Home Power Outage Calculator

There is no single answer, because it depends on which devices you want to keep running and for how long. The method is always the same: list each essential, estimate its wattage and how many hours it will actually run during the outage, add up the watt-hours, then add headroom for inverter losses and the reserve you keep on the battery. The calculator above does this for you and maps the result to a rough capacity class.

As a rough guide, keeping phones, Wi-Fi and a few lights going for a day needs only a few hundred watt-hours, so a 300–500 Wh unit can be enough. Add a refrigerator and the daily total usually climbs past 1,500 Wh, which points at a 2,000 Wh or larger power station for a full 24 hours. Multi-day outages either need a much larger unit or a way to recharge, such as solar panels.

For one device in isolation, the main Power Station Size Calculator and the device-specific calculators can give a more focused estimate.

The calculator works in 24-hour days: it totals the watt-hours your devices use in a day, multiplies by the number of days, then adjusts for efficiency and reserve. For a 24-hour outage covering a refrigerator plus phones, Wi-Fi and lights, the recommended minimum is commonly in the 2,000–3,000 Wh range once losses and reserve are included — the worked example above lands near 2,550 Wh. A lighter load without a refrigerator can drop well under 1,000 Wh.

For an outage shorter or longer than a day, adjust the inputs like this:

Portable power stations have a limited budget of watt-hours, so it helps to rank devices by how essential they are and how much energy they draw:

• Medical equipment first. A CPAP or other prescribed device is the priority for anyone who depends on it. It usually draws little power, so it is inexpensive to back up — the CPAP Power Station Calculator covers CPAP wattages and overnight sizing in detail.
• Food safety. A refrigerator or freezer is the next common priority. It is a larger load, but running it a few hours at a time is often enough to keep food cold. A microwave for reheating is brief but high-wattage — the Microwave Power Station Calculator checks whether an inverter can supply it. For an unattended hot meal, a rice cooker is a low, steady load — the Rice Cooker Power Station Calculator sizes its cook cycle and keep-warm energy, and the Slow Cooker Power Station Calculator sizes an all-day slow cook, where the runtime rather than the wattage sets the battery size.
• Water and flood protection. If a storm outage could flood a basement, a sump pump is a priority; if your home draws water from a well, the well pump is your water supply. Both are motor loads, so a power station needs the right startup output, not just watt-hours — the Sump Pump and Well Pump backup calculators cover running versus starting watts.
• Communication and light. Phone charging, a Wi-Fi router and a handful of LED bulbs together draw very little and keep you informed and safe.
• Comfort loads last. A fan, TV or laptop is fine to include if capacity allows. High-wattage resistive appliances — space heaters, kettles, hair dryers — drain a portable unit in well under an hour; the Electric Heater Power Station Calculator shows how fast. An air conditioner is a heavy load with a compressor surge of its own; the Air Conditioner Power Station Calculator sizes battery capacity and the output it needs. For staying warm, an electric blanket usually draws far less than heating a room with a space heater, because it warms the person directly — often tens of watts — though the blanket manual has to allow inverter power. The Electric Blanket Power Station Calculator sizes an overnight run.

Add the devices in that order in the calculator above and watch the recommended capacity climb, so you can see where your own cutoff sits.

A typical residential refrigerator draws roughly 100–250 watts while the compressor runs, but the compressor is only on for part of each hour. Averaged over a day that often works out to around 1,000–1,500 Wh. On that basis a 1,000 Wh power station might cover somewhere near half a day to a day, a 2,000 Wh unit around a day and a half, and a 3,000 Wh unit two days or more — before accounting for anything else you plug in.

Startup surge matters too. The compressor motor briefly pulls several times its running wattage when it kicks in. Battery capacity in watt-hours and inverter output in watts are separate specs: a unit can have plenty of stored energy yet still trip on the surge. Check the refrigerator's starting wattage and the power station's surge rating before relying on it.

The Refrigerator Power Station Calculator covers compressor cycling and surge in more detail, and the Power Station Runtime Calculator estimates hours for a specific unit and load.

A 1,000 Wh power station delivers roughly 650–800 Wh of usable energy after inverter loss and a reserve. That is a good match for phones, a Wi-Fi router, lights, a laptop and a CPAP for a night, and it can nurse a small or efficient refrigerator through part of a day. It is generally not enough to carry a full-size refrigerator plus other loads through a whole 24-hour outage.

A 2,000 Wh unit provides around 1,300–1,600 Wh usable, which is usually enough for a refrigerator plus the small essentials for a day, or the small essentials alone for two to three days. It is heavier and costs more, and many 2,000 Wh units have a higher continuous and surge output, which helps with a refrigerator's startup.

If your must-run list includes a refrigerator, the 2,000 Wh class is the safer starting point; if it is phones, Wi-Fi, lights and a CPAP, 1,000 Wh often covers it. Run your own device list through the calculator above to see which side of the line you fall on, and confirm the specific unit's usable capacity and output ratings.

Every figure this calculator produces is an estimate, not a guarantee. The result assumes each device draws a steady, average wattage for the hours you enter, and that the battery delivers close to its rated capacity. Real outages rarely match that exactly.

• Temperature. Cold reduces how much energy a battery can deliver, and a warm room makes a refrigerator's compressor run more often, raising its real consumption.
• Battery age and health. Cells lose capacity over years and cycles, so an older unit stores less than its label states.
• Actual power draw. Nameplate wattage is a starting point. Measuring a device with a plug-in watt meter gives a far better number than a generic example.
• Inverter and standby losses. Conversion to AC loses energy, and the power station's own electronics draw a few watts even with nothing plugged in.

Treat the recommended capacity as a floor to shop above, keep some extra margin, and test your setup before you have to depend on it.

Past a day or two, buying enough battery to ride out the whole outage gets expensive and heavy. Pairing a mid-size power station with a solar panel is a common alternative: the panel replaces some of what you use each day, so the battery only has to cover the gap and the overnight hours.

Solar output swings with weather, shade, panel angle and daylight, so plan for partial rather than full recharges and keep a reserve as a buffer. The Solar Charge Time Calculator estimates how long a given panel takes to refill a unit, and the Solar Panel Size Calculator works out how many watts of panel you need to keep pace over a set number of days.