Home energy · off-grid
Off-grid power load calculator.
Add up daily watt-hours and the simultaneous load, then size the inverter for the surge from the biggest motor.
Inputs
Your load
Enter your numbers.
How do you calculate an off-grid power load?
Daily watt-hours = watts x quantity x hours used per day, summed over every appliance. Watts times hours giving watt-hours is the standard unit relationship described by the U.S. EIA. The inverter must carry the loads that run at the same time (the sum of their running watts) and also the start-up surge of the largest motor, which is higher than its running watts. This tool adds the extra surge of the single biggest motor on top of the combined running load; read surge watts from the nameplate or manual.
Worked example. The placeholder rows are a fridge (100 W for 8 hours), lights (6 x 10 W for 5 hours), a laptop (60 W, 4 hours) and a pump (300 W, 1 hour, 900 W surge): 800 + 300 + 240 + 300 = 1,640 Wh per day. Running together is 100 + 60 + 60 + 300 = 520 W, and the pump start adds 600 W for a 1,120 W peak.
Frequently asked questions
How do I calculate how much power I need off-grid?
Multiply each appliance's watts by how many you have and the hours it runs per day, then add them up. That is your daily watt-hours; divide by 1,000 for kWh.
What inverter size do I need?
At least the combined running watts of everything that can be on at once, plus headroom, and a surge rating that covers the biggest motor starting. Motors such as pumps and compressors start at several times their running watts.
Why uncheck "runs with the other loads"?
Some loads, like a well pump and a microwave, you can choose never to run together. Unchecking them keeps their energy in the daily total but leaves them out of the inverter size.
Where do I find an appliance's watts?
On its rating label or in the manual. If the label gives only amps, multiply by volts.
Sources
Checked
The rows are placeholder loads, not typical figures. Headroom is a rule of thumb. Replace everything with values from your own appliance labels. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Planning estimate only. Surge figures vary by appliance and brand, so confirm with the manufacturer before buying an inverter.