Short answer: A conservative runtime estimate is your station's rated watt-hours, reduced by a safety margin, divided by the average watts of everything you plan to run at the same time. Label wattage is a ceiling, not an average, so this worksheet has you measure or estimate real use, then decide what earns power first. If you need help reading the numbers on the station itself, start with Power Station Labels: Watt-Hours, Watts, and Runtime Estimates Explained.
The Runtime Formula in Plain Language
Estimated hours = (rated watt-hours × planning factor) ÷ average watts running together.
- Rated watt-hours (Wh): the capacity printed on the station's label or in its manual.
- Planning factor: a margin you choose, because you should not assume every rated watt-hour reaches your devices. This article uses 0.75 in its examples. That is a planning assumption, not a published figure. Adjust it after you run your own test.
- Average watts: what your devices actually draw while running, added together for the devices on at the same time.
Step 1: Find Real Watts for Each Device
The U.S. Department of Energy describes several ways to find how much electricity a device uses. You can plug the device into an electricity usage monitor, which displays the watts it uses. If wattage is not listed on the appliance, you can estimate it by multiplying the current draw in amperes by the voltage. The same page also gives formulas for turning watts and hours into energy use. (U.S. Department of Energy, Estimating Appliance and Home Electronic Energy Use)
For outage planning, keep the result in watt-hours (watts × hours) so it matches your station's label. A printed nameplate figure may not match what a device draws in normal use, so a meter reading over a real stretch of time is more reliable, especially for anything that cycles on and off.
Step 2: Running Watts Versus Startup Watts
Running watts are what a device draws while operating. Some devices with motors or compressors draw more for a moment when they start. Your station has to cover both: its continuous output has to cover everything running together, and its surge or peak rating has to cover the startup moment.
Hypothetical example. These numbers are invented for illustration and are not measurements of any product.
- Suppose a station lists 600 W continuous output and 1,200 W surge.
- You plan to run a refrigerator (120 W running, with a 500 W startup listed in its manual), a router (20 W), and a lamp (10 W). Running together: 120 + 20 + 10 = 150 W, which is under 600 W.
- At the moment the refrigerator starts, add only its extra startup draw (500 − 120 = 380 W) to the 150 W already running: 530 W, which fits under both limits.
- Now add a 1,500 W kettle. Running total: 1,650 W, over the 600 W limit. The station may shut off even though its battery holds plenty of watt-hours. Watt-hours decide how long you can run. Watts decide what you can run at once.
If a startup figure is not in the manual or from the manufacturer, treat it as unknown and test that device at home while you can safely stop the test.
Step 3: Room-by-Room Load-Priority Worksheet
Print this section or copy it into a notebook. For each device, fill in one line in this order: Device | Label watts | Measured watts | Hours needed | Startup watts (if known) | Priority 1, 2, or 3
Priority 1 is what you cannot do without. Priority 2 is what you would like. Priority 3 is what you skip if power runs short.
Kitchen
- Refrigerator or freezer: ______ | ______ | ______ | ______ | ____
- Coffee maker or small cooker: ______ | ______ | ______ | ______ | ____
- Other: ______ | ______ | ______ | ______ | ____
Living Room and Home Office
- Modem and router: ______ | ______ | ______ | ______ | ____
- Laptop or TV: ______ | ______ | ______ | ______ | ____
- Other: ______ | ______ | ______ | ______ | ____
Bedrooms
- Phone and tablet charging: ______ | ______ | ______ | ______ | ____
- Lamp or fan: ______ | ______ | ______ | ______ | ____
- Other: ______ | ______ | ______ | ______ | ____
Utility Areas
- Sump pump or similar: ______ | ______ | ______ | ______ | ____
- Other: ______ | ______ | ______ | ______ | ____
Totals
- Add the watt-hours needed (average watts × hours) for all Priority 1 devices: ______ Wh
- Add Priority 2 devices: ______ Wh
- Station rated Wh ______ × planning factor ______ = usable Wh ______
- If Priority 1 alone exceeds usable Wh, cut hours, cut devices, or plan a second power source.
- Most running watts that could be on at the same moment: ______ W (compare to the station's continuous output)
- Largest extra startup draw (startup watts minus running watts): ______ W. Add it to line 5 and compare to the station's surge rating.
A Worked Example With Hypothetical Numbers
Every number here is invented for illustration. Replace them with your own.
Assume a 1,000 Wh station, a 0.75 planning factor (750 usable Wh), and a 12-hour overnight plan.
- Modem and router: 20 W × 12 h = 240 Wh (Priority 1). See Home Wi-Fi During an Outage: Router, Modem, and Backup-Power Planning for what else in a home network needs power.
- Two phones charging: 15 W × 2 devices × 2 h = 60 Wh (Priority 1)
- Bedside LED lamp: 10 W × 5 h = 50 Wh (Priority 2)
- Refrigerator: 120 W while running, on about 40% of the time, so 120 × 12 × 0.4 = 576 Wh (Priority 2)
The total is 926 Wh, which is 176 Wh more than the 750 usable. Priority 1 (300 Wh) fits easily, and the refrigerator is the item that does not. With Priority 1 and the lamp covered (350 Wh), 400 Wh remain. At the refrigerator's 48 W average (120 W × 0.4), that covers about 8 of the 12 hours. The plan now shows a decision to make before the outage: accept that the refrigerator is unpowered for about 4 hours, or add capacity. Food-safety time limits, covered below, should drive that decision.
Food and Medical Planning Limits
Ready.gov says a refrigerator will keep food cold for about four hours, and to throw away food that has been at 40 degrees or higher for two hours or more, or that has an unusual odor, color, or texture. (Ready.gov, Power Outages) Those time limits matter more than any runtime estimate, so plan around them and check food temperatures when power returns.
The same page advises talking to your medical provider about an outage plan for electrically powered medical devices and refrigerated medicines, and having alternate plans for both. Do not let a worksheet like this be your only plan for anything medical. Ask your provider and equipment supplier what backup they require.
Safety Limits and When to Get Professional Help
- If you also own a fuel-burning generator, Ready.gov says generators, camp stoves, and charcoal grills belong outdoors and at least 20 feet from windows to avoid carbon monoxide poisoning. (Ready.gov)
- Do not connect a power source to your home's wiring on your own. Ask a licensed electrician about any connection to a home circuit or panel.
- Follow your station's manual for output limits, ventilation, and operating temperature.
- Do not depend on the station for anything where a failure could harm someone until you have tested it under your real load.
What to Do Next
- Get a plug-in electricity usage monitor and record real watts for your Priority 1 devices.
- Fill in the worksheet above.
- Run a test at home: load the station with your Priority 1 devices, watch it for the first stretch, and time how long it lasts. Compare the result to your estimate and adjust your planning factor.
- Write down the final plan and keep it with the station.
Sources
- U.S. Department of Energy, Estimating Appliance and Home Electronic Energy Use
- Ready.gov, Power Outages
By InfinitePowerSolutions.com Technology Desk. This article is general educational information, not electrical, medical, or food-safety advice, and it does not recommend any product.
