Sizing a generator comes down to two checks. Can it carry everything that runs at once, and can it start the biggest motor while the rest is already on? This calculator answers both for a home, a home office or a small business that wants to keep a refrigerator, a sump pump, lights and the internet going through a multi-day outage. Facilities leads and continuity planners can use it to check the unit already in the shed before the storm, when that check is still cheap.
How the calculator works
Each appliance gets two figures. Running watts are the steady draw. The extra at start is what a motor adds on top of that for the few seconds it needs to get up to speed, often two to three times the running figure for compressors and pumps. Lights, electronics and coffee makers add nothing at start; leave their extra at zero.
- Running total (W) = sum of running watts × quantity
- Peak (W) = running total + the single largest extra at start
- Recommended rated (running) watts = running total ÷ 0.8
The peak assumes motors start one at a time. The 0.8 keeps the generator near 80 percent of its rated output in continuous use, a margin makers and electricians commonly apply to long runs. You then need two numbers from the spec sheet: running (rated) watts at or above the recommendation, and starting (peak) watts at or above the peak.
Presets are typical values from makers' published wattage guides; your nameplate, or a clamp meter with an inrush setting, wins. Those guides print the same refrigerator figure (2,200 W) sometimes as "starting watts" and sometimes as "additional starting watts". The calculator reads it the cautious way, as additional, because a generator that comes up short stalls or trips the moment the compressor kicks in. If your chart clearly gives the total starting draw, enter starting minus running instead.
Worked example
The calculator opens with a Refrigerator (700 W running, 2,200 W extra at start), a 1/2 hp Sump pump (1,050 W running, 2,150 W extra), LED lights for 10 bulbs (100 W) and a Modem and router (25 W).
- Running total: 700 + 1,050 + 100 + 25 = 1,875 W.
- Largest extra at start: the refrigerator's 2,200 W (the pump's 2,150 W comes close).
- Peak: 1,875 + 2,200 = 4,075 W.
- Recommended rated watts: 1,875 ÷ 0.8 = 2,343.75, about 2,344 W.
So this house needs at least about 2,350 running watts and 4,075 starting watts. A 2,200 or 2,350 W class inverter unit falls short on both, since those headline numbers are starting figures and the running rating sits lower. If the refrigerator and pump could kick on in the same second, the peak becomes 1,875 + 2,200 + 2,150 = 6,225 W, which is why the larger motor should start first and the second one after it settles. Take the pump off the list and the picture changes: 825 W running, a 3,025 W peak and about 1,031 W rated.
Safety and what the tool does not model
Generator carbon monoxide poisonings follow major storms with grim regularity, usually from a unit run in a garage or too close to the house. The CDC rules are short:
- Never run a generator inside a home, garage, basement or crawl space, even with doors and windows open.
- Run it outdoors, at least 20 feet from doors, windows and vents, exhaust pointed away from the building.
- Install battery-powered or battery-backup CO alarms on every level and near sleeping areas; a battery CO alarm keeps working when the mains is down. If one sounds, or anyone feels dizzy, weak or sick, get outside and call 911.
The Consumer Product Safety Commission also recommends models with an automatic CO shut-off.
Never back-feed the house by plugging the generator into a wall outlet or dryer receptacle. That can energize utility lines and electrocute line workers or neighbors, and it bypasses your panel's protection. Have a licensed electrician install a transfer switch or a breaker interlock, or run outdoor-rated extension cords sized for the load straight to the appliances.
The calculator also leaves out:
- Voltage. Many well pumps run on 240 V, and many small inverter units offer 120 V only.
- Altitude and heat. Engine output drops at elevation and in high temperatures; check the derating note in the manual.
- Electronics. A line-interactive UPS fed by a conventional open-frame generator may keep switching to battery as the frequency wanders. Inverter generators hold voltage and frequency more tightly.
- Fuel. Runtime per tank falls as load rises. Refuel only after shutting the engine down and letting it cool.
Choosing a generator in this size class
The 2,200 to 2,350 W inverter class is light and quiet, and suits lights, a modem, laptops and often a refrigerator, provided the fridge's real start-up draw fits under the unit's starting figure. The chart value is deliberately pessimistic, and many newer refrigerators start well below it; the nameplate or a clamp meter settles it. Once a sump pump, furnace fan or well pump joins the list, as in the worked example, move to the 4,000 to 5,000 peak watt inverter class and confirm its running rating clears your recommended figure. Whole-building backup is a standby generator with an automatic transfer switch, a job for a licensed installer.
No hands-on trials here: the generators below were shortlisted from manufacturer data and published reviews. Check running and starting watts on the current spec sheet.
Readiness kit
| Category | Pick | Why it is on the list | Link |
|---|---|---|---|
| Inverter generator, 2.2 kW | Honda EU2200i | The quiet, long-proven inverter class for a fridge, lights and electronics. | Shop on Amazon |
| Inverter generator, 2.3 kW | WEN 2350-watt inverter generator | Similar size to the Honda class, with a fuel shutoff that drains the carburetor for storage. | Shop on Amazon |
| Inverter generator, larger | Westinghouse 5000-peak-watt inverter generator | Electric and remote start, and enough output for a sump pump or furnace fan on top of the basics. | Shop on Amazon |
| CO alarm | Kidde battery CO alarm | Battery powered, so it keeps working in the outage when a generator or heater is most likely to run. | Shop on Amazon |
| Fire extinguisher | First Alert heavy-duty fire extinguisher | A rechargeable ABC unit with a metal valve for kitchens, garages and a server corner. | Shop on Amazon |
For fuel, cords and placement before a storm, see hurricane season readiness for small business. For short outages and indoor use, a power station may do the job without fumes.
Sources
- CDC: Carbon Monoxide Poisoning Basics
- CDC: What to Do to Protect Yourself During a Power Outage
- Ready.gov: Power Outages
- U.S. Consumer Product Safety Commission: Portable Generator Hazards
- Generator makers' published wattage guides (Honda, Champion, Generac) for typical running and starting figures.
Frequently asked questions
What size generator runs a refrigerator and a sump pump?
With the chart figures in the worked example, a refrigerator, a 1/2 hp sump pump, lights and a modem need 1,875 running watts and a 4,075 W start-up peak. Look for at least about 2,350 running watts and 4,075 starting watts, and start the two motors one after the other.
What is the difference between running watts and starting watts?
Running watts are the steady draw once an appliance is going. Starting watts are the higher draw for the few seconds a motor needs to reach speed; charts give either the total or the extra above running. A generator has to supply the running total plus the biggest motor's extra at the same moment.
How far from the house should a generator be?
Outdoors only, at least 20 feet from doors, windows and vents, per the CDC, and never in a garage, basement or crawl space, even with the doors open. Keep battery-powered or battery-backup CO alarms inside.
Can I plug a generator into a wall outlet to power the house?
No. Back-feeding through an outlet can energize utility lines and electrocute line workers or neighbors. Use a transfer switch or interlock installed by a licensed electrician, or run extension cords rated for the load to the appliances.