Calculator · Sizing
How Big a Generator Do I Need? An Outage Runtime Calculator
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Two numbers decide the size of any backup power system: watts (how much power your appliances draw at the same moment) and watt-hours (how much energy they use over the length of the outage). Gas generators are sized mostly by watts. Battery "solar generators" have to clear both bars. The calculator below works out both for your own list of appliances.
Outage runtime calculator
Tick what you want to keep running and adjust the watts to match your appliances' labels. "Hours/day" means hours the appliance is actually drawing power in 24 hours. A fridge compressor cycles on and off, for example. Every default is an editable starting point, not a measurement of your home.
Optional: your climate. Pick your state and county to see your 2021 IECC climate zone and which loads to plan for in your region[10].
Watts vs watt-hours, in one minute
Watts (W) are a rate: how hard an appliance pulls power right now. If you have a 1,000W microwave and a 500W furnace fan running together, you need a source that can deliver at least 1,500W at that moment. A battery's "continuous output" and a gas generator's "running watts" are both watt ratings.
Watt-hours (Wh) are an amount: watts multiplied by time. A 100W load running for 10 hours uses 1,000Wh, which is 1 kilowatt-hour (kWh). A battery's capacity is measured in Wh. A gas generator doesn't have a fixed capacity; its limit is how much fuel you have.
Virginia Cooperative Extension (Virginia Tech) gives the basic formula: daily energy use in kWh = (wattage × hours used per day) ÷ 1,000. If an appliance lists only amps, multiply amps by the voltage it uses to estimate watts. Most U.S. appliances run on 120V, while large ones like clothes dryers and electric cooktops use 240V[1]. The same guide notes that a refrigerator, although plugged in all day, cycles on and off. To estimate the hours it actually runs at full wattage, it suggests dividing the time it's plugged in by three. That's where our 8-hours-per-day refrigerator default comes from[1].
Starting watts: why your fridge needs more than its label says
Anything with a motor or compressor (refrigerators, freezers, sump and well pumps, furnace blowers, air conditioners) briefly draws a surge of power when it starts. Champion Power Equipment's wattage chart puts typical numbers on this[2]:
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 150–400W | 800–1,200W |
| Freezer | 100–500W | 500–1,000W |
| Furnace fan (1/2 HP) | 300–800W | 800–1,600W |
| Sump pump (1/3 HP) | 500–800W | 1,000–1,600W |
| Sump pump (1/2 HP) | 800–1,050W | 1,300–2,150W |
| Well pump (1 HP) | 1,000–2,000W | 2,000–4,000W |
| Window AC (10,000 BTU) | 900–1,200W | 1,800–2,400W |
| Central AC (3 ton) | 3,000–3,500W | 5,000–6,000W |
| Microwave | 600–1,200W | No extra starting watts |
| Internet router/modem | 10–20W | No extra starting watts |
| CPAP (without heat) | 30–60W | No extra starting watts |
The calculator estimates your peak as everything running plus the single biggest starting surge. That assumes two motors rarely start at the exact same instant. It's a common sizing shortcut, but it's an assumption, so leave some headroom.
Worked example: a typical "essentials" outage
The calculator loads with an "essentials" list ticked: refrigerator, router and modem, five LED bulbs, phone and laptop charging, a furnace fan and a 1/3 HP sump pump. Using the default values:
| Item | Running W | Hours/day (assumed) | Wh/day |
|---|---|---|---|
| Refrigerator | 200 | 8 | 1,600 |
| Router + modem | 15 | 24 | 360 |
| LED lights (5) | 50 | 6 | 300 |
| Phone + laptop charging | 60 | 4 | 240 |
| Furnace fan | 500 | 6 | 3,000 |
| Sump pump | 650 | 2 | 1,300 |
| Total | 1,475 | 6,800 |
- Everything on at once: 1,475W.
- Estimated peak: the biggest single starting surge is the refrigerator's extra 800W (1,000 − 200; the furnace fan adds 700W and the sump pump 650W), so 1,475 + 800 = 2,275W.
- Energy per day: 6,800Wh, an average load of about 283W (6,800 ÷ 24).
- Battery runtime: a 4,096Wh battery at 80% usable gives 3,277Wh, which lasts about 11.6 hours at 283W.
- For a full 24-hour outage you'd want about 8,500Wh of rated capacity (6,800 ÷ 0.80), or a way to recharge partway through.
Two things jump out. First, peak watts are modest: almost any of the big battery units, or a mid-size portable gas generator, covers 2,275W. Second, the furnace fan uses almost half the daily energy. In a winter outage, heating is usually what drains a battery fastest, and in summer it's air conditioning.
