HoldBackup / Planning
What size power station do you need?
Pick what has to stay on. This works out the watt-hours and watts to look for, and shows every assumption so you can change it. It's arithmetic, not a test.
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How it's worked out
daily Wh = each load's watts × hours + standbyrated Wh = daily Wh × days × headroom ÷ usable sharepeak W = all running W + the highest starting W
The result above starts with a top-freezer fridge and a modem and router: Loads: 1,424 Wh a day. Standby: 18 W × 24 h = 432 Wh. 1,856 Wh × 1 day × 1.5 headroom ÷ 0.85 usable = 3,274 Wh rated. Output: 210 W running + 1,200 W highest start = 1,410 W. A DC load, like a CPAP on its DC cable, adds energy but no standby and no inverter watts, because it doesn't use the inverter.
Why a fridge has three numbers
Average watts set the energy, because a compressor cycles on and off. Running and starting watts set the output. An EnergyGuide label gives none of the last two, and the makers' charts disagree: Honda lists a refrigerator or freezer at 132 to 192 running watts and 1,200 starting, Champion lists 150 to 400 running and 800 to 1,200 starting, and Generac lists 700 running and 2,200 starting. We start from Honda's figures. If a station can't start your fridge, Generac's are the ones to try next.
The sump pump keeps the runtime table's 1,116 W and takes the top of Champion's 1,000 to 1,600 starting watts for a 1/3 hp pump, the nearest size it lists. Champion lists no extra starting watts for the other loads here. For anything with a motor that isn't listed, Generac says starting watts are typically 2 to 3 times running watts, so the form multiplies by 3 unless you change it. If your label gives a starting figure, use that.
Why these assumptions
- Usable share. 85% is the factor the runtime table explains and uses. It stands in for depth of discharge and inverter efficiency, which we found no maker listing separately.
- Headroom. The 1.5 is our planning margin for hot weather, opened doors and an aging battery. It isn't a maker's rule. Set it to 1 to match the runtime table.
- Standby. 18 W is the middle of the four stations on the runtime table. It runs for as long as your longest AC load, up to 24 hours a day.
- Hours. The hours are our starting examples, not measurements. The sump pump's 2.4 hours is 6 minutes an hour. Yours depends on the water coming in.
What it leaves out
- Running things at once. The peak adds every load you picked, as if they all run at the same moment. Honda's guide suggests starting big loads one at a time, which lowers the peak you need.
- Battery age and cold. Both cut what a station delivers, and the headroom only partly covers them.
- Your own numbers. A plug-in watt meter beats every preset here.
If a medical device has to stay on, ask your medical provider and the device's maker what they recommend. This page isn't medical advice.