in winter

Winter is the limit of solar. A PVGIS-based calculation for central Germany puts November to February at about 14 per cent of the annual yield, with December alone at 2.6 per cent against 12.6 per cent in June, and with less in Hamburg than in Munich. A 200 W panel on a flat roof may therefore give only 50 to 100 Wh on a good December day and almost nothing under snow, against a 600 Wh daily load.

The practical answer is a second source: a DC-DC charger from the alternator while driving, mains charging when on a site, and leaner consumption, for instance gas heating and a fridge that is not oversized. Solar still helps in winter by topping up the battery and covering standby loads, and it pays off from spring to autumn. Planning for the weakest month rather than the best avoids unpleasant surprises.

## Sources

- [What is the difference between a PWM and MPPT charge controller? (Silicon Solar)](https://www.siliconsolar.com/what-is-the-difference-between-a-pwm-and-mppt-charge-controller/)
- [AGM vs LiFePO4 Batteries: Lifespan, Weight and Value Compared (Vatrer Power)](https://www.vatrerpower.com/en-de/blogs/news/agm-vs-lithium-battery-life)
- [Why you should not charge a lithium battery below 0 °C or 32 °F (REDARC Electronics)](https://us.support.redarcelectronics.com/hc/en-us/articles/13856244101007-Why-you-should-not-charge-a-lithium-battery-below-0-C-or-32-F)
- [Photovoltaik im Winter: 14 % Ertrag von November bis Februar (reduco)](https://reduco.ai/blog/solar/photovoltaik-im-winter-ertrag)
- [Solaranlage Ertrag (Aroundhome)](https://www.aroundhome.de/solaranlage/ertrag/)