Solar Load Calculator
Add up your appliances to find daily energy, peak watts and start-up surge. Use it to size panels, a battery and an inverter.
A load is the electricity your appliances use. Before you size panels, a battery or an inverter for a cabin, an RV, a shed or a backup circuit, you need that number. This calculator builds it from a list.
The list starts with a few common items. Add yours from the menu, then replace the watts with the figure on each appliance’s label. Label numbers beat typical values every time.
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Planning estimates only. Your roof, equipment, utility rules and weather change the real numbers, so get a written quote before you buy. Data as of October 2026.
How the load is worked out
For each row, energy per day is the number of units times the watts times the hours. The total is your daily watt-hours. The tool then divides by one minus your losses, because the inverter, wiring and battery waste about 15 to 25% on the way from the panels to the load.
Peak running power adds up every appliance that runs at the same time. The surge figure is that same total plus the extra kick of the hungriest motor starting, such as a well pump or a refrigerator compressor. Motors often pull two to three times their running watts for a moment, and the inverter has to cover that.
Set “Runs at once” to No for things you’ll never use together, like a pump and a microwave. It keeps you from buying an inverter that’s too big.
If you add a ZIP code or a state, the tool also finds the worst month for sun and shows how many panels run the load all year. An off-grid system has to work in that month, not on the yearly average.
Where to find the watts
Look for a label on the back or bottom of the appliance. It lists watts, or volts and amps (multiply them). A plug-in power meter shows the real use of anything you can plug in. Check heating loads first: an electric dryer, water heater or space heater can use more than everything else combined.
Where the numbers come from
- NASA POWER, daily data 2006–2025 at the center of each 3-digit ZIP area, data as of October 2026.
Questions
How do I calculate my solar load?
Multiply each appliance’s watts by the hours it runs each day, then add them all up. The calculator does this for you. Use the label on each appliance, or a plug-in power meter for the ones you can’t find.
What’s the difference between watts and watt-hours?
Watts measure power at one moment. Watt-hours measure energy over time. A 100 W bulb running for 5 hours uses 500 Wh. Panels and inverters are sized in watts, and batteries in watt-hours.
Why does a refrigerator need a bigger inverter than its label says?
When the compressor starts it pulls two to three times its running power for a second or two. The surge column covers that.
Should I count appliances I rarely use?
Count anything you’ll run on the system, but use hours you’d really run it. A microwave used for 10 minutes a day is 0.2 hours, not 1.
Does this include losses?
Yes. The default is 20%, which is typical for an off-grid system with a battery. Lower it for a grid-tied system without a battery.
What do I do with the result?
Use the buttons under the result to size a battery or an inverter with the same numbers.
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