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Generator Sizing Calculator

Select circuits to back up → running watts, surge allowance, kW generator, fuel burn, and transfer switch type.

Circuits to back up

800 W each

600 W each

800 W each

900 W each

2000 W each

400 W each

1500 W each

1200 W each

3500 W each

7200 W each

1.25

Multiplier on running watts for motor starts

Generator size

7.5 kW

2,000 W running · 2,500 W surge planning
Transfer switch
Manual interlock or 6–10 circuit transfer switch
Propane (planning)
0.5 gal/hr @ load
Natural gas (planning)
0.44 therms/hr

Shareable summary

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Generator sizing snapshot

Recommended kW
7.5 kW
Running load
2,000 W
Transfer
manual-interlock

Quick answer

List the circuits you need in an outage; most essential-load homes land around 7–12 kW running watts before surge, which often points to a 12–16 kW standby or large portable with a transfer switch. Use the checklist below — then confirm with nameplate loads and an electrician.

What this assumes

  • Circuit wattages are planning defaults — read nameplate amps × volts for critical loads.
  • Surge factor accounts for motor starts (sump, well, AC) — default 25% unless you know soft-start.
  • Fuel burn is approximate at 65% load factor for planning.

Important

Generator and transfer equipment must be installed per NEC and local utility requirements. Never backfeed a panel without an approved transfer device.

Understanding the result

Running watts versus surge

Motors draw several times running amps at start. Size the generator for running load plus realistic surge, or stagger starts manually.

Transfer switch types

Portable setups use a manual interlock or small transfer panel. Standby generators at 20 kW+ typically need an automatic transfer switch (ATS).

How the math works

Running watts = sum of selected circuits × quantity. Surge watts = running × surge factor. Recommended kW = max(running×1.15, surge) rounded up to standard generator sizes. Fuel burn uses partial load factor at that kW.

Where to find each input

  • Nameplate amps × volts on fridge, well, and AC
  • Furnace blower label — often 6–10 amps at 120V
  • Sump pump nameplate — surge matters
  • Service panel main breaker — caps whole-house options

Worked examples

Essentials only

Fridge, furnace blower, lights, sump — no AC.

Often 4–6 kW running → 9–12 kW class generator with surge headroom.

Add 3-ton AC

Includes central AC circuit.

Jumps to 14–20 kW+ class — verify LRA on condenser nameplate.

Well + rural pump

2 kW well pump plus basics.

Well pump surge drives transfer switch and generator tier — stagger starts.

What this does not include

  • Utility standby fees
  • Gas meter upgrade
  • Concrete pad
  • Maintenance contract

Regional adjustment

Coastal and wildfire-prone areas see higher install labor — pair with generator install cost tool.

Accuracy and references

NEC Article 702; generator manufacturer sizing guides; typical residential nameplate loads.

Counterpart bridge

Pricing this on the contractor side?

This page interprets quotes and budgets from the buyer’s chair. The matching pro tool uses the same scope logic to build a defensible price.

Open Whole-House Generator Installation Cost Calculator

For trade owners

Pricing this job as a contractor?

Homeowners see national ranges on this page. If you are building the estimate, use the same scope assumptions — then price to burdened labor, overhead recovery and a real margin.

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Common questions

What size generator for a whole house?
Whole-house often means 22–26 kW for 200-amp service with AC — but list your circuits; many homes back up essentials on 12–16 kW.
Can I run central AC on a portable generator?
Only if generator kW and surge capacity exceed AC locked-rotor amps — often needs 14 kW+ for a single stage 3-ton unit.
Manual interlock vs transfer switch?
Interlock is cheaper for portables; ATS is standard for permanently mounted standby units.
How many watts does a refrigerator need?
Plan 600–800 running watts with higher surge when the compressor starts.
Do I need to size for EV charging?
Backup EV charging at Level 2 adds 7,200+ watts — usually excluded from outage backup unless you size for it.
Propane vs natural gas burn?
This tool shows approximate gallons per hour (propane) and therms per hour (natural gas) at partial load.
Is a sump pump essential?
In finished basements, yes — include it if the pit is active during storms.
Who installs the transfer switch?
A licensed electrician — permit and inspection are typical.
Is this a quote from a contractor?
No — it is a planning band from published medians and engineering rules. Always collect written, itemized bids.
Can I save my results?
Use Print / save PDF on the shareable card or save to your homeowner account when signed in.
Why does my metro change the number?
Labor and permit costs vary by market. We apply a regional cost index from our metro table.
What vintage are the cost ranges?
2024–2026 residential medians cross-checked with True Trade Path cost guides and RSMeans-style repair data.
Does this include tax?
Unless your quote says otherwise, assume sales tax may apply to materials in your state.
Should I get three quotes?
Yes — three comparable scopes are the standard check before you sign.
What if my house is unusual?
Historic homes, steep sites, and commercial-grade equipment need site-specific quotes — treat this as a screen.
Where is the pro counterpart?
See the counterpart bridge at the bottom — contractors use the paired tool to build defensible pricing.

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