What Size Generator Do I Need for My House?

The right generator size is based on the electrical loads you want available during an outage—not on square footage alone. For a useful planning estimate, total the loads that may run at the same time, account for the extra starting demand of the largest motor or compressor, and then consider whether high-demand equipment will be managed rather than allowed to run simultaneously. Final sizing should be verified against the actual equipment labels, the generator’s output on the chosen fuel, the transfer/load-management design, and the requirements that apply to the installation.

Current residential standby product lines include models from roughly 10 kW through the mid-20s kW and beyond, but that market range is not a sizing rule. Two houses of the same size can need very different generator capacity because their heating, cooling, water, cooking, laundry, pumps, EV charging, and other loads may be completely different.

If you are still deciding between backing up most of the house and only critical circuits, start with the whole-house generator guide and define the outage experience you actually want before trying to pick a kW rating.

Start with the loads you actually want during an outage

Generator sizing becomes much easier once you separate “must run,” “nice to have,” and “can wait.” The important question is not whether an appliance exists in the house; it is whether you expect it to be available at the same time as other loads while the generator is carrying the home.

Common loads that change a standby-generator calculation include:

• Central air conditioning and heat pumps, especially when compressor starting demand is high.
• Electric resistance heat or auxiliary heat strips, which can be a very large steady load.
• Well pumps, sump pumps, sewage or grinder pumps, and other motor-driven equipment.
• Electric water heaters, ranges, ovens, clothes dryers, and other high-wattage resistance loads.
• Pool pumps, spas, workshops, detached buildings, and other optional loads that may or may not need outage power.
• Level 2 EV charging, if you intend to charge during an outage rather than lock out or manage that load.

Fuel type can matter too. Some standby generators have different continuous-output ratings on natural gas and propane, so the same model name or enclosure size does not always mean the same usable kW on both fuels. Use the rating for the exact model and fuel you plan to use.

Why square footage is not enough to size a generator

Square footage can be a useful screening input because larger homes often have more equipment, but it cannot tell you how much power the home needs. A 2,000-square-foot home with gas heat, gas water heating, one air conditioner, and city water can have a very different backup load from a similar-size all-electric home with a heat pump, auxiliary heat, a well pump, electric cooking, and an EV charger.

The same is true of your electrical service rating. A 100-amp or 200-amp service describes the service equipment and available utility capacity; it is not a direct generator-size recommendation. A generator can be sized for the loads you intend to support, and load management can change which loads are allowed to operate together.

Running load and starting demand are different

Running watts are the steady power a load uses after it is operating. Starting watts—or motor-starting demand—are the additional power some equipment needs briefly when a motor or compressor starts. Refrigerators, well pumps, sump pumps, furnace blowers, air conditioners, and heat pumps are common examples of loads where startup behavior matters.

A common manufacturer worksheet method for rough planning is:

That formula is useful for a first-pass estimate, but it is still an estimate. Motor-starting performance depends on the actual equipment and generator, and professional sizing may need to consider voltage dip, load sequencing, load management, and the way multiple loads can cycle during an outage. Do not assume one generic “2×” or “3×” multiplier is correct for every motor.

A homeowner-safe way to build a sizing worksheet

You can collect the information an installer needs without opening electrical equipment or doing live electrical work. The goal is to document the loads, not to perform the final design yourself.

The factors that most often change the final generator size

Central air conditioning and heat pumps

HVAC is often one of the largest motor loads in a home, so both running power and compressor starting behavior matter. Do not size from tonnage alone. The installer should use the actual equipment data and the generator manufacturer’s sizing method. If the home has multiple systems or electric auxiliary heat, the design decision becomes even more important.

Electric resistance loads

Electric water heating, ranges, ovens, clothes dryers, resistance heat, and similar loads can add several kilowatts each while operating. You may decide that some of these loads do not need to run during an outage, or that they can be managed so they are not all available at once.

Pumps and other motors

Well pumps, sump pumps, pool pumps, and other motors can have starting demand above their steady running load. When a pump is critical, its nameplate and control setup deserve the same attention as the HVAC equipment.

Fuel, altitude, and temperature

The generator’s usable output is not always identical in every installation. Some models have different ratings on natural gas and propane, and manufacturer documentation may specify output changes at higher elevation or temperature. Final sizing should use the exact model’s current specification sheet rather than the number printed in a product family name alone.

Future electrification

A future EV charger, heat-pump conversion, electric water heater, addition, or second HVAC system can materially change the load picture. That does not always mean buying a much larger generator today; it can also mean deciding which future loads will be excluded or managed during backup operation.

