An Automatic Backup Generator Powers a House During an Outage

How to Size a Generator

The most common question we answer: what size generator to buy? As elementary as it sounds, without going into details, there’s only one correct way to answer.

Buy the generator with enough capacity for everything that needs power.

Details matter, which makes it a little more complicated to answer the question than ‘buy x-generator to power your house.’

We start by determining what will receive power during an outage. Will the generator power everything or just a few essentials?


In This Guide

Whole House Generator or Just the Essentials?

This is a good place to start. Will the generator power all or most of the house, or only the most important appliances and circuits? In most cases, a whole house generator has enough capacity to run any appliance in the house. It may not run all of them at once, but it can handle the usual number of appliances and lights at any given time, plus one or two major appliances like an air conditioner or an electric range. Exceptions include electric heating and electric instant water heaters.

Best Natural Gas or Propane Generators for Whole House Power

Generators that only power essentials have enough capacity to power the most critical circuits—usually all at the same time. Essentials might include a refrigerator, freezer, sump pump, well pump, furnace, garage door opener, one or two lighting circuits, and probably a kitchen circuit. Some homeowners consider air conditioning critical.
Infographic explains how a standby generator system works

How an Automatic Backup Generator Works

A standby generator, often called a backup generator, is automatic. Most of the time, they monitor the power from the utility. On a regular schedule, weekly, bi-weekly, or monthly, they start and run for a short exercise cycle. No engine should sit for a long time without running. Exercise lubricates the parts, clears out moisture, and keeps the seals in good shape. It is common wisdom that starting an engine regularly and warming keeps it easy to start when you need it. Exercise and regular maintenance according to schedule ensure a backup generator starts and runs when the power goes out.

Most standby generators for a home will run on natural gas or propane. Fuel comes from the home’s existing natural gas line or propane tank. Natural gas or a correctly sized tank keep the generating running through any outage. Propane tanks may need filling during long outages.

When the utility power shuts off, the generator senses the outage and starts the engine. It takes a few seconds to stabilize the engine speed. When it’s ready to accept the load, it signals the automatic transfer switch. The ATS disconnects the utility first, then it connects the generator and restores power to the house.

The generator senses when the utility restores power and reverse the process. After a few seconds to ensure utility power is stable, the ATS disconnects the generator, then connects the utility. The generator engine runs in a cool-down cycle—usually about five minutes.

A Power management system allows a generator with less capacity to power all the appliances in a home, but not necessarily at the same time.

Watts and Kilowatts – What You Need to Know

For sizing, the important numbers are watts (W) or kilowatts (kW). Note: Don’t confuse kW with kWh (kilowatt-hours). The first is a measure of power, the second a measure of energy. Most appliances have a tag that provides the information required. Look for watts, kilowatts, or possibly volt-amps (VA) which for home appliances we can consider as watts. For any appliance with a motor larger than a window fan, we also consider starting watts. Since a home generator capacity rating is in kilowatts, convert watts or volt-amps to kilowatts. Volt-Amps = Watts. Watts / 1000 = Kilowatts. You can also multiply volts by amps to get watts. 120 volts x 10 amps = 1200 watts / 1000 = 1.2kW. It takes more power to start a motor than it does to run it. We differentiate starting watts running watts. In general, it takes two to three times as much power to start a motor as it does to run the motor. For example, a refrigerator might require 900 running watts and 1800 starting watts.
7.5kW PowerPact Generator Home Backyard Installation

