Choose your backup type, select appliances below, then click Calculate. Formula: Recommended Size = (Total Running Watts + Largest Single Starting Surge) × Safety Margin, rounded up to a standard generator size.
Backup Type
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Your Appliances & Loads
| Appliance | Running (W) | Starting (W) | Qty |
|---|---|---|---|
| Refrigerator | — | ||
| Sump Pump (1/3 HP) | — | ||
| Well Pump (1/2 HP) | — | ||
| Garage Door Opener | — |
| Appliance | Running (W) | Starting (W) | Tons / Qty |
|---|---|---|---|
| Central Air Conditioner (per ton) | — | — | |
| Window AC Unit (10,000 BTU) | — | ||
| Gas Furnace Blower | — | ||
| Electric Water Heater | — |
| Appliance | Running (W) | Starting (W) | Qty |
|---|---|---|---|
| Microwave | — | ||
| Dishwasher | — | ||
| Washing Machine | — | ||
| Electric Clothes Dryer | — |
| Appliance | Running (W) | Starting (W) | Qty |
|---|---|---|---|
| TV / Home Electronics (per unit) | — | ||
| Medical Equipment (CPAP / Oxygen Concentrator) | — | ||
| EV Charger — Level 1 (120V) | — | ||
| EV Charger — Level 2 (240V) | — |
Safety Margin & Fuel Type
Your Results
Both numbers matter when choosing a generator: it must run everything continuously at your Running Watts total, and it must also handle the brief surge shown as Starting/Peak Watts when your largest motor switches on.
Results are for reference and estimation only. Verify all values before use in any generator purchase or installation. See full disclaimer below.
Calculate a generator size above to enable this section.
Using fuel type: Gasoline — change this above in Safety Margin & Fuel Type. Enter your tank size and fuel price below to estimate consumption, runtime, and cost.
Fuel consumption varies by generator model, engine efficiency, altitude, and temperature — these are planning estimates, not a substitute for your manufacturer's published fuel consumption curve.
Common home backup profiles mapped to a recommended generator size, computed with this calculator's own formula at the default 20% safety margin. The first three rows use the Portable size list; the last uses the Whole-Home Standby size list, since a load this size is realistically a standby installation. Use this to sanity-check a calculated result at a glance — your own appliance selections above will differ from these fixed profiles.
| Profile | Peak Demand | Recommended Size | Type |
|---|---|---|---|
| Refrigerator, Garage Door Opener, Gas Furnace Blower | 3,050 W | 4 kW | Portable |
| + Well Pump + Sump Pump | 5,300 W | 6.5 kW | Portable |
| + 2-Ton Central Air Conditioner | 9,300 W | 12 kW | Portable |
| Whole home: above + Electric Water Heater + 3-Ton AC + EV Charger (Level 2) | 24,000 W | 30 kW | Whole-Home Standby |
A deeper look at what goes into correctly sizing a generator, and how to choose between backup types and fuels — written for first-time buyers and job-site reference alike.
Why Starting Watts Drives Sizing More Than Running Watts
Running watts tells you what a generator has to sustain all day. Starting watts tells you what it has to survive for a fraction of a second. A generator sized only for the sum of everyone's running watts will often stall or trip the instant a well pump, central AC compressor, or furnace blower motor tries to start — because that single motor can momentarily demand two to three times its running wattage. This is usually the single biggest reason a "big enough on paper" generator fails in practice.
Why Only the Single Largest Surge Counts — And Its Limits
This calculator adds the running watts of everything you select, then adds only the largest individual starting-watt surge on top — not everyone's surge added together. That's standard practice because motors are assumed to start one at a time: everything else is already running at its steady-state wattage when the next motor kicks on. The limit of this assumption is simultaneous starts — if your well pump and central AC happen to start in the same instant, actual peak demand could briefly exceed this estimate. A generous safety margin and, for standby systems, a load-management transfer switch are the two common ways installers protect against that edge case.
Portable vs. Whole-Home Standby — The Real Trade-off
Portable generators cost far less upfront, run on gasoline, propane, or diesel you supply yourself, and require you to manually connect a transfer switch or inlet box every time the power goes out. Whole-Home Standby generators install permanently, start automatically within seconds of an outage, typically run on natural gas or propane from an existing utility line or tank, and cost several times more installed — but need no manual setup at all. The deciding factor is usually how often you actually lose power and how much manual effort you're willing to accept during an outage, not just the sticker price.
Choosing a Fuel Type
Gasoline is the most widely available fuel for portable units but degrades within a few months without a stabilizer, so it's a poor choice for a generator that sits unused for long stretches. Propane stores indefinitely and burns cleaner, at a slight cost in energy density per gallon. Diesel is typically the most fuel-efficient option per gallon and holds up well in commercial duty, at a higher equipment cost. Natural Gas offers effectively unlimited runtime for a standby unit tied into a utility line, at the cost of being fixed in place and dependent on that utility supply staying up during the outage.
Common Sizing Mistakes
The most frequent errors are: sizing only for running watts and ignoring the largest motor's starting surge, forgetting a well pump or central AC entirely because it isn't "always on," skipping the safety margin because the running-watts math already "adds up," and connecting a portable generator directly into household wiring instead of through an approved transfer switch — which risks backfeeding the grid and endangering utility workers. Always size for the specific appliances you'll actually run, not a rough guess at square footage.
This guide is provided for educational and reference purposes. Always confirm system specifications, permitting, and transfer equipment with a licensed electrician or generator installer before purchasing or installing equipment.