Most people shop for backup power by picking a big number — a 2,000 Wh power station, a 7,500 W generator — and hoping it's enough. It's cheaper and far more reliable to do the math first. The good news: it takes one formula and a short list.
The only formula you need
Watts (W) measure how fast something uses power. Watt-hours (Wh) measure how much energy a battery holds, or how much a device uses over time. The link between them:
Watts × Hours = Watt-hours
A 10 W LED bulb running for 5 hours uses 50 Wh. A 2,000 Wh battery can supply 400 W for about 5 hours (2,000 ÷ 400).
Step 1: List what you actually need to run
Not what you'd like to run — what you must. For most households in a multi-day outage that means the refrigerator, a few lights, phone and laptop charging, internet, and possibly a medical device, sump pump, or well pump. Typical running loads are below. They're approximate, so check the label on your own equipment.
| Device | Typical running watts | Notes |
|---|---|---|
| LED light bulb | 8–10 W | Cheap to run; use these |
| Wi-Fi router + modem | 15–30 W | Runs 24/7, so it adds up |
| Phone charging | 5–20 W | |
| Laptop | 45–90 W | |
| Efficient refrigerator | 100–200 W while running | Cycles on and off; roughly 1–2 kWh per day |
| Sump pump | 400–1,000 W | Large startup surge |
| Well pump | 750–1,500 W | Large startup surge |
| Microwave | 1,000–1,500 W | Short bursts |
| Window air conditioner | 500–1,500 W | Budget killer |
| Space heater | 1,500 W | Budget killer |
Step 2: Check surge (startup) watts
Anything with a motor — a refrigerator compressor, a pump, an air conditioner — pulls several times its running wattage for a split second when it starts, commonly two to three times or more. A power station or generator has two ratings: continuous (what it can sustain) and surge (what it can handle briefly). Both need to cover your biggest motor load. A refrigerator that runs at 150 W may need 600 W or more to start.
Step 3: Add up your daily energy use
Here's a worked example for a minimal "keep the food cold and stay connected" setup:
| Load | Calculation | Energy per day |
|---|---|---|
| Refrigerator | Typical efficient model | ~1,500 Wh |
| Router + modem | 20 W × 24 h | 480 Wh |
| LED lights | 40 W total × 5 h | 200 Wh |
| Phones + laptop | Estimate | ~300 Wh |
| Total | ≈ 2,480 Wh |
Add about 20% for inverter losses and real-world inefficiency: 2,480 × 1.2 ≈ 3,000 Wh per day.
Step 4: Match it to a battery
A 2,000 Wh station would run that whole list for roughly 16 hours, or run just the refrigerator (about 1,800 Wh/day with the same 20% cushion) for around a day. A bigger battery buys more hours — but a battery by itself only delays the problem, which is why recharging matters.
Step 5: Plan the recharge
A battery buys time; solar or a generator buys days. Rough solar math:
Panel watts × peak sun hours × 0.75 ≈ daily Wh
400 W of panels × 4 peak sun hours × 0.75 ≈ 1,200 Wh per day. Peak sun hours vary with location and season; in winter at northern latitudes they can fall to 2–3 or lower. In our example, 400 W of solar covers only about 40% of the 3,000 Wh daily need. You'd either cut loads, add panels, or pair the battery with a generator.
Portable station, whole-home battery, or generator?
- Portable power station: the simplest option. Quiet, safe to use indoors, ideal for essentials. Limited by capacity.
- Whole-home battery: wall-mounted systems that power selected circuits automatically and pair well with solar. Higher cost and professional installation.
- Generator: a lot of power for the money, but noisy, fuel-dependent, and it must run outdoors. Often best paired with a battery so it only runs when needed.
- Run generators outdoors, well away from windows, doors, and vents — the CDC advises at least 20 feet. Carbon monoxide kills. Use battery-powered CO alarms.
- Never plug a generator into a wall outlet ("backfeeding"). It can energize utility lines and seriously injure line workers.
- To power household circuits, use a properly installed transfer switch or interlock. Hire a licensed electrician and follow local code and permitting requirements.
Ready to shop? See power stations, solar, and whole-home battery options on our Power page.