EngineerHub.orgWhat this calculator does
Simplified mode uses monthly kWh and a household profile to create a cautious generator-size range. Appliance list mode totals actual running watts and allows several motor-start assumptions. A foldable candidate check compares a generator’s rated and starting watts against the result.
Inputs
Results
Simplified mode derives average kW from monthly kWh, then applies representative scope, peak and surge multipliers. Because those multipliers cannot reproduce a specific home, the result is displayed as a range.
Generator, transfer equipment and household loads
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Background
Optional reading — open any section below.
A generator must satisfy two different demands. Running watts are the continuous watts needed after appliances are operating. Starting watts are a short burst required by some motors and compressors while they accelerate.
In detailed mode, the calculator totals every selected appliance’s running watts and then adds the motor-starting extras allowed by your simultaneity choice. The sizing margin is applied to both the continuous and starting requirements.
Rated output = running watts × (1 + margin)Starting output = starting watts × (1 + margin)
Simplified mode uses monthly electricity consumption to infer a broad generator class. Monthly kWh describes energy used over time, not the largest momentary load, so the page intentionally reports a range rather than a precise purchase size.
Detailed mode is the better final screening method. Use appliance nameplates or manuals and include only loads you genuinely plan to operate during an outage. The calculator is most useful when the list reflects an actual backup strategy rather than the entire house by default.
Refrigerators, freezers, well pumps, sump pumps and air-conditioners may need several times their running power for a short starting interval. A generator with enough continuous watts can still stall, trip or produce poor voltage if its short-duration output is insufficient.
One largest motor is reasonable when loads can be started deliberately in sequence. Two largest motors is more conservative. All motor extras represents an automatic restart scenario where several loads may return together after power is restored.
Wattage alone does not make a generator compatible with a home. The generator must provide the required voltage and phase, and any connection to fixed household wiring needs approved transfer equipment that isolates the home from the utility supply.
Connecting a generator to a normal wall outlet can energise utility conductors unexpectedly. This practice, commonly called backfeeding, can electrocute utility workers, damage equipment and create a fire hazard.
Suppose essential appliances total 1,600 W while running. The largest refrigerator or pump adds another 700 W during startup. The starting requirement is therefore about 2,300 W.
With a 20% sizing margin, the preliminary selection becomes at least 1,920 W rated output and 2,760 W starting output. A real product must meet both figures, provide the correct household voltage, and have suitable receptacles or transfer equipment.
Portable generators produce carbon monoxide. Operate them outdoors only and far from doors, windows and vents. Never use one in a home, garage, basement, crawlspace, shed or other enclosed area, even with doors open.
- U.S. Consumer Product Safety Commission — generators and engine-driven tools
- Ready.gov — power-outage preparedness
- Generator, transfer-equipment and appliance manuals — final product ratings and connection requirements
- Local electrical code and a qualified electrician — fixed-wiring installation requirements
Frequently asked questions
Open a question below for a practical explanation.
Monthly kWh is energy averaged over hundreds of hours. Generator size is controlled by the highest simultaneous power and motor-starting demand, which a monthly bill does not show. Simplified mode therefore produces a broad screening range.
Usually not for a first household estimate. Detailed mode can assume one motor starts at a time, the two largest start together, or all listed motors start together. The correct assumption depends on controls, automatic restart behaviour and how carefully loads will be sequenced.
Rated or running output is the power a generator can supply continuously. Starting or surge output is available only briefly. A suitable product must meet both requirements; a large “maximum watts” number does not compensate for an inadequate continuous rating.
No. That is dangerous backfeeding. Use approved transfer equipment for household circuits, or power individual appliances directly with correctly rated cords in accordance with the generator manual and local requirements.
Follow the manufacturer and local emergency guidance. The essential rule is outdoor operation far from doors, windows and vents, with exhaust directed away from occupied buildings. Carbon monoxide is colourless, odourless and potentially fatal.
Include them only if you deliberately intend to power them. Resistance heating, electric cooking and large air-conditioning loads can dominate generator size. An essentials-only plan is usually far smaller and easier to operate safely.