Watt calculator
Which appliances do you want to run during an outage or on a camping trip? Select them, adjust the hours of use, set how many days you need to bridge without recharging, and get the capacity, the simultaneous load and the start-up surge your power station has to handle.
How the calculation works
For every selected appliance the calculator multiplies watts by hours and sums that up to the energy per day in watt-hours (Wh). For cooling appliances only the time the compressor actually runs counts: about a third for a refrigerator, 35% for a chest freezer or a cooler, and a cautious half for a medication refrigerator. The daily figure is multiplied by the days you want to bridge without recharging, and a safety margin of 20% is added on top. In practice power stations rarely deliver their nominal capacity as usable energy because of inverter losses and battery protection circuits.
For power we distinguish two values. The simultaneous load is everything that can run in parallel (refrigerator, router, lights, TV) plus the strongest short-burst appliance such as a kettle or microwave. Burst appliances run for a few minutes and rarely at the same time, so only the strongest one counts. The start-up surge comes on top when a motor starts while the rest is already running: refrigerator and freezer compressors briefly pull up to five times their running wattage, pumps and other motors about 1.5 to 3 times. Induction hobs and heaters start gently. That is why power stations quote two ratings, continuous AC output and surge (typically twice the AC output). Your power station has to clear both.
Example: 24 hours of home backup
A family of four typically needs for 24 hours without grid power: a refrigerator all day (about 400 Wh, the compressor runs a third of the time), a Wi-Fi router all day (about 360 Wh), LED lighting for 6 hours (about 180 Wh), two smartphone charges (about 80 Wh) and a radio for 6 hours (about 48 Wh). That adds up to 1,068 Wh per day. With the 20% margin the calculator recommends a power station with at least 1,282 Wh of nominal capacity, the Wh figure on the data sheet. The simultaneous load is 123 W, rising briefly to about 323 W when the refrigerator compressor starts. For three days without recharging the demand is 3,204 Wh and the nominal capacity 3,845 Wh.
Why LiFePO4 batteries
The figures assume LiFePO4 (lithium iron phosphate) power stations. They last 3,000 to 4,000 charge cycles before dropping to 80% of their initial capacity, which for occasional backup use works out to decades of service life. NMC batteries, found in older models and many budget brands, manage only about 500 cycles and age faster even when unused.
Frequently asked questions
How many watt-hours does a power station need to run a refrigerator?
A modern refrigerator draws roughly 200 to 500 Wh over 24 hours because the compressor only runs about a third of the time. The calculator assumes 50 W for a mid-size A++ refrigerator, which gives 400 Wh per day and, with a 20 percent margin, 480 Wh of nominal capacity. The start-up surge matters as well: when the compressor kicks in it briefly pulls up to five times its running wattage, and the power station has to deliver that peak.
Is a 1,000 Wh power station enough for a power outage?
For one night with a refrigerator, Wi-Fi, lights and phones, 1,000 Wh is enough. For a full 24 hours with refrigerator, router, lights, two phones and a radio the calculator arrives at about 1,070 Wh of demand and recommends just under 1,300 Wh of nominal capacity with the 20 percent margin. Nominal means the Wh figure on the data sheet: only about 80 to 85 percent of it is usable because of inverter losses and battery protection. For three days without recharging you are already at about 3,850 Wh.
What does the surge or peak rating of a power station mean?
The surge rating is the short-term maximum output during appliance start-up. Compressors, pumps and motors draw up to five times their running wattage when switched on, while induction hobs and heaters start gently. The continuous AC output of the power station has to carry your simultaneous load, the surge rating has to cover the start-up peak. If either is too low the power station shuts down even though the watt-hours would have been sufficient.