The answer is whatever covers your essential daily watt-hours, plus about a 25 percent buffer, for the number of days you want to last. That is the whole secret. Most people plan for far more capacity than they will ever use, or too little and get caught short, both because nobody showed them this one simple sum. Here it is, step by step.
Battery capacity is measured in watt-hours. To size a system you only need to answer three questions: what do I need to run, how much energy does it use in a day, and how many days do I want to cover? Put plainly: daily watt-hours, times days, plus a buffer, equals the capacity you need.
Multiply each device's running watts by the hours you use it per day, then add them together. Watts times hours equals watt-hours. Do it only for the things that genuinely matter in an outage, not your whole house.
| Device | Running watts | Hours per day | Watt-hours per day |
|---|---|---|---|
| Refrigerator (cycles on and off) | ~150 average | 24 | ~1,500 |
| LED lights (a few rooms) | ~40 | 6 | ~240 |
| Phones and laptop charging | ~50 | 4 | ~200 |
| Wi-Fi router and modem | ~15 | 24 | ~360 |
| CPAP (humidifier off) | ~40 | 8 | ~320 |
| Daily total | ~2,620 Wh |
Using the table, our example home needs about 2,620 watt-hours per day. For a single day of backup, add the 25 percent buffer: 2,620 times 1.25 is about 3,275 watt-hours, so a power station around 3,000 watt-hours, or a smaller one recharged by solar during the day, would cover it. For three days with no recharging, multiply first: 2,620 times 3 is 7,860, plus the buffer is about 9,825 watt-hours, which means a large expandable system or pairing a mid-size unit with solar panels to refill it daily.
Capacity in watt-hours tells you how long a unit lasts. The inverter rating in watts tells you how much it can power at once. A unit can hold plenty of energy but still refuse to start a device that draws more than its inverter allows. Add up the running watts of everything you might run simultaneously, check for any high-surge devices like a fridge compressor or a pump kicking on, and make sure the inverter rating comfortably clears that peak.
A typical home refrigerator uses roughly 1,000 to 2,000 watt-hours per day because it cycles on and off rather than running constantly. Plan for the higher end in hot weather or with an older unit.
Many households can cover core essentials, a fridge, lights, phones, and internet, on roughly 2,000 to 3,000 watt-hours per day. A power station in the 2,000 watt-hour range with a 2,000 watt or higher inverter is a common starting point.
Real systems lose energy to inverter inefficiency, batteries deliver less capacity in the cold, and real usage is always a little higher than the estimate. A buffer keeps you from coming up short at the worst moment.
A small window unit can run on a larger power station, but it is one of the heaviest common loads and can use as much energy as everything else combined. If cooling is essential, size for it deliberately or plan on a generator.
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