What Size Power Station You Need: The Sizing Math, Worked Through

• Watts (W): how much power a device draws while running. A station's *continuous* rating must exceed the total of everything running at once.
• Starting / surge watts: the brief spike when a motor or compressor kicks on, often two to three times its running watts. The station's *surge* rating must cover it.
• Watt-hours (Wh): energy over time: watts × hours. This is the battery's capacity, and it decides runtime.

Write down only what you truly need during an outage. Find each device's watts on its label or power brick (amps × volts = watts). If you can't find it, use a published typical value, but your label is always better.

Add the running watts of everything that could be on at the same time. Multiply by 1.2 for margin. That's your minimum continuous rating. Then take the single biggest starting-watt jump (starting minus running) and add it to your running total. That's your minimum surge rating. Motors rarely all start in the same instant, so you only need to cover the largest one.

Example: fridge (200 W) + router (20 W) + 4 LED bulbs (40 W) + laptop charging (50 W) = 310 W running. × 1.2 = 372 W continuous minimum. Fridge starting 1,200 W − 200 W = 1,000 W extra, so surge needed ≈ 1,310 W. A 600 W / 1,200 W surge station is borderline; a 1,500 W / 3,000 W station is comfortable.

For each device: watts × quantity × hours per day. For cycling appliances like fridges and freezers, DOE's method divides plugged-in hours by three, because the compressor isn't running constantly.

Our fridge figure is deliberately conservative. DOE's federal purchasing guidance lists an ENERGY STAR 18.1 cu ft top-freezer at 363.5 kWh per year (about 1,000 Wh a day), so a newer fridge may need less.

Batteries store DC power; your appliances use AC. The inverter loses some energy in that conversion. A DOE-submitted Florida Solar Energy Center report models efficiency at 0.84–0.93 depending on load. We divide by 0.85 to stay conservative. Then we multiply by 1.2 so you're not planning to run the battery to zero, and to cover battery ageing, cold weather and estimate error.

That's why a 1 kWh station, which suits phones, a router and a CPAP, only covers part of a day once a fridge is involved. For the example household, the choices are: about 3.5 kWh of battery per day of outage (an expandable whole-home-scale station), a 1–2 kWh station recharged daily by solar panels or a generator, or trimming the list.

See our station picks by size in best portable power stations, or let the quiz match one for you.

• Ignoring surge. A station can have plenty of watt-hours and still trip when the fridge compressor starts.
• Using capacity as usable energy. Count on roughly 85% of the label Wh reaching your devices.
• Forgetting heat. Space heaters, electric blankets and heated CPAP humidifiers are big energy users.
• Assuming solar will refill everything. One 200 W panel replaces roughly 600 Wh on a good day under our planning assumptions.
• Planning to power hard-wired loads with cords. Furnaces and 240 V well pumps need a transfer switch installed by a licensed electrician. See transfer switch and interlock basics.