A few winters ago, I had no backup power plan when a storm knocked out our electricity for four days. The refrigerator started warming within hours. By day two, we were making decisions about what food to eat fast and what to throw out. I had a portable battery pack designed for phone charging and no generator. When the outage finally ended, I sat down and actually calculated what I would have needed to keep critical appliances running — and the answer was both surprising and fixable.
The Power Outage Calculator does that math for you. Here is how to use it to build a genuinely useful backup power plan.
Step 1 — List Your Critical Appliances
Start by identifying which devices and appliances actually matter during an outage. Typically that means: refrigerator, freezer, phone and device chargers, a few lights, and any medical equipment (CPAP, oxygen concentrator, insulin pump cooler). Secondary items might include a small fan, radio, or laptop.
For each item, you need its wattage. This is usually printed on a sticker on the back or bottom of the appliance, listed in the user manual, or searchable online. Common reference points: a modern refrigerator runs 100 to 400 watts. A LED bulb is 8 to 12 watts. A box fan is 40 to 100 watts. A CPAP machine is 30 to 60 watts.
Step 2 — Enter Wattage and Daily Hours into the Calculator
Open the Power Outage Calculator and enter each appliance with its wattage and how many hours per day you would run it during an outage. The calculator multiplies wattage by hours to produce watt-hours (Wh) — the standard unit for backup power capacity.
Example: a 200-watt refrigerator running continuously is 200W x 24 hours = 4,800 Wh per day. Three LED lights at 10 watts each, running 6 hours per night = 180 Wh. Phone charging at 20 watts, 2 hours = 40 Wh. Total in this example: roughly 5,020 Wh or about 5 kWh per day.
Step 3 — Select Your Backup Power Source
Now specify what backup power you have or are considering:
- Generator: Enter the rated wattage output and fuel tank size. The calculator estimates runtime.
- Battery bank / whole-home battery (e.g., LiFePO4): Enter total amp-hours and voltage (multiply to get watt-hours).
- Solar + battery: Enter solar panel wattage and battery capacity.
A 5kWh daily need with a 100Ah/12V LiFePO4 battery (1,200 Wh usable at 80% depth of discharge) means you would drain it in roughly 5 to 6 hours. Add a generator or multiple batteries to fill the gap.
Pro Tip
Reduce your daily watt-hour need before sizing up your backup power. Replacing a 60-watt incandescent with a 9-watt LED is not just efficiency — it is 51 fewer watts running potentially 8 hours a day, saving 408 Wh daily. Small substitutions add up significantly over a multi-day outage.
Step 4 — Review the Output
The calculator outputs your total daily watt-hour requirement, estimated runtime for your backup source, and a prioritized list of what to run and when. The runtime estimate tells you how long your generator fuel or battery charge will last given your entered usage pattern.
If the runtime is less than your target outage duration, you have options: add capacity, reduce consumption, or accept that some appliances will go offline partway through.
Interpreting Results and Setting Priorities
The output should drive a power priority decision for your household. Medical devices go first — always. Then food preservation (refrigerator and freezer). Then lighting. Communication devices. Everything else is secondary and should be disconnected during an outage to reduce load and extend backup power duration.
For broader emergency lighting options beyond backup power, the Emergency Lighting Power Outage guide covers candles, battery lanterns, solar lights, and hand-crank solutions that can stretch your power budget further.
The Complete Emergency Preparedness Guide ties backup power into the larger framework of household emergency readiness including food, water, and communication.
Run your numbers now, before the next outage hits. The calculation takes ten minutes and can save you thousands of dollars in spoiled food, discomfort, or under-sized equipment purchases.