Add up the watt-hours your devices use in a day, add the inverter’s own draw, multiply by the days you need, then divide by 0.85. That is the battery capacity to look for. The calculator below does the math for you and also shows the output watts you need, how long common sizes last and how many solar panels refill your daily use.
How this calculator works: it uses typical wattages from the U.S. Department of Energy and our own sizing guides, plus the same assumptions we use in every article (85% usable capacity, a 10 W inverter draw, solar panels at 75% of their rating). Results are calculated estimates, not measurements. For the most accurate answer, replace the default watts with the numbers on your own devices’ labels. See How We Research.
Power station size calculator
1. Pick your devices
Defaults are typical values. For the best estimate, replace them with the watts on your device’s label.
| Use | Device | Qty | Watts | Hours/day |
|---|
2. Your plan
How the calculator works
The math is the same one we use in every guide on this site. You can check it by hand:
- Energy per device: watts × hours per day × quantity. A refrigerator uses the kWh per day from its EnergyGuide label (kWh per year ÷ 365), because its compressor cycles on and off.
- Inverter draw: add 10 W for every hour the AC outlets are on (up to 24 hours). The inverter uses power even when your devices are small.
- Days: multiply the daily total by the number of days you want to cover.
- Usable capacity: divide by 0.85. We assume you get about 85% of the rated watt-hours after conversion losses.
- Output: add the running watts of everything that could be on at the same time. We count a refrigerator as 200 W while its compressor runs.
- Solar: daily watt-hours ÷ (peak sun hours × 0.75) gives the panel watts that refill one day of use.
Formula: capacity needed (Wh) = (device Wh per day + inverter Wh per day) × days ÷ 0.85
Worked example: refrigerator plus essentials for one day
These are the calculator’s default devices, the same list as in our sizing guide:
| Device | Calculation | Wh per day |
|---|---|---|
| Refrigerator (average) | 1.5 kWh/day label | 1,500 |
| Wi-Fi router + modem | 15 W × 24 h | 360 |
| 4 LED bulbs | 4 × 10 W × 5 h | 200 |
| 2 phone charges | 2 × 15 Wh | 30 |
| Laptop | 50 W × 4 h | 200 |
| Inverter draw | 10 W × 24 h | 240 |
| Total | 2,530 |
Capacity needed: 2,530 Wh × 1 day ÷ 0.85 = about 2,980 Wh, so a 3 kWh-class power station covers a full day. A 2,048 Wh unit lasts about 16.5 hours with this load: 2,048 × 0.85 ÷ (2,530 ÷ 24 h). The output needed is modest, about 335 W with everything on, and about 750 W of solar panels would refill a day’s use at 4.5 peak sun hours.
Our sizing guide’s version of this example leaves out the inverter’s own draw. The calculator adds it, so its answer is about 10% more conservative.
Three example setups
| Scenario | Daily use | Capacity to look for | Solar to refill (4.5 sun hours) |
|---|---|---|---|
| One CPAP night with a humidifier (55 W × 8 h + inverter) | 520 Wh | About 610 Wh per night; a 1 kWh unit covers about 1.7 nights | About 160 W |
| Camping weekend: 2 phone charges, 1 LED lamp 4 h, fan 8 h, laptop 3 h | 700 Wh | About 1,650 Wh for 2 days, or a 1 kWh unit plus solar | About 210 W |
| Outage day: refrigerator, router, 4 LEDs, 2 phones, laptop | 2,530 Wh | About 2,980 Wh (3 kWh class) | About 750 W |
For the CPAP numbers, see our physician-written CPAP runtime guide. For the refrigerator, our refrigerator runtime chart shows every battery size.
Typical watts used as defaults
| Device | Default in calculator | Typical range |
|---|---|---|
| Refrigerator | 1.5 kWh/day (200 W while running) | 1.0–2.0 kWh/day |
| CPAP | 55 W with humidifier | 30 W without humidifier; 90 W with heated tubing |
| Wi-Fi router + modem | 15 W | 10–20 W |
| LED bulb | 10 W | 8–12 W |
| Phone charge | 15 Wh | 10–20 Wh |
| Laptop | 50 W | 30–70 W |
| Box or pedestal fan | 50 W | 30–75 W |
| LED TV (50-inch) | 90 W | 60–120 W |
| Microwave | 1,500 W (input) | 1,000–1,500 W |
| Coffee maker | 1,000 W | 900–1,200 W |
| Space heater | 1,500 W | 750–1,500 W |
Common mistakes
- Multiplying a refrigerator’s running watts by 24 hours. 200 W × 24 h is 4,800 Wh, about three times what an average fridge really uses. Use the label’s kWh per year ÷ 365.
- Forgetting the inverter. Ten watts sounds small, but over 24 hours it adds 240 Wh, enough to charge a laptop several times.
- Sizing only the battery. A 3 kWh unit is useless for a microwave if its output is below the microwave’s input watts. Check both numbers. See Can a Power Station Run a Microwave?
- Using the microwave’s cooking power. A “1,000 W” microwave can draw about 1,500 W from the outlet.
- Expecting panels to deliver their full rating. Heat, angle and clouds cut output; we plan for about 75%.
Frequently asked questions
How many watt-hours do I need for a refrigerator?
Most full-size refrigerators use about 1 to 2 kWh per day. Divide the kWh per year on the EnergyGuide label by 365, then divide by 0.85 to get the capacity for one day: about 1,800 Wh for an average 1.5 kWh/day fridge, or about 2,050 Wh once you add the inverter’s own draw.
Why divide by 0.85?
A power station never delivers all of its rated watt-hours to your devices. Converting battery power to 120 V AC loses energy as heat. We plan on 85% usable capacity so our estimates stay on the safe side.
What matters more: watt-hours or watts?
You need both. Watt-hours (Wh) decide how long your devices run. Watts (W) decide what you can plug in at all. A small, efficient load needs capacity; a microwave, kettle or heater needs high output.
How accurate is this calculator?
It is as accurate as the watts and hours you enter. Typical values can be off by half or more for your specific device. For a precise number, read the label or measure your devices with a plug-in watt meter for a day.
Does it work for 240 V appliances?
No. The calculator assumes 120 V household devices. Many well pumps, dryers and ranges run on 240 V; read our well pump guide before planning for one.
What to read next
- 1 kWh or 2 kWh Power Station? A side-by-side of two sizes from the same brand.
- Best Portable Power Stations for Home Backup: our picks from 1 to 3 kWh.
- How Many Solar Panels Do You Need? The full solar math, with a chart for every battery size.
Sources
- U.S. Department of Energy, Energy Saver: Estimating Appliance and Home Electronic Energy Use (formula and typical wattages).
- CPAP, refrigerator and device figures: our own guides linked above, each with its manufacturer and government sources.
This calculator gives general estimates, not medical advice. If you rely on a powered medical device, confirm its power needs with the manufacturer and your doctor.