Whole-House Generator Sizing Guide

The right whole-house generator size is not determined by square footage or by the biggest unit that fits the budget. It comes from the loads you need during an outage, which loads can run together, and the extra capacity required to start motors and compressors.

That distinction matters. A generator can cover a home’s steady demand yet still struggle when an air conditioner, well pump, or other motor starts. This guide provides a planning calculation you can check with a qualified generator installer; it is not a final electrical, fuel, or permit design.

Quick answer: size a generator in five steps

1. Define the outage plan. List the loads that must run, the loads that are optional, and the loads that can be locked out or shed.
2. Record verified inputs. Use each appliance’s nameplate or manual for running demand and motor-starting information. Note voltage, fuel-burning heat versus electric heat, and any manufacturer-approved soft-start equipment.
3. Calculate two requirements. Total the running demand of loads that may operate simultaneously, then calculate the momentary demand when the largest motor starts.
4. Apply load management. Document which large loads the transfer or energy-management system will prevent from operating together. Do not subtract a load based only on a promise to remember to turn it off.
5. Validate a candidate system. Compare both requirements with the generator manufacturer’s continuous and motor-starting ratings for the intended fuel, then have the installer confirm fuel supply, transfer equipment, controls, and local requirements.

Manufacturer sizing guidance follows the same basic sequence. Briggs & Stratton tells users to choose simultaneous loads, obtain values from manuals or data plates, total running watts, and add the single largest additional starting requirement. Its current whole-home guidance also says sizing is about electrical load rather than floor area. See the manufacturer’s generator sizing method and whole-home sizing guidance.

Running capacity vs momentary capacity

Treat continuous operation and motor starting as separate checks:

Required running capacity = sum of simultaneous running loads

Required momentary capacity = simultaneous running loads + largest additional start demand

The word additional prevents a common double-counting error. Some documents list “additional starting watts,” while others list the motor’s total starting watts.

• If the source gives additional starting watts, add that value to the total running load.
• If it gives total starting watts, subtract that motor’s running watts first to find the additional demand. Alternatively, add the total starting value to the running demand of all the other loads.

For example, a motor with 600 running watts and 1,800 total starting watts contributes 1,200 additional starting watts—not another 1,800 on top of its 600 running watts. These figures illustrate the math only; they are not appliance specifications.

The single-largest-start method assumes motors will be sequenced rather than starting together. If controls can start multiple motors at once, or if the documentation gives locked-rotor current, kVA, or another value instead of watts, ask the installer and equipment manufacturer to evaluate the starting event. Do not invent a conversion from a generic online table.

Generac’s official home standby sizing worksheet likewise separates running watts from additional starting watts and tells readers to verify estimates against the equipment data plate or owner’s manual.

Build the load inventory from reliable sources

Use this hierarchy for every input:

1. Equipment nameplate and owner or installation manual. Record the exact model and the manufacturer’s running and starting data.
2. Generator, transfer-equipment, and load-management documentation. Confirm what the proposed system can supply, transfer, shed, and restart.
3. Installer calculations or measurements. Ask the contractor to document any field measurement, assumption, or manufacturer consultation.
4. Generic wattage charts. Use these only to identify missing loads or create an early screening estimate. Never use them as final quote inputs.

For HVAC equipment, photograph the indoor and outdoor unit labels and collect the manuals. For a private well, record the pump and control-box details described in the well pump electrical requirements guide. For every load, capture enough information that an installer can trace the number to its source.

Load-planning field	What to record
Equipment	Type, manufacturer, model, and location
Priority	Must run, optional, managed, or locked out
Running input	Verified watts, amps, VA, or other nameplate value with its source
Starting input	Additional starting watts, total starting watts, LRA, inrush data, or manufacturer guidance
Supply details	Voltage, phase, circuit, and transfer method
Control plan	Loads allowed together and required start sequence

Worked generator-sizing examples

The numbers below are hypothetical planning inputs, not typical appliance ratings or product recommendations. Replace every value with documentation for the actual equipment.

