Generator sizing calculator
Check the appliances you want to run during an outage and set quantities where you have more than one. Totals update as you go, and the full-coverage size accounts for motor surge plus headroom. Equipment labels usually print amps, not watts. Tap custom on any row to enter the amps off your nameplate (or your own watts) instead of our typicals.
Amps view converts the watt figures at each appliance’s usual voltage (shown per row), handy for breaker and inlet sizing. They are conversions of typical figures, not readings off your equipment; tap custom on a row to enter the amps printed on your nameplate.
Managed outage list loaded. Food, heat, water, light, and communications are in; air conditioning, range, dryer, EV charging, and electric water heat are out. Add your well pump if that is where your water comes from, and watch the size move. The playbook is in the managed outage guide.
Real-house example loaded. One real house’s outage list: two ACs (the 1.5-ton rides our 3-ton row with its own plate figures entered as custom), twin sump systems, a septic pump, four fridges/freezers and whole-home lighting. Two-of-a-kind nameplates are averaged onto our rows; septic and lighting use our typicals. That is the everything-at-once list. The real house does not run it that way: battery sump pumps bridge the first hours, and the generator covers a managed list. Tick the soft-starter box below to watch the surge problem change.
Loading appliance data…
Your totals
Select at least one appliance and we'll work out the size.
Full coverage vs managed outage: the full-coverage size above assumes every checked load can run without you managing starts. The cheaper alternative is further down: a smaller unit that covers the running load while you manage the starts. Read the managed outage guide.
Soft-start scenario is an estimate, not a guarantee: the AC’s starting watts are figured at 40% of the unsoftened surge (floored at its running watts), following Hyper Engineering / Eltwin SureStart application guidance, which estimates reduced start current at 0.4 × rated LRA. Voltage sag, other loads starting at the same moment, and compressor condition can still prevent a start; when in doubt, shop the unsoftened size.
Generators that fit your load
Select appliances above and we’ll match real models to your load.
Managed-outage matches
Matched on the fuel you pick, not the brochure number: most standby units put out less on natural gas than on propane. Top three cheapest matches are flagged as best matches; the full highlighted list lives in the model database.
How this calculator works
Every appliance has two numbers. Running watts are what it draws once it is going. Starting watts are the brief surge a motor pulls in the first seconds after switch-on, a well pump or air conditioner can pull two to three times its running load while starting.
Your generator has to survive both. We total the running watts of everything selected, then add the single largest surge increment, the biggest (starting − running) difference among your selected appliances, on the assumption that motors rarely all kick on in the same second. That sum is the starting load.
Finally we apply roughly 20% headroom so the generator is not running flat-out at its rating (which shortens engine life and leaves nothing for the load you forgot), and round up to the next standard home size class: 7.5, 10, 14, 18, 22, 24, or 26 kW. Treat the result as the size class to shop in; the model database shows what that class actually costs and burns.
The connection can be the real limit. A generator can only deliver what the inlet or transfer device will carry: at 240 V, 30 A is 7,200 W, a 40 A GenerLink is 9,600 W, and 50 A is 12,000 W. Pick your connection in the totals panel and the calculator checks your running load and starting peak against it. A bigger generator does not push more power through a smaller connection.
What “whole home” really means. A whole-home standby feeds the entire panel, and load management decides what actually runs together: compressors get sequenced so two ACs never start in the same second, and the electric range, dryer, and EV charger are the loads that get shed or simply left off until the power is back. That is how a 22 kW unit honestly covers a house whose everything-at-once list adds up to much more. Our full-coverage figure is the strict version of the same idea: everything you checked, unmanaged, plus headroom. If it comes out larger than a “whole home” size you were quoted, that gap is the load management, not a math error.
A load-management module or smart thermostat can stagger motor starts and sometimes let you drop a size class. Worth asking your installer about before you buy up.
Not every list needs full coverage. The number above is the automatic version: everything checked is available, and the generator absorbs the biggest start. A managed outage takes the other path: protect the loads that can hurt you, let battery backups bridge short outages, run one big motor at a time, and leave the range, dryer, EV charger, and second AC out of the plan. The managed outage guide lays out that cheaper playbook, including a starter list it can load into this calculator for you.
Your equipment, your numbers: typical wattage tables are a starting point, not your house. Tap custom beside any appliance and enter your own figures: either watts, or the amps printed on the nameplate (that is what most labels show; starting amps are rarely printed). We convert amps to watts at the voltage you pick. If your label prints LRA (locked-rotor amps), enter it and the starting watts come straight from it; without LRA we estimate the surge from the typical multiple for that appliance and mark the figure (est.). Thinking in amps instead (breaker panels, inlets, cords)? The Figures switch above the list converts every row, and your running total, at each appliance’s usual voltage. New to nameplates altogether? The nameplate reading guide shows where the amps, RLA and LRA hide on your equipment and how to convert them. Edited rows are marked custom, starting is never figured below running, and reset to typical restores our compiled figures. Your numbers are never mixed into the typicals silently.
A soft starter on a central AC or heat pump attacks the same problem from the other end: it ramps the compressor up instead of slamming it, cutting the start surge itself. Tick the soft-starter box above and the calculator refigures your AC’s starting watts at 40% of the unsoftened surge, the planning estimate in Hyper Engineering’s SureStart application guidance (reduced start current ≈ 0.4 × rated LRA; their datasheet quotes a 60–70% reduction range for scroll compressors). It applies only to conventional single- or two-stage compressors, never inverter/variable-speed units, and it is an estimate, not a promise that a given generator will start a given AC.
Typical appliance wattage
The same figures the calculator uses. Check the nameplate on your own equipment where you can. Older appliances, especially fridges and pumps, often draw more than the typical values below.
| Appliance | Category | Running watts | Starting watts | Notes |
|---|---|---|---|---|
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Next steps: take your recommended size to the generator model database, then the cost guide and fuel & runtime guide.