How much solar and battery does a campervan need?

Get three units straight first

Watts (W) describe power at a moment. Watt-hours (Wh) describe energy over time: a steady 20W load running for five hours uses 100Wh. A kilowatt-hour is 1,000Wh. This is the useful common currency for a fridge, a laptop and the energy arriving from solar.

Amp-hours (Ah) describe electrical charge. Include nominal voltage to estimate stored energy: 12.8V × 100Ah is 1,280Wh. That is nominal capacity, before choosing a discharge window or accounting for losses. Its maximum current and surge ratings set separate limits on the appliances it can supply. [1]

Keep those questions separate: energy determines how long a load can run; current and power limits determine whether the system can run it at all.

Build a daily load list

List the appliances you use during a typical day. Include lighting, refrigeration, water pump, ventilation, heating controls, phones, computing and equipment left on standby. Note which are DC and which require an inverter. Measure a normal day where possible, then a more demanding one.

On a narrow screen, scroll the table sideways to compare every column.

The fridge's assumed 12W average includes time with its compressor off. Actual use changes with ambient temperature, ventilation, thermostat setting and door opening. If you only have running watts, estimate duty cycle explicitly and replace it with measured daily energy when you can.

Add losses, then check the biggest loads

Make the budget battery-side before sizing storage. AC appliances take additional battery energy through an inverter; its standby draw also matters. DC converters and cabling have losses too. Use efficiency figures from the equipment documentation or your own measurements. Avoid adding the same loss twice.

Now check the worst simultaneous demand and starting surge. A short burst from a kettle can use less daily energy than a fridge, while demanding much higher current. Inverter, battery, BMS, wiring and protection limits need checking together. The battery and BMS must be able to supply the inverter's current and surge demand. [2]

Ask whether changing an appliance or reducing the time it runs would simplify the system more effectively than buying extra storage.

Choose how long you need to manage without useful charging

Suppose your completed battery-side budget is 600Wh a day. Two days without useful charging need 1,200Wh of planned usable storage, before an additional reserve. At a chosen 80% operating window, that corresponds to 1,500Wh nominal, or about 117Ah at 12.8V. You can adjust those assumptions to suit your own trips.

Different discharge windows change the nominal capacity needed. Our battery guide compares a 12V 110Ah AGM at a chosen 50% window, 660Wh, with a 12.8V 100Ah LiFePO4 at 80%, 1,024Wh. These are chosen planning windows; use the limits and operating guidance for your own battery. Temperature, age, discharge rate and the exact model's instructions matter. [3] [1]

Decide what the reserve is for: a delayed journey, poor weather or a load you cannot switch off. A plan that spends its entire reserve on every normal night has little flexibility left.

Panel watts are not daily watt-hours

Rated panel power is measured under specified test conditions. Daily collection depends on the light available, mounting, temperature and losses. Panels can generate from diffuse light on cloudy days, but nearby shade still reduces output. Estimate collection from the intensity of the sunlight as well as the hours available. [4]

For an explicitly hypothetical illustration, 400W operating for the equivalent of two full-output hours gives 800Wh before further losses. Those two equivalent full-output hours are an assumption for this example. A different location, season or parking position can make that assumption inappropriate.

Also plan recovery. If a poor spell leaves a 1,000Wh deficit and ordinary daily use consumes everything the next day's solar supplies, the battery cannot catch up. You need surplus generation, another charging source or reduced demand.

Use seasonal evidence, not an annual average alone

Investigate the months and places where you will travel. The European Commission's PVGIS tool provides long-term solar-resource and PV-performance information. Its grid-connected calculation reports monthly and annual averages from multi-year weather data, with location and mounting inputs. [5]

Use monthly averages to understand seasonal patterns. For a particular trip, check the near-term forecast and allow for days below that average. Battery acceptance, loads and charge control need to be considered separately from the grid-connected model. PVGIS offers a separate off-grid tool for systems with storage, which still depends on its assumptions. [6]

For mobile use, include flat roof mounting if that is your real arrangement and revisit shade at the parking spot. Long-term evidence helps set expectations; near-term weather and terrain information help with day-to-day choices.

Use the worksheet to compare assumptions

Enter your loads and daily operating times, then change one assumption at a time. Try a longer laptop day, a higher fridge average or another day without charging. It helps you see which loads and assumptions have the biggest effect on demand and reserve.

Keep energy estimates and equipment limits separate. The next stage is to check cold-weather panel voltage, roof mounting, alternator compatibility and protection for the actual equipment. Record uncertain inputs and replace them with measurements or manufacturer specifications before turning the plan into a build.

What an actual system specification needs next

• Daily and peak loads, essential loads and required reserve.
• Travel locations, seasons, parking shade and expected driving or hook-up.
• Usable roof dimensions, vents, roof-load and mounting constraints.
• Exact panel voltage/current data and controller limits, including cold conditions.
• Battery chemistry, current limits, temperature protection and permitted configuration.
• Compatible solar, alternator and mains charging, plus wiring and protection design.
• Complete delivered and installed budget, access for maintenance and monitoring.

Solar Outlook can help explore terrain-aware daylight and weather-based potential, with supported equipment readings adding context. Use that information alongside the equipment and installation checks above. Start with a transparent energy budget, then have the complete installation specified around the way you will use it.