Van and RV Solar + Battery Sizing: How Many Panels and How Much Battery for Your Daily Use
Size a van or RV solar and battery system from your real daily watt-hours: panels from peak sun hours and derating, battery from usable depth of discharge and inverter loss, and charge controller from array watts.
Step-by-Step Procedure
List your loads and add up daily watt-hours
List each load, its watts and its hours per day, then multiply and add. For example, a 12 V fridge at about 400 Wh per day, lights 100 Wh, a laptop 150 Wh and a fan 100 Wh total about 750 Wh, and a small margin brings a planning total to about 1,000 Wh per day. Do not size from panel wattage alone; size from daily energy.
Size the solar array from peak sun hours and derating
Divide daily watt-hours by peak sun hours, then divide again by a derating factor of about 0.75 to 0.78 for real-world losses from temperature, wiring, controller efficiency and imperfect angle. For 1,000 Wh per day at 4 peak sun hours: 1,000 / (4 x 0.75) = 333 W, so round up to about 400 W of panels, for example two 200 W panels. Peak sun hours change with location and season, so winter sizing needs a lower value.
Size the battery from usable energy, not nameplate energy
A battery's nameplate watt-hours are not all usable. For a lithium iron phosphate battery, plan on about 80% usable depth of discharge, and for lead-acid about 50%. Also allow for inverter loss of about 10%. For 1,000 Wh per day with one day of autonomy: 1,000 / (0.80 x 0.90) = 1,389 Wh nominal, which is about 116 Ah at 12 V. A 200 Ah 12 V lithium battery gives more margin for cloudy days and winter, as shown in the worked example.
Size the charge controller from array watts
For an MPPT controller, divide array watts by battery voltage and multiply by 1.25. For a 400 W array on a 12 V battery: 400 / 12 = 33.3 A, x 1.25 = 41.7 A, so use a 50 A controller (the next standard size up). Check the controller's maximum input voltage against the panels' cold-weather open-circuit voltage as well.
Check DC wiring, fusing and voltage drop
Keep 12 V runs short and thick, because voltage drop bites hardest at low voltage. Fuse each major circuit close to the battery, use a battery monitor or shunt so you can see real state of charge, and treat any fixed or mains-connected work as a job for a qualified installer. This guide is a planning estimate for a leisure system, not a substitute for product manuals or local safety rules.
Worked Practical Example
1,000 Wh per day, 4 peak sun hours, 12 V system
A van using about 1,000 Wh per day, with 4 peak sun hours in the planning season.
- •Solar: 1,000 Wh / (4 peak sun hours x 0.75 derating) = 333 W, round up to 400 W of panels
- •Battery minimum: 1,000 Wh / (0.80 usable x 0.90 inverter efficiency) = 1,389 Wh nominal
- •Battery minimum at 12 V: 1,389 / 12 = 116 Ah, so a 120 Ah battery is the bare minimum
- •Recommended with margin: 12 V 200 Ah lithium, 2,400 Wh nominal, about 1,920 Wh usable at 80%, about 1,728 Wh delivered after inverter loss
- •Charge controller: 400 W / 12 V x 1.25 = 41.7 A, so a 50 A MPPT controller
Common Mistakes to Avoid
- ✕Confusing panel watts with daily watt-hours: a 400 W panel does not give 400 Wh in an hour unless conditions are ideal.
- ✕Using nameplate battery capacity as usable capacity, especially with lead-acid at 50% usable depth of discharge.
- ✕Forgetting inverter loss and controller derating, then wondering why the battery runs flat early.
- ✕Sizing for summer sun hours and then running short in winter, when peak sun hours are much lower.
Frequently Asked Questions
How many solar panels do I need for a van?
It depends on your daily watt-hours and your peak sun hours. Divide daily watt-hours by (peak sun hours x 0.75) to get array watts, then round up to standard panel sizes. A 1,000 Wh per day van at 4 peak sun hours plans to about 400 W.
How long will a 100 Ah lithium battery run a 12 V fridge?
A 100 Ah 12 V lithium battery holds about 1,200 Wh nominal and about 960 Wh usable at 80% depth of discharge. At about 400 Wh per day for a fridge alone, that is roughly two days before other loads and inverter losses, so treat it as a planning estimate, not a guarantee.