Watt-hours explained for homeowners: the one unit every backup decision runs on

Every page on this site ends in the same unit, and most of the confusion in backup power comes from mixing it up with a different one that sounds the same. A watt is how fast. A watt-hour is how much. Once those two are separate in your head, every runtime table, every product sticker and every calculator on this site reads as arithmetic rather than marketing.

Watts: the speed of energy

A watt is a rate of energy use, the way miles per hour is a rate of travel. Every appliance has one, printed on a nameplate or a label: a refrigerator compressor runs at a couple of hundred watts while it is running, a CPAP draws about 53 W with its humidifier on per ResMed’s user guide, a 1,500 W space heater draws 1,500 W every second it is switched on. Watts answer the question “can the battery’s inverter handle this,” which is the running versus surge watts question, and nothing else.

Watt-hours: the amount

A watt-hour is one watt sustained for one hour. Ten watts for an hour, or one watt for ten hours, or a hundred watts for six minutes: all one watt-hour. It is the unit a battery is sized in, because a battery is a container of energy, and it is the unit a bill is written in, a thousand at a time as kilowatt-hours. The average American household bought about 855 kWh a month in 2023 per EIA, roughly 28,000 Wh a day. A 2 kWh power station holds about 2,048 of those.

Runtime is one divided by the other

Watt-hours in the battery, divided by watts drawn, gives hours. That is the entire formula behind every runtime table here, with two corrections that the usable capacity guide explains: the inverter loses about 15% converting battery energy to outlet energy, and most appliances do not draw their rated watts all the time, so a duty cycle scales them. A 52 W refrigerator (the ENERGY STAR annual average for an 18 cu ft top-freezer, compressor cycling included) on a 2,048 Wh battery: 2,048 times 0.85, divided by 52, is about 33 hours.

Converting what the label gives you

Labels rarely say watts outright. The conversions:

The label says	The arithmetic	Example
Amps at a voltage	amps times volts equals watts	Zoeller M53 sump pump, 9.7 A at 115 V, about 1,116 W
kWh per year (EnergyGuide)	divide by 8,760 hours for average watts	453 kWh a year is 52 W average
Amp-hours at a voltage (batteries)	amp-hours times volts equals watt-hours	40 Ah at 51.2 V is 2,048 Wh
Milliamp-hours at 3.7 V (power banks)	mAh times 3.7, divided by 1,000	20,000 mAh is 74 Wh

The last row is why a “20,000 mAh” power bank and a “288 Wh” power station are not in the same class: the first is 74 Wh, and, per the flying guide, the only one of the two the FAA lets on a plane. Amp-hours without a voltage are a number without a meaning, which is also why this site never quotes a battery in amp-hours.

The two mistakes

Sizing from watts alone. A reader sees a 1,500 W heater and a 2,000 W inverter and concludes the battery “handles” the heater. It does, for a little over an hour; the space heater page is that arithmetic. The inverter rating says whether the load can run; the watt-hours say for how long.

Sizing from the sticker alone. A reader divides 2,048 Wh by 52 W and plans on 39 hours of refrigerator. The inverter’s loss takes about six of those hours away before the fridge sees them. Divide the sticker by the load after multiplying by 0.85, or read the usable figure off the spec strip on any product card here.

Where to take it next

The sizing calculator does this arithmetic for a whole list of appliances from sourced figures, and prints every step. The 1 kWh and 2 kWh roundups show what those amounts buy in a box.