Work out roughly how much electricity your campervan uses in a day, then turn that into a sensible starting point for battery capacity, solar panel wattage and inverter size. Change every figure to match your own van.
How to use this calculator
You do not need to understand campervan electrics before you start. Work through it in this order:
- Choose your battery type. If you already know whether you plan to use lithium or AGM/lead-acid, select it. If you are only exploring, leave the default lithium setting for now.
- Choose how many days you want to manage without charging. One day means the battery only needs to cover roughly one day’s use before being recharged.
- Leave the solar settings alone at first. The default 3 full-sun hours and 75% system yield give you a useful starting point. You can experiment with them later.
- Edit the appliance list. Change the example wattages and hours to match what you expect to use. Add anything missing and remove anything you will not have.
- Choose the supply for each appliance. Most lights, pumps, USB chargers and 12V fridges are 12V/DC. Appliances plugged into a normal household-style socket while off-grid are usually 230V via inverter.
- Read the four results. They give you a starting point for daily energy use, leisure-battery capacity, solar-panel wattage and inverter size.
Not sure about a number? Use the wattage printed on the appliance or its power supply. For hours/day, estimate how long it actually runs in a typical day. You can come back and refine the figures later.
1. Your setup
2. What do you use each day?
The rows below are examples only. Replace the wattage and usage time with figures for your own appliances. Watts × hours per day gives watt-hours. For appliances that cycle on and off, use the average time they are actually drawing their rated power, or enter your own measured daily Wh as an equivalent watt/hour combination.
| Appliance | Watts | Hours/day | Qty | Supply | Battery Wh/day | Remove |
|---|
3. Planning result
Planning tool, not a wiring design. Solar output changes dramatically with season, weather, shading, panel angle and location. The solar result does not add a separate allowance for battery charge/discharge losses or inverter standby draw, so reduce the system-yield setting or add those loads if you want more margin. Inverter surge loads can be much higher than running watts. Cable size, fusing, earthing, RCD protection and mains hook-up work need to be designed for the actual installation.
How the calculator works
The useful place to start is the energy you actually expect to use. A 20W light used for five hours is 100Wh. A 700W appliance used for ten minutes is about 117Wh. That is why the biggest appliance is not always the biggest daily drain.
12V appliances are counted directly. For 230V appliances the calculator allows for the inverter efficiency selected above, because the battery has to supply more energy than reaches the appliance. That is why the table shows battery Wh/day rather than just the appliance’s own watt-hours.
How much leisure battery do I need?
The calculator converts the daily watt-hours into amp-hours at 12V, multiplies by the number of days you want to cover, then allows for the usable portion of the battery. The default is 80% usable for LiFePO4 and 50% for AGM/lead-acid. These are planning assumptions rather than promises about a particular battery, so use the manufacturer’s limits for the battery you actually buy.
If you are still deciding how to build the system, our DIY camper electrical system guide shows the sort of components that have to work together.
How much solar do I need on a campervan?
Solar sizing starts with the same daily energy figure. The calculator divides that by the full-sun-hours figure you enter and by the selected system yield. The result is the panel wattage that would, in those assumed conditions, replace about one day’s calculated consumption.
That does not mean a panel will produce that amount every day in Britain. Winter, cloud and shade can reduce output severely, so try the calculator with a lower full-sun-hours figure to see what happens. For installation choices, see our campervan solar panel guide.
What size inverter do I need?
Inverter size is about peak AC power rather than daily energy. The calculator adds together the rated watts of every appliance marked 230V via inverter, then adds 25% headroom as a simple starting point. That assumes those AC appliances could be running together. If you know you will never run some of them at the same time, remove or change those rows while sizing the inverter. Motors, compressors and some electronic appliances can have a much larger start-up surge, so the continuous and surge ratings of the actual inverter both matter.
Our campervan inverter guide goes into the choice in more detail. If a microwave is one of your biggest loads, the campervan microwave guide is worth reading before sizing the whole system around it.
What about campsite electric hook-up?
This calculator works backwards from your everyday loads to help with the 12V side and off-grid planning. A campsite hook-up is a separate mains supply with its own protection and limits. Our campervan electric hook-up guide covers that side of the installation.
Use the result as a starting point
The useful result is not one magic number. It is seeing which appliance is driving your energy use and what changes when you alter the assumptions. Try removing the microwave, reducing laptop time or changing the number of days without charging. That makes it much easier to decide whether you need a bigger battery, more solar, or simply a different way of using power.