How to read the battery results
"Usable energy at the outlet (%)": batteries lose some energy converting stored DC power to household AC, and they also draw power just being switched on. We default to 80%. That's our assumption, but it lines up with EcoFlow's own example, which says a DELTA Pro 3 (4,096Wh) runs a 120W refrigerator for 27.3 hours[3]. Dividing 4,096 by 120 would give 34.1 hours, so EcoFlow's example implies about 80%. Raise or lower the figure if your manufacturer publishes something different for your setup.
Continuous output check: if the "everything on at once" number is higher than the battery's continuous rating, the unit will shut off or refuse the load. You can stagger big loads (don't run the microwave while the well pump is filling), choose a higher-output unit, or connect two units in parallel where the brand supports it. The three flagship units differ a lot here: 4,000W for the EcoFlow DELTA Pro 3, 3,840W for the BLUETTI Apex 300 and 6,000W for the Anker SOLIX F3800 Plus, per their spec pages[3][4][5]. See our full EcoFlow vs Bluetti vs Anker SOLIX comparison.
Runtime: runtime is usable watt-hours divided by your average load. This assumes the battery starts full and nothing recharges it. Solar panels or a gas generator can stretch runtime a long way. EcoFlow, BLUETTI and Anker all sell generator and solar options for exactly that reason.
EcoFlow DELTA Pro 3(paid link) Anker SOLIX F3800 Plus(paid link) BLUETTI Apex 300(paid link)
How to read the gas generator results
For a portable gas or dual-fuel generator, look at two numbers on the spec sheet: running (rated) watts and starting (peak) watts. The running number should be at least your "everything on at once" total, and the starting number at least your estimated peak, ideally with 10–20% headroom so the generator isn't working flat out.
As one example of how manufacturers list this, Westinghouse rates its WGen9500DF dual-fuel portable at 9,500 running and 12,500 peak watts on gasoline, and 8,500 running and 11,200 peak watts on propane[6]. Runtime for a gas generator depends on fuel and load. Westinghouse lists up to 12 hours at 25% load and 8 hours at 50% load on its 6.6-gallon tank[6].
Whole-home standby generators are sized by an installer, often for the whole electrical panel. Generac, for example, rates its 24kW Guardian at 24,000W on propane and 21,000W on natural gas[7]. Costs for each option are in Solar vs gas vs standby: real costs compared.
Browse portable generators at The Home Depot(paid link) Browse portable generators on Amazon(paid link)
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Getting more accurate numbers
- Read the label. Most appliances have a nameplate showing watts, or volts and amps (watts ≈ volts × amps)[1]. Nameplate figures are usually maximums, so real average use is often lower.
- Measure with a plug-in meter. An inexpensive plug-in energy meter shows exactly how many watt-hours your fridge or freezer uses in 24 hours. That's far better than our "hours/day" guess. Enter the measured Wh by setting running watts × hours to match.
- Check your well or sump pump's horsepower. Pumps vary a lot, and their starting surge is often the biggest single number in the whole calculation.
- Think about seasons. Winter outages add furnace blower time; summer outages add AC. Run the calculator for both.
- Plan for longer than average. The EIA reports that U.S. customers averaged about 11 hours without power in 2024, with major events such as hurricanes causing about 80% of that time[9]. The outages you actually prepare for are the long ones.
What to do next
- Run the calculator for your "must-have" list and your "nice-to-have" list.
- If peak watts are under about 4,000W and you mostly see short outages, a single battery unit may cover you. Compare the three leading units.
- If you need multi-day coverage or 240V whole-house loads, read the cost comparison and get quotes from licensed electricians.
Sources
- Virginia Cooperative Extension, "Energy Series: Estimating Appliance and Home Electronic Energy Use" (BSE-301): pubs.ext.vt.edu (PDF)
- Champion Power Equipment, Generator Wattage Chart: championpowerequipment.com/generator-wattage-chart
- EcoFlow, DELTA Pro 3 product pages (US specs; CA runtime examples): us.ecoflow.com · ecoflow.com/ca
- BLUETTI US, Apex 300 product page: bluettipower.com/products/apex-300-home-battery-backup
- Anker SOLIX US, F3800 Plus product page: ankersolix.com/products/f3800-plus
- Westinghouse, WGen9500DF product page: westinghouse.com/products/wgen9500df-generator-dual-fuel
- Generac, 24kW Guardian Series (model 7210) specifications: generac.com, 24kW Guardian 7210
- U.S. CPSC, Generators and Engine-Driven Tools: cpsc.gov
- U.S. EIA, "Hurricanes in 2024 led to the most hours without power in the United States in 10 years": eia.gov/todayinenergy/detail.php?id=66744
- Pacific Northwest National Laboratory for DOE, Guide to Determining Climate Zone by County: Building America and IECC 2021 Updates (PNNL-33270, 2022): doi.org/10.2172/1893981