Watts (W) are how fast a device uses power right now; watt-hours (Wh) are how much energy a battery holds.
A power station has one of each on its label: its watt-hours say how long it lasts, its watts say how much it can run at once. A 1,024 Wh unit delivers about 870 Wh through its outlets after inverter losses, so it runs a TV drawing 89 W for
about 9.8 hours.
Work it out
For several devices at once, with start-up and a reserve, use the calculator; it carries your list to the units that fit.
An explanation, not a measurement: watt-hours are what the tank holds, watts how wide the tap is open. The same tank lasts half as long when the tap draws twice the watts.
Watts: how much at once
A device's watts are on its label or charger: an LED bulb about 10 W, a laptop charging about 40 W, a 50-inch TV about 90 W, a space heater
1,500 W. A power station's continuous output in watts is the most it can run at the same time, and its surge
figure is the brief extra it can give to start a motor (the running and surge watts guide covers that check).
Watts are volts times amps: a device drawing 1 A from a 120 V outlet uses 120 W.
Watt-hours: how long
Watt-hours are watts multiplied by hours. A 60 W device for 5 hours uses 300 Wh; a 100 W device for 3 hours uses the same 300 Wh. A power
station's capacity in Wh is the energy stored in its cells. Through the AC outlets you get less, because the inverter turning battery power into
120 V loses some as heat: this site assumes 85%, so a 1,024 Wh unit gives about 870 Wh to a plugged-in device. USB and 12 V
outputs skip the inverter and lose less. A kilowatt-hour (kWh) on your power bill is 1,000 Wh.
Amp-hours: a battery's other label
Batteries sold for RVs, boats and phones are often rated in amp-hours (Ah) or milliamp-hours (mAh). Multiply by the battery's voltage to get
watt-hours: a 100 Ah, 12.8 V lithium battery holds 1,280 Wh; a 10,000 mAh phone power bank at 3.7 V holds 37 Wh. Amp-hours alone cannot be
compared across batteries of different voltages, which is why power stations are rated in watt-hours.
Putting the two together
Choose the watt-hours for how long you need power, then check the watts for what you will run at once. A 300 Wh unit can run a 250 W
blender, but only for about an hour; a 2,000 Wh unit with a 1,000 W inverter can keep a fridge going for well over a day but cannot run a 1,500 W heater at all.
Hours on a 1,024 Wh power station
Rated draws from the device table; hours = 1,024 Wh × 0.85 ÷ watts, with no reserve
Only if you need to run bigger things at once. Watts decide what it can run; watt-hours decide for how long. For a fridge through an outage, watt-hours matter more; for a power tool or a microwave, the watts and surge rating decide whether it works at all.
How many watt-hours do I need?
Add each device's watts times its hours of use, then allow for inverter losses and a reserve. The calculator does it for your list and shows the units that fit.
How do I convert mAh to Wh?
Multiply milliamp-hours by the voltage and divide by 1,000: 20,000 mAh at 3.7 V is 74 Wh. In the US the FAA allows lithium batteries up to 100 Wh in carry-on bags, and 101 to 160 Wh with the airline's approval, which is why most power banks stop just under 100 Wh; nearly every power station is above 160 Wh and cannot travel as passenger baggage.
Why does my power station run out sooner than Wh ÷ W says?
Because of inverter losses (this site assumes 85% efficiency), the inverter's own idle draw while it is on, a battery that is not quite full, and cold, which lowers what lithium cells deliver. Use the AC outlets only when you need them; USB and 12 V outputs waste less.