Load management can change how much generator you need

A whole-home transfer arrangement can be designed so the generator is connected to the home while selected high-demand loads are automatically delayed, shed, or locked out when necessary. That can prevent generator overload and may allow a smaller generator to support a larger set of connected circuits than a design that assumes every major load can run at once.

This is why “whole-house generator” and “generator large enough to run every electrical load simultaneously” are not always the same thing. Ask the installer to identify any managed loads in the proposal and explain what happens when the generator is heavily loaded. The transfer equipment and load-management strategy should be reviewed together; see the generator transfer-switch guide for the equipment side of that decision.

Your panel rating is not the generator-size answer

It is tempting to turn a 200-amp service into a generator wattage number and shop from that, but that skips how residential load calculations and demand work. Your service and panel still matter because the transfer equipment, breaker arrangement, available space, and existing electrical system affect the project. They just do not tell you the generator kW by themselves.

If the project raises questions about the existing panel or service, use the home electrical upgrades guide as the planning reference. A licensed electrician should verify the actual service configuration and whether enabling electrical work is needed.

Essential-load backup and whole-home backup can lead to very different sizes

If your goal is refrigeration, a furnace blower, a sump or well pump, a few lights, internet, and selected outlets, your generator requirement can be much smaller than a plan that keeps central air, electric water heating, laundry, cooking, pool equipment, and EV charging available. The most useful sizing decision is often deciding which loads truly need backup before deciding how many kilowatts to buy.

If you are unsure which loads belong in the backup plan, compare an essentials-only scope with a broader whole-home scope before final sizing. The generator recommendation should follow that decision, not force it.

Common generator-sizing mistakes to avoid

• Choosing a generator from square footage alone.
• Treating the main-breaker or service amp rating as the generator requirement.
• Using generic appliance wattage charts when actual model/nameplate data is available.
• Adding only running watts and overlooking motor/compressor starting demand.
• Assuming “whole house” means every high-demand load can run at the same moment.
• Ignoring the generator’s exact natural-gas or propane output rating.
• Buying extra capacity for every possible future load instead of deciding whether some loads should be managed during outages.

Where the planning estimate ends

A homeowner worksheet is useful for comparing proposals and spotting assumptions, but the final system is a professional design decision. The generator installer and/or licensed electrician should verify the connected loads, starting requirements, transfer equipment, load-management strategy, generator model and fuel rating, existing electrical system, and the code and permitting requirements that apply where you live. Fuel-supply work may also require the appropriate qualified fuel contractor.

Do not open service equipment, move conductors, bypass transfer equipment, or modify fuel piping as part of a sizing check. The safe homeowner role is to gather labels, photos, outage priorities, and future-load plans so the professionals can design from accurate information.

Questions to ask before you accept a generator-size recommendation

• What loads did you include in the calculation, and which did you exclude?
• What is the largest motor or compressor starting load in the design?
• Are any loads automatically managed, delayed, or locked out during generator operation?
• What is the generator’s continuous output on the fuel I will actually use?
• Did altitude, temperature, or other manufacturer derating rules affect the selection?
• What transfer equipment is included, and how does it coordinate with load management?
• Does the proposal assume future loads such as an EV charger, heat pump, addition, pool/spa, or workshop?
• What electrical, fuel, permit, or site work is separate from the generator itself?

Next step: turn the load list into a project scope

Use the Home Generator Size Calculator to organize your loads and compare planning scenarios.

Once you have a realistic load list, the useful next step is not simply to “round up.” Compare installer recommendations by the loads they assume, the starting demand they account for, and the way they use transfer equipment or load management. Then use the whole-house generator cost guide to understand how generator capacity and enabling work affect the project budget.

Sources

1. Cummins Inc. — Home Generator Sizing Calculator | Size Your Whole House Generator Today (opens in a new tab). Accessed September 23, 2026.

2. Rehlko / KOHLER Home Energy — Standby Generator Sizing Guide (opens in a new tab). Accessed September 23, 2026.

3. Generac Power Systems — Home Standby Generator Buyer’s Guide — Wattage Worksheet (opens in a new tab). Accessed September 23, 2026.

4. Generac Power Systems — 50A Smart Management Module — Owner’s/Installation Manual (opens in a new tab). Accessed September 23, 2026.

5. Generac Power Systems — 18 kW Home Standby Generator — Current Specifications (opens in a new tab). Accessed September 23, 2026.

6. Eaton — Power Distribution Systems — Consulting Application Guide, Motor Starting (opens in a new tab). Accessed September 23, 2026.