How to Size a Power Station for a Power Outage
By OutageMath · Updated September 24, 2026
The right size depends on three things: what you need to keep running, for how long, and how you'll recharge. Here's how to work it out in five steps.
Step 1: List only what you actually need
Outages are about essentials. Typical priorities: the fridge and freezer, medical devices like a CPAP, internet and phones, a few lights, and in some homes a sump pump or the furnace blower. Heating and cooling are the big energy users, so decide early whether they're on the list.
Step 2: Turn it into watt-hours per day
For each device, multiply its average watts by the hours you'll use it. Fridges and pumps cycle on and off, so use their average, not their running watts. Add everything up, then divide by 85% for inverter losses.
Battery Wh per day = Σ (average W × hours) ÷ 0.85
Step 3: Check peak and surge
Add the running watts of everything that could be on at once; that's your minimum continuous output. Then add the biggest motor's extra starting watts for the minimum surge. See running vs starting watts.
Worked examples
| Setup | Battery per day | Continuous | Surge |
|---|---|---|---|
| Basic kit Fridge, router, 2 phones, 3 LED bulbs | 2,296 Wh | 192 W | 1,242 W |
| Basic + CPAP + TV Adds a CPAP (8 h) and a 55" TV (3 h) | 2,932 Wh | 322 W | 1,372 W |
| Basic + sump pump Adds a 1/2 HP sump pump running 20% of the time | 8,226 Wh | 1,242 W | 2,342 W |
Calculated with typical wattages. Your numbers will differ; enter your own devices.
The sump pump is the lesson here: it multiplies the battery you need and pushes the surge requirement past what smaller stations can deliver.
Step 4: Decide on days and recharging
Most outages are short, but storms can take out power for days. Multiply your daily figure by the days you want to cover without recharging, then add a 10–20% margin for battery aging and cold weather.
Recharging changes the math. Solar panels can put back a large share of a day's use in good weather (try the solar calculator). Many stations can also charge from a car's 12 V outlet, slowly, or from a gas generator run safely outdoors.
Step 5: Pick a model
Look for a model that meets all three numbers: battery, continuous output and surge. If you need 240 V for a well pump, only a few large models qualify. The size finder does this matching across 48 models.
Safety
- Plug devices directly into the power station or use a properly rated extension cord.
- Never plug a power source into a wall outlet to power your house (“backfeeding”). Connecting to home circuits requires a transfer switch or power inlet installed by a licensed electrician.
- If you also use a gas generator, the CDC says to never run it inside your home or garage, even with doors and windows open, and to keep it more than 20 feet from windows, doors and vents. CDC guidance.
- If someone depends on powered medical equipment, talk to their provider about a backup plan that doesn't rely on a single battery.