GCGenerator Cost Calculator

Capacity planning

What Size Whole-House Generator Do I Need?

Plan essential, managed or whole-home backup from actual running and starting loads rather than square footage alone.

Direct answer

There is no safe universal kW answer from house size. Generac's published worksheet method adds the running watts of the loads that may operate together to the single highest additional starting-watt demand, then uses that total only to narrow the equipment class. A professional design must confirm the result and fuel capacity.

Free running + starting watts worksheet

Calculate the outage load before pricing a generator

Select only equipment that may operate together. The worksheet totals continuous running watts and then adds the single highest additional motor-start demand—the published manufacturer method—while keeping professional verification explicit.

Loads expected to run during an outage
Add one exact nameplate or manual value

Published appliance figures are examples only. Replace or supplement them with the exact data plate or owner-manual values. If multiple motors can start together, a professional must model that sequence rather than applying this single-start worksheet.

What changes the result?

A useful calculation starts with the physical scope and the written commercial terms. These are the factors to make explicit:

  • Exact running watts
  • Largest motor starting demand
  • HVAC, well, sump and sewage pumps
  • Electric heat, cooking and water heating
  • Load management and fuel derating

Use one scope for every quote

Give each business the same information and ask it to resolve the same boundaries. That makes the totals more comparable and exposes work left as an allowance or exclusion.

  1. List each required outage load
  2. Record nameplate running and starting data
  3. Decide which loads may be shed
  4. Require electrical and fuel calculations

Use running watts plus the highest starting demand

Motor-driven equipment can need extra power for a few seconds when it starts. The manufacturer's worksheet demonstrates the method as total simultaneous running watts plus the highest single additional starting-watt figure, not every motor surge added together. Its worked example is 4,250 running watts plus 2,350 starting watts, or 6,600 watts total.

  • Total the loads intended to run together
  • Add the highest additional starting requirement
  • Convert watts to kW by dividing by 1,000
  • Retain design margin and manufacturer rules

Choose the backup scope before the equipment

Essential-circuit backup can exclude central air conditioning, electric resistance heat, ranges, dryers, EV charging and other large loads. Managed whole-home designs may temporarily shed selected loads to keep a smaller generator within capacity. A whole-panel transfer switch does not mean every load can run simultaneously.

Square footage is only context

Generac's current sizing tool asks about home size, heating fuel, EV charging, whole-home versus selected-circuit coverage and expected simultaneous use. That is more useful than floor area alone, but its result still directs the homeowner to a dealer evaluation. The installed-cost calculator therefore treats kW as an entered price scenario, never a recommendation.

Questions people ask

Can house square footage determine generator size?

No. Square footage does not reveal HVAC starting current, well pumps, electric heat, cooking, water heating or which loads will run together.

What is the basic generator sizing formula?

For an early worksheet, add simultaneous running watts and the highest single additional starting-watt demand. Then have a qualified professional verify load, transfer and fuel design.

Can load management reduce the generator size?

It can prevent selected large loads from running together, but the controls and priorities must be designed and commissioned for the actual equipment.

Does a 22 kW generator always produce 22 kW on natural gas?

Not necessarily. Ratings can differ by fuel and operating conditions. Check the exact model specification and design conditions rather than using the marketing size alone.

Method and sources

The model is a planning tool, not a quotation. These are the published sources used for this page; read the full assumptions before relying on a range. Source review: 21 September 2026.

Review full methodology