Backup Power for a Long Outage: What Will It Really Run?

In short

Start with a short list of what must run, not with a product.
Watts tell you if it will run. Watt-hours tell you for how long.
Battery power stations are safe indoors. Fuel generators never are.
Keep a generator outside, at least 20 feet (6 metres) from any door, window or vent.
Never plug a generator into a wall outlet. It can kill a line worker.

Start with a list

Write down what truly has to run, in order. For most homes it looks like this:

Medical devices
Phones and a radio
A few lights
The fridge or freezer, part of the time
The furnace blower, if an electrician has set up a safe connection

Watts and watt-hours

Watts (W) are how hard something pulls right now. Watt-hours (Wh) are how much energy is stored. A power source has to supply enough watts to start the device and hold enough watt-hours to keep it going.

The sum is simple:

Hours of running time = battery watt-hours × 0.85 ÷ device watts

The 0.85 covers the energy lost in the battery's own electronics. A 1,000 Wh power station running a 50-watt device lasts about 17 hours.

Device	Typical draw	Energy per day
Phone, one full charge	n/a	about 15 Wh
LED lantern, 5 hours	5 W	25 Wh
Internet modem and router	10 to 20 W	240 to 480 Wh
CPAP machine, 8 hours, no heated humidifier	30 to 60 W	240 to 480 Wh
Modern fridge	100 to 200 W when running	1,000 to 2,000 Wh
Gas furnace blower	300 to 600 W when running	2,500 to 5,000 Wh in deep cold
Portable electric heater	1,500 W	Too much for any battery you can carry

These are typical figures. The label on your own device is the real number. The calculator adds them up for you.

Battery power stations

A power station is a large rechargeable battery with wall-style outlets and USB ports. No fuel, no exhaust, no noise, safe indoors. The limits are capacity and recharging: once it is empty you need the grid, a car, solar panels or a generator to refill it. In a northern winter, solar panels produce a small fraction of their rated output.

See the power station guide for sizes and picks.

Fuel generators

A generator makes far more power for far longer, as long as you have fuel. It also makes carbon monoxide, and generators kill people in almost every large outage.

Outside only. Never in a house, basement, garage, shed or on a balcony, even with doors open.
At least 20 feet (6 metres) from every door, window and vent, including the neighbours'. Point the exhaust away from buildings.
Never plug it into a wall outlet. This is called backfeeding. It sends power back up the lines and can kill a utility worker or burn your house down. To power house circuits you need a transfer switch installed by a licensed electrician.
Let it cool before refuelling, and store fuel outside the living space in approved containers.
Keep it dry under an open canopy made for the purpose, never in an enclosure.
Run a battery CO alarm indoors the whole time.

Fuel is the real limit. A typical portable generator burns 10 to 20 litres (3 to 5 gallons) of gasoline a day, and gas stations cannot pump without power. In a long outage, a generator run for a few hours a day to recharge batteries and cool the fridge goes much further than one run all day.

Your car

A car can charge phones and small devices from its 12-volt socket or USB ports. See your car as backup, including why you never idle it in a garage.

Charging safely

Charge batteries on a hard, flat surface, away from bedding and away from your exit. Use the charger that came with the device. Do not charge a swollen, damaged or very cold battery.

Know when the backup ends

Every backup runs out. Work out the day it does, and decide now what you will do on that day. For someone on a medical device, that decision is the most important line in the plan.

Sources

Official guidance this page was checked against. If your local authority says something different, follow them.