Quiet, safe indoorsPortable power station
Battery and inverter in one box. Silent, no fumes, recharges from a wall socket, car or solar.
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Home backup power calculator
What size generator or power station do you need? Add what you want to keep running and for how long. Results update as you type.
- Counts starting watts
- Free, no sign-up
- Runs in your browser
Backup power calculator
What do you want to keep running?
How long do you need backup?
What you need
Generator running and starting watts, power station battery and inverter size, solar recharge and grid cost.
How it works
Backup power has two separate limits. Watts (W) is how much power flows at once and sizes a generator or an inverter. Watt-hours (Wh) is how much energy is used over time and sizes a battery.
- 1Running wattsrunning W × quantity, added up for everything that could be on at the same time.
- 2Starting wattsrunning total + largest single surge. Motors in fridges, pumps and AC units draw 2–3× their running watts for a second or two as they start. Usually only one starts at a time.
- 3Battery energyrunning W × hours it runs × days ÷ 0.85. The 0.85 covers inverter and battery losses.
For outages shorter than 24 hours, each item counts its hours a day, up to the length of the outage, so the battery estimate errs on the safe side. Generators get 25% headroom and inverters 20%, so neither runs flat out. Preset wattages are labelled examples. The rating label on your appliance always wins.
Worked example: fridge, sump pump and essentials for 24 hours
A refrigerator, a 1/2 hp sump pump, router and modem, six LED bulbs and a CPAP, using the example figures in the calculator:
Running load150 + 1,000 + 20 + 60 + 60 = 1,290 W
Peak at start-up1,290 W + 2,000 W sump pump surge = 3,290 W
Generator1,290 W × 1.25 = 1,613 W → 2,000 W running, 3,300 W starting
Energy over 24 h1,200 + 2,000 + 480 + 360 + 480 = 4,520 Wh
Power station4,520 Wh ÷ 0.85 and 1,290 W × 1.2 → 5,400 Wh battery, 1,600 W inverter (3,300 W surge)
Same load from the grid4.52 kWh × $0.18 = $0.81 a day
Leave out the sump pump and the peak drops to 1,340 W (the fridge's surge becomes the largest) and the battery to about 3,000 Wh.
Frequently asked questions
What’s the difference between running and starting watts?
Running watts is what an appliance draws once it’s going. Starting watts is the short burst a motor needs to get moving, often 2 to 3 times higher for a second or two. A generator or inverter that can’t cover the burst will stall, trip or shut down, even if the running total looks fine.
Why does a fridge need so much more power to start?
Its compressor is an electric motor. A motor at rest draws a large current until it reaches speed, then settles to its running draw. A fridge cycles on and off all day, so it hits that surge many times, not just once.
What’s the difference between W and Wh?
Watts (W) is power at one moment. It decides the size of a generator or inverter. Watt-hours (Wh) is energy over time. It decides the size of a battery. A 100 W load running for 10 hours uses 1,000 Wh.
Should I get a generator or a portable power station?
A generator gives more power for longer, as long as you have fuel, but it is loud and must run outdoors. A power station is silent and safe indoors, but it stops when the battery is empty unless you recharge it from solar, a car or a generator. Many homes use a station for overnight essentials and a generator for big loads.
Can I run a generator indoors or in the garage?
No. Generators produce carbon monoxide, which you can’t see or smell and which can kill within minutes. Never run one inside a home, garage, basement or shed, even with doors open. Keep it at least 20 feet (6 m) from the house with the exhaust pointing away from doors, windows and vents, and fit battery-powered CO alarms.
Can I plug a generator into my home’s wiring?
Only through a transfer switch or interlock fitted by a licensed electrician. Feeding power back through a wall outlet is illegal in most places and can electrocute utility workers repairing the lines. Otherwise, run appliances from the generator with heavy-duty outdoor extension cords.
More backup power tools
Backup power that fits your result
Three ways to cover an outage. Sizes appear once the calculator has a result.
We may earn a commission from links on this page. How this works
For multi-day outagesSolar generator kit
A power station bundled with folding solar panels, so it can refill itself during a long outage.
Add appliances to the calculator to see the size you need.
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Most output, outdoors onlyDual-fuel inverter generator
Runs on gasoline or propane and handles motor surges. Use outdoors only, at least 20 ft from the house.
Add appliances to the calculator to see the size you need.
AmazonCheck price