Essentials Only Home Generators

A generator for essentials doesn’t need the capacity to power appliances and circuits we think of as conveniences or entertainment. Examples might be a game room, dishwasher, electric range, or the televisions in bedrooms. Automatic Transfer Switches for an essential-circuits generator install as a separate circuit breaker panel connected to the main panel. A circuit breaker in the main panel feeds utility power to the transfer switch. A second connection feeds power to the ATS from the generator. All-in-one transfer switches have the switch and the circuit breakers. Twelve or 16 circuits are common, but eight circuit and 10 circuit transfer switches are available. Another method installs a single load transfer switch between the main breaker panel and a separate sub-panel with as many breakers as required. Make a list of everything that must run during an outage with its running watts and starting watts. Total the running watts and add 10 to 20 percent because a generator should not run at its fully loaded rating all the time. Manufacturers may include an 80/20 clause in their warranty. Take the highest starting watts in your list and add it to the running watts. If a load starts and stops frequently, like a sump pump during a storm, add its starting watts to the total starting watts required. Choose a generator that can provide both the running watts and the starting watts required. Essential Circuits with Power Requirements.  
Circuit Running Watts Starting Watts
Furnace Fan 1/2 HP 875 3200
Well Pump 1 HP 2000 4100
Sump Pump 1/2 HP 1000 3000
Garage Door Opener 1/4 HP 700 1900
Freezer 800 1300
Refrigerator 800 1300
Lighting Circuit A 1500 1500*
Lighting Circuit B 1500 1500*
Kitchen Circuit 1900 3800
Total 11075 Watts or 11.075kW 4100 + 3000 = 7.1 kW**
*If the running watts are equal to the starting watts, don’t include the number in the starting watts. **Add the Well Pump starting watts with the sump pump starting watts. Calculate running watts required within the 10-20 percent margin. Running Watts = 11.075kW + 10% = 12.18kW           Round Up 12.2kW Running Watts = 11.075kW + 20% = 13.29kW           Round Up 13.3kW Starting Watts = 13.29kW + 7.1kW = 20.39kW          Round Up 20.4kW We can make a judgement call for one load that never puts a continuous load on the generator. In this case, the garage door opener only runs for a few seconds at a time. Relegate its power requirements to fall within the margin and reduce our running watts requirements by 700 watts to 11.5kW to 12.2kW. However, don’t reduce the starting watts capacity required. Generator running watts rating = 11.5kW to 12.6kW. Choose a generator with a rated capacity that falls within this range, plus the motor starting capability required. Standby generators by the major manufacturers have motor starting capability close to double the rated capacity. Here, the homeowner could select a 12kW generator, but could also choose a 13kW or 14kW generator for additional margin.
Portable Generator for Home Use connected to Natural Gas and Manual Transfer Switch

Portable Generators for Essentials and Critical Circuits

A Standby Generator isn’t practical for all situations. For those who rent, the landlord probably isn’t willing to spend the money for a standby installation. A temporary tenant has little incentive to make the same type of investment in the property as a homeowner.

Homeowner Associations and similar organizations that govern property use may object to the installation of a standby generator. They may not have authority to block medically necessary improvements, but it’s up to the homeowner to prove need.

For some homeowners, the economic considerations may prohibit the expense or they can’t justify it.

In these cases, a portable generator might be a better choice.

When using a portable generator, keep it at least 20 feet away from the home or neighboring home and where exhaust won’t blow against the houses. Never Backfeed through an appliance outlet. Never run a generator indoors. Carbon Monoxide in the exhaust can sicken or kill anyone in the house in just minutes. Always read and follow manufacturer guidelines and safety considerations. Follow the rules for portable generator safety. Every home should have a carbon monoxide detector on each level.

The safest, easiest, best connection for portable generators is a manual transfer switch with an inlet plug. Plug the generator into the inlet, and that is the only connection you make. A manual switch allows the generator to power hardwired appliances like the furnace or well pump. It eliminates running cords from the generator indoors to the house and then to appliances throughout the house. The main difference between a manual transfer switch and an automatic transfer switch for essentials centers on automatic vs manual.

Planning a Manual Transfer Switch Installation

The next option relies on Generator Extension Cords. These heavy duty cords can carry the full output current of the generator receptacle. Need 30 amps? Choose a 30-Amp cord that matches the receptacle type. For example, a 30-amp  120-volt cord can carry the same power as two standard household outlets.

Obviously, the second option can create a trip hazard with multiple cords coming into the house.

Less obvious—cords entering through open windows, even if only open enough to admit the cord, can increase the danger of carbon monoxide poisoning. Differences in air temperature and pressure can draw exhaust into the house.

Extension cords won’t power hardwired appliances like a furnace or well pump.

Determine portable generator size requirements using the same guidelines as a standby generator for essential circuits.

