Generator Sizing Calculator

Find the perfect backup generator for your home

Takes about 60 seconds • No email required

START HERE

What do you want to keep running during an outage?

Choose the closest fit. You can fine-tune your loads on the next step.

Essentials onlyFridge, lights, internet, and a few critical circuits Most of the houseEssentials plus selected comfort and convenience loads Whole homePlan for broad coverage, including major loads

Takes about 60 seconds • No email required

Planning estimate only. Final generator selection and electrical design should be verified by a qualified electrician or generator professional.

Coverage selected ✓

Next: Enter your home details, then review your loads.

Home details

Step 2 of 5

Home ZIP code (US, 5 digits) Uses a representative postal location and nearby climate station to estimate elevation and the hottest-month 1991–2020 normal daily high. Leave blank to assume sea level & 60 °F. Sources: GeoNames and NOAA Climate Normals.

Home square footage (required) NEC general lighting load = 3 VA per sq ft × this number. Include garage & basement.

Appliances & circuits

✅ Preset applied. 0 items selected.

Step 3 of 5

HVAC & EV loads

Step 4 of 5

❄️ Outdoor equipment

Add one row per outdoor unit. Enter nameplate voltage, minimum circuit ampacity (MCA) when listed, and compressor rated-load amps (RLA) or manufacturer running current. Running VA uses MCA when provided and otherwise uses the running-current value. The running-current value selects a sourced planning start estimate; do not use MCA or locked-rotor amps (LRA) for that estimate. Direct LRA belongs in the separate motor-start section and replaces the estimate.

🔥 Heating

🔌 EV chargers

Included at 100%. Enter each charger's actual charging power in kW (for example, 7.2 kW), not breaker amps.

⚡ Largest motor or compressor start

Required when an unmanaged sump, septic grinder, well or pool pump, A/C, or heat-pump outdoor unit is active. Well-pump voltage, running amps, and starting amps come from the three pump inputs above. Other supported sources may begin with a clearly identified screening estimate that you can replace with equipment data.

Starting-load source Only active, unmanaged qualifying motors and outdoor units appear.

No qualifying starting load is active.

Locked-rotor amps (LRA) or measured starting current Enter nameplate LRA or manufacturer-provided/electrician-measured starting current. Well pumps never receive an automatic fallback here.

Running amps used for sizing Outdoor units use MCA for running VA when provided, otherwise their rated/manufacturer running current. For pumps, use nameplate running current when available.

Voltage Select a starting-load source to use its voltage.

Peak startup replaces this source's running VA with the applicable starting VA. Soft-start scenario inputs affect only the selected A/C or heat-pump compressor-start scenario.

Load management system (LMS) Each eligible outdoor unit is managed by its stable equipment record. A/C condensers are removed only from cooling; heat-pump condensers are removed from cooling and applicable heat-pump-heating scenarios. Auxiliary electric heat remains a separate manageable load.

Settings & safety margin

Step 5 of 5

Fuel type

Headroom buffer (%) ?

Default is 20%. Most homes use 10–30% headroom.

Your estimate is in progress

Planning estimate only. Final generator selection and electrical design should be verified by a qualified electrician or generator professional.

Load details

How this estimate works

For a heat pump with electric backup heat, the planning calculation uses 100% of the compressor load plus 65% of supplemental electric heat.
The calculator checks the largest expected motor or compressor starting event separately instead of adding every starting value together.
A load marked as managed receives sizing credit only when an applicable installed load-management system or other qualifying control actually controls it.
Where applicable, voltage and current entered from equipment or nameplate data are used to calculate apparent demand.
Generator capacity may be adjusted for elevation and temperature where the applicable generator-family data support it.

What your electrician should verify

Actual 120/240 V loads may not be evenly distributed between panel legs. Verify the installed load distribution and neutral loading.
Review whether multiple thermostatically controlled motors or compressors may restart after an outage, including equipment-specific heat-pump defrost and backup-heat staging.
Some variable-speed equipment, UPS systems, electronics, and controls may have tighter voltage or frequency requirements than normal generator transient/start criteria.
Verify the actual generator model, transfer equipment, installed load-management controls, fuel supply, equipment compatibility, and locally adopted code/AHJ requirements.

How to use this calculator

Choose your coverage level

Select whether you want to power essentials only, most of your house, or everything during an outage.

Enter your home details

Provide your ZIP code (for elevation/temperature adjustments) and square footage for accurate calculations.

Review & adjust appliances

We'll pre-select common appliances based on your coverage level. You can customize the list and wattages as needed.

Add HVAC, heating & EV chargers

Include any air conditioning units, heating systems, or electric vehicle chargers you want to power.

Get your recommendation

Use the summary card to see planning load, needed amps and peak surge, then talk to a licensed electrician.

Finding appliance wattage & starting amps

Look for the metal nameplate or data tag on your appliance (usually on the back or bottom).

Find "Watts" or "W" — this is the running wattage you'll enter here.

For single-phase equipment, Volts × Amps gives apparent demand in VA. Use a published watts value or a supported power factor for real watts.

For motors/compressors, look for "LRA" or "Starting Amps" to use in the Max Starting Load section.

📸 Video tutorial and photo examples coming soon!

Heads up: This calculator is for general information only and does not replace a site visit or detailed load study. Always consult a licensed electrician and follow local codes before purchasing or installing a generator.

Planning to buy a generator?

Tell us what you are considering and when you expect to buy. We will review your information and reply with the most useful next step. You can also request local installer introductions as that service becomes available.

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Optional, but useful if you request installer introductions.

Project notes (optional)

Tell us anything useful about your home, equipment, or outage needs.

No obligation. We will use your information to respond to this request and will share it with installers only when you select the installer-introduction option.

⚡ Generator Sizing Calculator