Generator Size Calculator
This free generator size calculator sizes backup power the way it actually fails in the field: not on running watts, but on the starting surge. Motors — well pumps, sump pumps, AC compressors, furnace blowers — draw several times their running current for a moment at startup, and a generator that carries the running load fine will stall when the well pump kicks on. The calculator totals your running watts and rides the single largest surge on top.
Every load uses sourced wattage defaults from manufacturer reference charts, shown as honest ranges where published figures vary — and your appliance nameplates always govern. The recommendation adds 25% headroom so the generator runs at roughly 80% of capacity, which is where fuel economy, voltage stability, and engine life all live.
What this calculator will not do is pretend the connection is simple: a transfer switch or interlock is required by code to connect any generator to household wiring, backfeeding through a dryer outlet is illegal and lethal to utility line workers, and transfer equipment is permitted, licensed work nearly everywhere. Sizing is the DIY part — use it to buy the right machine and brief your electrician. Free, no signup.
Generator Size Calculator
What size generator do you need? Check the loads you want to keep running, and get the recommended kW — running watts plus the largest motor-starting surge, with proper headroom. Free, no signup.
What do you need to keep running?
Calculation Formulas
The steady-state demand with everything you selected on at once.
Example:
1 HP well pump (2,000 W) + freezer (500 W) = 2,500 W running.
Motors draw a large inrush at start. Sizing assumes motors start one at a time — the biggest single surge rides on top of the running load. If two large motors can start together, add both surges.
Example:
Well pump surge (4,000 − 2,000 = 2,000 W) on 2,500 W running = 4,500 W peak.
Industry guidance sizes 20–25% above peak so the generator loafs at roughly 80% of capacity — better fuel economy, voltage stability, and lifespan.
Example:
4,500 W peak × 1.25 = 5,625 W → 5.5–6 kW generator.
Direct-on-line motor starts draw several times running current for a moment; in appliance-level sizing this shows up as the starting-watts column (typically 2–3× running watts).
Example:
A 3-ton AC: ~3,500 W running, ~8,500 W starting.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Headroom factor | × 1.25 | The 20–25% margin — generator runs at ~80% capacity. |
| Refrigerator | 150–700 W run / 800–2,200 W start | Sourced range (Champion charts vary by document) — nameplate governs. |
| Well pump 1 HP | 2,000 W run / 4,000 W start | Champion owner's-manual chart. |
| Central AC 3-ton | 3,500 W run / 8,500 W start | HVAC Base — the largest common residential surge. |
| EV charger (L2) | 7,200–11,520 W continuous | 32–48 A × 240 V — no surge, but a huge continuous draw most portables cannot carry alongside a house. |
| Load management | −25–40% | Shedding large loads while motors start can cut the required size substantially. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
NEC Article 702 — Optional Standby Systems(NFPA 70 (2023) Art. 702)
View StandardHome standby/portable backup falls under Article 702: transfer equipment is required, and 702.5 requires the system be sized for the load managed. Backfeeding without transfer equipment is a code violation and a lethal hazard to utility workers.
Key Requirements:
- •Transfer switch or interlock required
- •System sized per the managed load
NEC 445.13 — Generator conductor ampacity(NFPA 70 (2023) 445.13)
View StandardConductors from the generator terminals are sized at not less than 115% of nameplate current (the equipment-side rule; the transfer-equipment install applies it).
Key Requirements:
- •115% of nameplate current for generator conductors
NEC Tables 430.248 / 430.7(B) — Motor data(NFPA 70 (2023) Art. 430)
View StandardMotor full-load currents and locked-rotor code letters — the code basis behind the starting-watts phenomenon this calculator models with manufacturer charts.
Key Requirements:
- •Single-phase FLC per 430.248
- •Locked-rotor kVA/hp per 430.7(B)
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
What the outage threatens
Climate drives the load list
The loads worth backing up differ sharply by region — which changes the size class entirely.
Regional Examples:
Transfer equipment rules
The legal connection
Every jurisdiction requires listed transfer equipment (switch or interlock) to connect a generator to house wiring; permits and inspection apply.
Regional Examples:
Fuel and runtime realities
Bigger is not always better
Oversized generators burn more fuel at light load and can wet-stack; the ~80% loading target is why the headroom is 25%, not 100%.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
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How to Use This Calculator
- Check every load the generator must carry at the same time — think about your actual outage: what must run simultaneously, not everything you own.
- Set quantities where you have more than one (rooms of lights, freezers).
- Click Calculate for the running load, the peak with the largest motor surge, and the recommended generator size in kW.
- Compare the recommendation against generator classes — and remember the transfer switch is part of the project, not an accessory.
Why the Surge — Not the Running Load — Sizes the Generator
A refrigerator runs on a few hundred watts but can demand over two thousand for the moment its compressor starts. A well pump doubles its draw at startup. That inrush — locked-rotor current of five to eight times running current — is why generators stall on paper-correct loads. The standard sizing method handles it honestly: total the running watts of everything on simultaneously, then add the single largest additional surge, on the assumption that motors start one at a time (if your well pump and sump pump can genuinely start together, add both surges). The 25% headroom on top is not padding — generators are happiest loaded to about 80%, where voltage holds steady and the engine is neither lugging nor loafing. And a practical note the charts do not show: an EV charger or electric tankless water heater is a continuous multi-kilowatt draw that dwarfs everything else on the list — backing those up moves you into whole-house standby territory.
Frequently Asked Questions
What size generator do I need to run a refrigerator, well pump, and furnace?
Roughly: refrigerator 150–700 W running, 1 HP well pump 2,000 W, furnace blower 500–800 W — call it ~3,000–3,500 W running. The well pump's starting surge adds ~2,000 W on top, putting peak demand near 5,000–5,500 W. With 25% headroom, that's a 6.5–7 kW generator. Check your actual loads in the calculator — the answer moves fast with AC or a second pump in the mix.
What's the difference between running watts and starting watts?
Running watts is the steady draw once an appliance is going; starting watts is the momentary inrush when a motor starts — locked-rotor current runs 5–8× the running current, which shows up as starting watts of roughly 2–3× running on manufacturer charts. Resistive loads (heaters, lights, water heater elements) have no surge. Generators are rated both ways too: the 'peak' rating handles surges for seconds, the 'rated' number is what it carries continuously.
Why do you add only the largest surge, not all of them?
Because motors almost never start at the same instant — the fridge compressor, the well pump, and the sump pump each kick on independently, so the generator only rides one surge at a time on top of the running load. That's the standard industry sizing method. The exception worth planning for: if two large motors genuinely can start together (a well pump and a sump pump during a storm), add both surges or stagger them with load management.
Can I just plug my generator into a dryer outlet to power the house?
No — that's backfeeding, and it's illegal everywhere for a reason: it energizes the utility lines outside your house and can kill the line worker restoring your power. Connecting a generator to household wiring requires a transfer switch or a listed interlock kit that physically prevents the generator and utility from being connected at once. It's an electrician install with a permit in nearly all jurisdictions, and it's the part of the project to price before you buy the machine.
Whole-house standby or portable — how do I choose?
Run the numbers both ways. If your must-run list is the survival core — refrigeration, well pump, heat, some lights — a 6–9 kW portable with an interlock covers it for a fraction of the cost. Once central AC, an EV charger, or an electric tankless water heater joins the list, peak demand jumps into the 15–25 kW range and you're in automatic-standby territory with its gas line, pad, and transfer switch. The calculator makes the jump visible: toggle the AC on and off and watch the recommendation.