A 26kW Whole House Generator Near a Large Home

Whole House Generator

The whole house generator has enough capacity to power both the conveniences and necessities, including the major and large appliances. A whole house generator system could have:
  • Enough capacity to power everything, at any time, including all air conditioners, electric heating, ranges, etc.
  • Manage the power in a way that allows everything to receive power, but not at the same time.
A whole house generator feeds the main circuit breaker panel through an automatic transfer switch. The ATS installs between the electric utility meter and the circuit breaker panel. Normal operation feeds the panel from the utility lines. During an outage, the ATS disconnects the utility first, then connects the generator. Most homeowners can choose a generator and transfer switch from 10kW to 26kW air cooled models. In most of the United States, air-cooled models provide reliable backup power. The generator engine spins a fan that blows air over the engine and alternator to keep them cool. Air cooled is the most economical to purchase, install, operate, and maintain. Their manufacturers rate them for installation down to 18 inches from the home—dependent on local codes and how the home is laid out with windows, doors, and vents. The generator and transfer switch packages ensure compatibility and provide the widest range of features, including power management. Larger homes and estates or those with multiple buildings may need a larger generator. Liquid cooled generator models start at 15kW all the way to 150kW or more. Liquid cooled generators have engines similar to vehicle engines. A pump circulates a coolant through the engine and radiator. A fan blows air through the radiator to dissipate heat. Liquid cooling is more efficient than air cooling and better suited for climates where temperatures regularly exceed 100 degrees. Liquid cooling like a car engine with a radiator, fan, and coolant pump is more efficient than fan-driven air cooling, especially in triple digit temperatures. A whole house generator has either the capacity to power everything at once, or it uses power management to limit the total load at any given time to fall within the generator’s capacity rating.
Champion Home Standby Generator Outside a Beautiful Home

Whole House Generator Load Calculation

The load calculation is a bit more complex than a generator for essentials.

  1. General Lighting and appliance.
  2. Cooking equipment.
  3. Special appliance.
  4. Major Appliance.
  5. Largest Motor.

General Lighting and Appliance Loads

The NEC requires an occupied residence to have an electrical capacity of 3 watts per square foot for general lighting and appliance loads. Multiply 3 by the number of square feet. For a 2000 square foot home, general lighting and appliance loads total 3 x 2000 = 6000 watts.

Small Appliance Loads

Small appliance circuits are those in the kitchen, dining room, pantry, and breakfast room. Two small appliance circuits must serve the rooms dedicated to cooking or eating. A third small appliance circuit serves the laundry room. Each circuit is 1500 Watts, for a minimum total of 4500 Watts.

A demand factor permitted by the NEC applies at 100 percent to the first 3000 watts, and 35 percent to the remaining load.

3000 Watts + (4500-3000) x 0.35 = 3000 + (1500 x 0.35) = 3000+525 = 3525 Watts.

Generac 24kW Generator Features

Special Appliance Loads

Many special appliance circuits don’t plug into a receptacle including dishwashers and disposals, water heaters, trash compactors, and similar loads. Microwaves and sump pumps plug into receptacles—usually on a circuit dedicated to each load.

The demand factor for special appliance loads is 0.75.

Dishwasher 1200 Watts

Disposal 960 Watts

Electric Water Heater 4500 Watts

Kitchen Microwave 1200 Watts

Sump Pump 1000 Watts

Furnace Motor 875 Watts

Total = 9735 Watts * 0.75 = 7300 Watts.

Best Natural Gas or Propane Generator for Whole House Power


Electric ranges, cook tops, and ovens have a demand factor of 0.65.

Electric Range 9600 Watts

Total = 9600 x 0.65 = 6240 Watts


If the home has electric heat, it won’t turn on the heat at the same time as the air conditioning. Drop the smaller of the two appliances from the calculation. However, add together the watts for multiple appliances, like two air conditioners.

For example, a 10,000 Watt electric heater and 5500 watt air conditioner. Only consider the electric heater. If the home has two 5500 watt air conditioners, the total is 11,000 watts. In that case, only consider the two air conditioners.

Briggs and Stratton PowerProtect Generator Features and Benefits

The NEC does not allow a demand factor for HVAC.

If a home has gas heat, then only include the air conditioner. This is a 2000-square-foot home. It’s probably a 2-ton or 3-ton air conditioner. We’ll call it a 2 Ton for the sake of example.

