Generator Size Calculator

Background.

A generator can run the lights, the router and the television without strain and still stall the moment the refrigerator's compressor switches on. Motors take a brief rush of power to get turning, often two or three times what they draw once running, and that moment, not the steady load, is what trips undersized generators. Sizing one means checking two numbers: the running watts of everything you want on at once, and the extra kick needed when a motor starts.

The example list here is a typical outage kit: a refrigerator at 700 running watts, a sump pump at 800, 300 watts of lights, a 50-watt router and a 150-watt television. Together they draw 2,000 watts once running. The refrigerator needs 2,200 watts for a moment to start, 1,500 more than it runs at; the sump pump needs 1,200 extra. On the assumption that only one motor starts at a time, the worst moment is the refrigerator starting while everything else is already on, which calls for 2,000 + 1,500 = 3,500 watts. A 20% margin turns those into 2,400 running watts and 4,200 starting watts, the two figures to hold against a generator's specification.

Generators are sold on exactly that pair. The running rating (sometimes called rated or continuous watts) is what the machine can deliver hour after hour; the starting rating (surge or peak watts) is what it can deliver for a few seconds. Both have to clear your figures. Since the one-motor-at-a-time assumption only holds if you make it hold, the usual practice is to start the largest motor first and switch other loads on one by one after it.

None of this makes a generator safe to run. Portable generators produce carbon monoxide, a gas you cannot see or smell, and FEMA, the CDC and the Consumer Product Safety Commission all warn that running one indoors or in a garage can kill. Placement, connection to the house and fuel storage come before any wattage question.

What is generator size calculator?

Running watts are the steady power an appliance draws once it is operating. Starting watts, also called surge watts, are the higher power some appliances need for a moment as they start: anything with an electric motor or compressor, such as refrigerators, freezers, pumps, air conditioners and many power tools. Lights, electronics and heating elements start at about the same power they run at, but heating is the quiet budget-breaker: a common 1,500-watt space heater has no surge to speak of, yet on its own it draws three-quarters of the example list's 2,000 running watts. Generator sizing compares the total of your simultaneous running loads, and that total plus the largest single starting surge, with the generator's running and starting ratings. A safety margin on top keeps the machine from working at its exact limit.

How to use this calculator.

1. 01Decide which appliances have to run together during an outage. The list should be the set you will power at the same time, not everything in the house.
2. 02Edit the load list. Each appliance is one entry written as {"name":"freezer","runningWatts":500,"startingWatts":1500}, with entries separated by commas inside the square brackets. Every entry counts toward the total unless it carries "simultaneous":false, which leaves that appliance out; deleting the entry does the same.
3. 03Take running watts from the appliance label or manual, and starting watts from the manual or manufacturer. For appliances without a motor, such as lights and electronics, make starting watts equal to running watts; starting watts can never be lower.
4. 04Keep the 20% safety margin or change it. A larger margin buys headroom for uncertain labels and future additions, at the cost of a bigger, heavier generator.
5. 05Check a generator by comparing its running rating with the safety-margin running watts and its starting rating with the safety-margin starting watts; both must be at least as large. Or enter the generator's two ratings in the optional fields and the pass-or-short lines at the bottom of the results make the comparison for you.
6. 06Before buying, check the outlets as well as the watts: the generator needs the 240-volt or high-current outlet your transfer equipment or largest appliance uses, and enough 120-volt outlets to spread the rest.

The formula.

The calculation has three steps. First it adds the running watts of every entry: ΣR = R₁ + R₂ + … + Rₙ. That is the load the generator carries once everything is on. Second, for each entry it finds the extra starting surge, Sᵢ = startingᵢ − runningᵢ, which is zero for loads that start at their running power, and keeps the largest, Sₘₐₓ. The minimum starting requirement is ΣR + Sₘₐₓ: every other load already running while the hungriest motor starts.

Third, both figures are scaled by the safety margin m: recommended running watts = ΣR × (1 + m), and recommended starting watts = (ΣR + Sₘₐₓ) × (1 + m). With m at 20% the multiplier is 1.2.

Adding only the largest surge is the central assumption. A starting surge lasts only a moment, so two motors overlap only if they happen to start at the same instant. That can happen: when a generator is switched on with everything already plugged in, or when a refrigerator and a freezer happen to cycle together. If it could happen with your loads, combine the motors that might coincide into one entry whose running watts are the sum of their running watts and whose starting watts are the sum of their starting watts. Their surges then add up instead of only the larger counting. Where one motor's surge dwarfs the rest, as a central air conditioner's often does, a soft-start kit fitted to the compressor lowers its inrush; enter the reduced starting figure the kit maker quotes and the starting requirement falls with it.

The arithmetic works in watts and ignores voltage. A 240-volt load such as a well pump needs a generator with a 240-volt output and a suitable connection, whatever the wattage says, and the margin does not account for generators losing output in high heat or at altitude, for which manufacturers publish their own derating.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Federal Emergency Management Agency (2024). "Use Generators Safely at Home." FEMA.
2. [2]Centers for Disease Control and Prevention (2024). "Carbon Monoxide Poisoning: Generator Safety." CDC.
3. [3]U.S. Consumer Product Safety Commission (n.d.). "Portable Generators." CPSC.
4. [4]National Institute of Standards and Technology (2026). "NIST Handbook 133, Appendix E: General Tables of Units of Measurement." U.S. Department of Commerce.
5. [5]National Institute of Standards and Technology (2008). "Guide for the Use of the International System of Units (SI), NIST Special Publication 811." U.S. Department of Commerce.