Off-Grid Solar Sizing Calculator
"What size solar kit do I need?" depends entirely on what you run and where you park. Enter your daily use and get the array wattage, battery bank and inverter size to match.
Array uses a 1.3x loss factor (panel heat, wiring, charge controller, battery round-trip). Inverter is sized at 1.25x your largest simultaneous load for surge headroom.
How the math works
array watts = daily Wh ÷ sun hours × 1.3 battery Wh = daily Wh × days of autonomy ÷ usable depth battery Ah = battery Wh ÷ system voltage inverter watts = largest simultaneous load × 1.25
Frequently asked questions
What size solar panel kit do I actually need?
Work backwards from consumption, not panel marketing. Add up the watt-hours you use per day, divide by your peak sun hours, then add roughly 30% for charging and inverter losses. A van running a fridge, lights, laptop and fan typically lands between 400 and 800 watts of panel.
How many batteries do I need?
Battery capacity is set by how long you must run without sun — days of autonomy. Two days is normal for an RV or van, three to five for a cabin. Lithium (LiFePO4) can be discharged to about 80-90%, lead-acid only to 50%, which is why lead-acid banks have to be roughly twice as large for the same usable energy.
What size inverter should I buy?
The inverter is sized by the largest simultaneous load, not by daily energy. Add the running watts of everything you might have on at once and add headroom for motor startup surges — a fridge or a power tool can briefly draw two to three times its running watts.
Why is the array bigger than my daily use divided by sun hours?
Real systems lose energy at every step: panel heat and soiling, wiring, charge controller conversion, and battery round-trip efficiency. This calculator applies a combined 1.3x factor, which is the common field rule for off-grid design.
Related
Get weekly eco swaps in your inbox.
Plastic-free picks, low-tox swaps, and sustainable finds — curated every week. No greenwashing. Unsubscribe anytime.
No spam. We hate it too.