Total 2400 Watts

Largest Motor

Not all homes will have a large motor. One example is a well pump. Add the largest motor with a demand factor of 0.25.

2 horsepower Well Pump 3750 Watts

Total = 3750 * 0.25 = 940 Watts.

Add it all up

General Lighting and Appliance          6000 Watts

Small Appliance                                  3525 Watts

Special Appliance                                7300 Watts

Cooking                                               6240 Watts

HVAC                                                   2400 Watts

Largest Motor                                     3750 Watts

Total                                                   32,215 Watts

The decision now centers on whether to purchase a generator that manages the 32kW requirement, or a generator than supplies the entire load without power management.

Understanding Demand Factor can help with this decision.

Cummins Quiet Connect Air Cooled Home Standby Generator

Demand Factor Explained

If you have an electric range, how often do you use all four burners at the same time set on the high setting? Certainly not all day and probably almost never. Remember that the oven cycles on and off for baking to maintain temperature but stays on for broiling. Few people use the range all day long, except in a commercial environment. Therefore, the NEC allows a demand factor for certain loads.

When the electric dryer is in use, it uses 5000 Watts on the high setting from start to finish. If an electric dryer is part of the calculation, apply the full 5000 Watts without a demand factor.

It is the appliance and the way it uses electricity that determine how to include it in the load calculation. If an appliance use is rare, it makes little sense to include that specific appliance in the load calculation.

Need to Know: Portable Generator vs Standby

A disposal, for example, only gets used for a few seconds at a time. A sump pump may run often during a storm, but for less than 30 seconds each time. It makes sense that we apply a demand factor to certain loads.

In the kitchen, we accounted for two appliance circuits with a demand factor of 100 percent. The load calculation already includes anything we run from those two circuits. The waffle iron on Sunday morning, making coffee, using the blender to make a smoothie. If you exceed the power allowed by the calculation, the circuit breaker for that circuit will trip—no different from when running on utility power.

A single home has power in a darkened suburb after a severe storm

To Manage or Not to Manage

Our load calculation for the entire house shows that a generator with a capacity of at least 32.2kW would power the entire house. That’s not the only way to do it, however.

A power management system allows loads to run based on priority.

For example, the removing the electric range and water heater reduces the total load by 9614 Watts to 22,586 Watts, substantially reducing the size of the generator.

Power management can handle this in several ways. The first simply “locks out” a load to prevent it from using any power. Another way allows the load to run, but only if the generator has enough power available.

Tankless water heaters are an example of a load that uses power at the extreme. Depending on size and capacity, whole-house tankless water heaters require 10,000 to 40,000 watts. Many homeowners would choose to lock out the water heater instead of adding to the power requirement. However, the right size generator could power a tankless water heater.

Air Cooled vs Liquid Cooled Generator

The HVAC load runs on a priority and usually has first priority. When it is not running, that adds 2500 watts to the power available. When the dishwasher is not in use, and the Air Conditioner is not running, the generator might have enough remaining capacity to run the water heater or possibly the electric range.

The Right Size Generator

We can choose a generator with enough capacity to allow those appliances to run when one or two other loads are not running.

A 26kW Home Generator would have 3800 watts of spare capacity (over 15 percent margin). If the AC (2500 watts) is not running and the furnace fan (875 Watts) is off (they usually run together), and the dishwasher (1200 Watts) is not in use, the system has at least 8375 watts available. The water heater could turn on or we might have use of the electric range.

The latest technology in power management provides control over priority settings via smartphone app of any power-managed appliance. For example, lower the priority of some managed appliances to enable the range for cooking, or the water heater before taking a shower.

Power management allows a smaller and more fuel-efficient generator that costs much less to purchase, install, operate, and maintain to take the place of a much larger generator.

There are still good reasons to consider the larger, liquid-cooled generator.

  • Larger or luxury homes may have multiple air conditioners and appliances that must run at the same time.
  • Air cooled generators may struggle in climates where temperatures over 100 degrees are the norm rather than the exception.
  • Using all the conveniences and luxuries is more important than the cost.


Bottom line: Buy the generator with enough capacity for everything that needs power.

A home with backup generator power during an outage while the neighborhood is dark.