Defaults last checked: October 29, 2026. Results are estimates.
The number on the box (1,000 Wh, 2,000 Wh and so on) is the battery size, not what you get out of the socket. Some energy is lost turning battery power into household AC, and the unit uses a little power just by being switched on. This calculator takes both into account, so the runtime it gives you is closer to what you’ll actually see.
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How to use it
- List what you want to run. For each appliance, enter its watts, how many hours a day it runs and how many you have. We start you off with an average US fridge-freezer (more on that below) and an empty row.
- Enter the power station’s capacity in Wh. The default is 1,152 Wh, the capacity of the BLUETTI AC180, used only as an example.
- Check the usable share and self-consumption. We use BLUETTI’s published runtime formula for the AC180: 90% depth of discharge × 85% inverter efficiency, which works out to about 76.5%, plus about 15 W of self-consumption. Other models differ, so use your unit’s figures if the maker publishes them.
- Set the number of days of backup you want.
- For the generator comparison, check the grid price, the fuel price and the generator’s fuel use.
The formulas
- Daily need = sum of (watts × hours per day × quantity)
- Usable energy = capacity × usable share
- Runtime = usable energy ÷ (average draw + self-consumption), where average draw = daily need ÷ 24
- Capacity needed = (daily need + self-consumption × 24 h) × days ÷ usable share
- Cost per usable kWh from the grid = electricity price ÷ usable share
- Generator fuel cost per kWh = fuel used per kWh × fuel price
Worked example: fridge and Wi-Fi through an outage
The U.S. Department of Energy puts the average refrigerator-freezer at 588 kWh a year, which averages out to about 67 W around the clock. Add a 10 W router running all day:
- Daily need: (67 × 24) + (10 × 24) = 1,848 Wh
- A 1,152 Wh unit gives about 881 Wh of usable energy, so runtime is 881 ÷ (77 + 15) ≈ 9.6 hours
- To cover a full day you’d want about (1,848 + 360) ÷ 0.765 ≈ 2,886 Wh of rated capacity
That’s why a single 1 kWh unit gets you through an evening but not a whole day, and why we go into fridges in more detail in how long will a power station run a refrigerator.
Power station vs generator: cost per kWh
Recharging that 1,152 Wh unit from the grid at the EIA’s U.S. average of 18.16 cents costs about 21 cents. Per usable kWh, that’s about 24 cents.
For a generator we use Honda’s published figures for the EU2200i inverter generator: its 0.95-gallon tank runs 8.1 hours at a quarter load (about 450 W), which is roughly 3.65 kWh per tank, or 0.26 gallons per kWh. At the EIA’s U.S. regular gasoline price of $4.269 a gallon (week of October 26, 2026), that’s about $1.11 per kWh, more than four times the grid cost. At full load the same generator is more efficient (about 0.165 gallons per kWh), but backup loads are usually light.
In the UK, with petrol at 174.07p a litre (UK government weekly average) and electricity at 26.32p, the gap is similar: about £1.72 per kWh from the generator against £0.34 from a recharged power station.
A generator still wins when an outage lasts days and you can’t recharge from solar. Our guide to power station vs generator for home backup covers that trade-off, and safety: never run a generator indoors or in a garage.
What the result doesn’t tell you
- Fridges and freezers cycle. They switch on and off and draw a surge at start-up. The runtime is an average, and the “everything on at once” figure helps you check the surge and continuous output ratings.
- Charging losses are not included, so the real recharge cost is a little higher.
- Battery age and cold reduce usable capacity.
FAQ
What size power station do I need for a fridge?
For an average fridge-freezer through a full day, plan on roughly 2–3 kWh of rated capacity. An efficient modern fridge needs less. Measure yours with a plug-in meter for the best figure.
How do I find an appliance’s watts?
Look for the rating plate or the power supply label. For devices that cycle, such as fridges, use yearly kWh from the energy label ÷ 8,760 to get an average.
Why does the power station use power when nothing is plugged in?
The inverter that makes AC power draws a small amount whenever it’s on. Many units let you switch the AC output off when you only need USB or 12 V. That saves energy, and running a CPAP from 12 V is a good example (our CPAP sizing guide explains how).
Can I add solar panels?
Yes. Most power stations accept solar input. The calculator doesn’t model recharging during the day, so treat its runtime as the worst case with no sun.
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