Example 1: essential loads with a well pump

Suppose the verified inventory shows 2,200 watts of simultaneous running load. The well pump has the largest additional starting demand at 2,000 watts.

• Required running capacity: 2,200 W
• Required momentary capacity: 2,200 W + 2,000 W = 4,200 W

The candidate generator must satisfy both checks under the proposed operating conditions. Selecting a unit only because its advertised peak number exceeds 4,200 W would be incomplete; its continuous output, voltage, fuel-specific rating, and motor-starting capability also have to match the system.

Example 2: managed cooling load

Suppose must-run loads and a central cooling system total 5,300 watts while operating. The cooling compressor adds 4,500 watts during its verified start event, producing a 9,800-watt momentary requirement.

Now suppose an electric water heater is excluded by an automatic load-control strategy. Its demand is not added while cooling operates—but only because the proposed controls enforce that sequence. If both loads may operate together, both belong in the simultaneous running total.

Use the generator and transfer-switch handoff worksheet now to record the verified inputs, exclusions, start sequence, and questions for each contractor.

Validate the proposed generator and transfer strategy

A calculated wattage is a shortlist, not an installation specification. For every candidate, ask the installer to document:

• Continuous output on the intended fuel. Do not assume the model name is its output on both natural gas and propane; compare the exact current specification sheet.
• Motor-starting capability. Ask how the manufacturer expresses it and how the largest verified motor start was checked.
• Load-management behavior. Identify what sheds first, what restarts automatically, any lockouts, and what happens after a fault or manual override.
• Transfer compatibility. Confirm the generator, transfer equipment, service configuration, and controls are designed to work together.
• Fuel adequacy. Have the appropriate contractor verify the natural-gas service or propane storage and piping for the chosen unit and other connected appliances.
• Environmental and permitting requirements. The installer must use the current product manual and locally adopted rules for placement, exhaust, electrical work, permits, and inspections.

Brand-neutral candidate comparison

Compare	Evidence to request
Output	Continuous rating for the intended fuel and voltage
Starting performance	Manufacturer data supporting the largest motor start
Managed loads	Compatible modules, number of priorities, and documented sequence
Transfer equipment	Model, rating, service arrangement, and compatibility statement
Fuel	Full-load demand and the contractor’s supply calculation
Support	Warranty terms, local service availability, and parts support

Avoid choosing by square footage alone. Two homes with the same floor area can have radically different loads because one may use combustion heating and modest essential circuits while another has electric heat, multiple compressors, a well pump, or EV charging.

Generator sizing is not a panel load calculation

This backup-power inventory asks, “What selected loads will the generator supply, and in what sequence?” A service or panel calculation asks whether the electrical service, feeder, or panel can support building loads under the applicable code method. The two calculations are not interchangeable. Use the panel upgrades and load calculations guide when evaluating service capacity or adding major electrical loads.

Keep adjacent decisions in their own lane

Once the size range is defensible, use the generator interlock vs. transfer switch guide to compare connection approaches and the manual vs. automatic transfer-switch guide for equipment and installation scope. Those choices can change which loads are transferred or managed, but they do not replace the load calculation.

For portable-generator placement, carbon-monoxide risk, cords, fuel handling, and backfeed prevention, follow the dedicated portable generator safety guide. Safety rules and installation instructions come from the current equipment manuals, authorities having jurisdiction, and qualified installers—not from the sizing examples above.

Turn the calculation into a quote-ready scope

A strong proposal should show the running-load total, largest start event, intended fuel rating, managed-load sequence, excluded loads, transfer strategy, and fuel-supply assumptions. Ask the installer to mark every change instead of presenting only a generator model number.

Complete the printable generator and transfer-switch handoff worksheet before requesting quotes. It gives each contractor the same inputs and makes differences in included loads, controls, and assumptions much easier to compare.