To size a solar generator you need three numbers: the running watts of everything you want to use at the same time, the surge watts of the hungriest device when it starts, and the watt-hours per day you will use. Enter your devices below and the calculator adds them up.
Enter each device's running watts (from its label or a plug-in watt meter), its starting surge if it has a motor or compressor, and how many hours a day you will run it.
| Device | Running W | Surge W | Hours/day |
|---|
- Total running watts (all on at once)
- 0 W
- Highest single startup (largest surge + others running)
- 0 W
- Energy per day
- 0 Wh
- Suggested battery capacity (with 15% inverter loss and 20% reserve)
- 0 Wh
Where to find the numbers
- Running watts: look for a label on the device or its power adapter. If it lists amps instead, multiply amps by volts (120 V for US household devices) to get an upper estimate. A plug-in watt meter shows the real draw, which is often lower than the label.
- Surge watts: devices with motors or compressors (refrigerators, pumps, power tools) briefly draw much more when they start. The manual sometimes lists it; if not, a watt meter that records peak draw helps.
- Hours per day: estimate honestly. A refrigerator is plugged in all day but its compressor only runs part of the time, so use its measured or labeled daily energy rather than running watts multiplied by 24 hours.
How the calculator works
Total running watts is the sum of every device’s running watts. Startup surge is that total plus the extra draw of the single device with the largest surge, because devices rarely start at exactly the same moment. Energy per day is each device’s watts multiplied by its hours, added together.
The suggested battery capacity divides daily energy by 0.85 to allow for inverter losses when running AC devices, then adds a 20% reserve so you are not draining the battery to zero. Both are planning assumptions; check your power station’s specifications for its actual efficiency and usable capacity.
What to do with the result
- Choose a power station whose continuous output is above your total running watts and whose surge rating is above your startup surge.
- Choose a battery capacity at or above the suggested figure, or plan to recharge during the day.
- Plan the recharge with our solar panel size calculator.
For a full walk-through with a worked example, read how to size a solar generator.
