Use this free off-grid solar and battery calculator to size your system. Pick what you want to power and for how long, and you'll get an estimated inverter size, battery bank and solar array, matched to products we carry. No signup needed.
How to size an off-grid solar system
Sizing comes down to three numbers: how much power you use at once, how much energy you use per day, and how long you need to run without sun.
- Inverter size (kW): add up the watts of everything that could run at the same time, then add about 25% headroom. Motors like well pumps and air conditioners need extra power for a few seconds when they start.
- Battery bank (kWh): multiply your daily kWh by the days of backup you want, then divide by about 0.83 to cover usable lithium capacity and inverter losses.
- Solar array (kW): divide your daily kWh by your area's peak sun hours and by about 0.75 for system losses. Size full-time off-grid systems for winter sun.
Browse off-grid inverters, hybrid inverters, server rack batteries and solar panels. Sizing a well pump too? Try our well pump sizing calculator.
Frequently asked questions
How many batteries do I need for an off-grid home?
It depends on how much energy you use per day and how many days you want to run without sun. Multiply your daily kWh by your days of backup, then divide by about 0.83 for usable lithium capacity and inverter losses. For example, a home using 10 kWh per day with 2 days of backup needs about 24 kWh of storage, which is five 5.12 kWh server rack batteries or two 14.3 kWh wall-mount batteries.
What size inverter do I need?
Add up the watts of everything that could run at the same time and add about 25% headroom. If your loads can reach 4,000 W at once, plan for about 5 kW, so a 6 kW inverter like the EG4 6000XP fits. If you have 240V loads such as a well pump, dryer or larger mini-split, you need an inverter with 120/240V split-phase output.
How many solar panels do I need?
Divide your daily kWh by your area's peak sun hours, then by about 0.75 for system losses. For 10 kWh per day with 4.5 sun hours, that's about 3 kW, or roughly 7 panels at 450 W. Full-time off-grid systems should be sized for winter sun, which raises that to about 10 panels.
What's the difference between off-grid and grid-tied with backup?
An off-grid system has no utility connection, so the inverter, batteries and solar carry every load. A grid-tied system with backup keeps the utility, uses a hybrid inverter such as the EG4 12kPV, 18kPV or FlexBOSS21, and switches to batteries during an outage. Solar is optional with backup systems.
How accurate is this calculator?
It gives an estimate based on typical appliance ratings and standard sizing rules. Your actual usage, appliance labels, wiring and local code all affect the final design, so have your installer or electrician confirm the system before you buy.