What size generator do I need for my house?

• You should buy a generator that can sufficiently cover your energy needs during power outages

• Calculate your energy needs by adding up the running wattage of your appliances and systems and adding a 20% buffer

• The size generator needed to run a home usually ranges from 5 kW to 20 kW, depending on the purpose and home size

Choosing the right size generator for your home means finding the sweet spot between not enough capacity and more capacity than you need. You don’t want to be left with inadequate power, but you also don’t want to overspend.

We created this guide to help you figure out how big of a generator you need to buy to get a reliable source of backup power. You’ll learn how to estimate your home’s energy needs, what size generators are recommended for different types of homes, and what else you should consider before buying a whole home generator.

Why generator size matters for homeowners

Capacity is the first and most important consideration when you select a generator for your home. If your whole house generator is undersized, it won’t supply sufficient power to run your essential appliances during outages. Too large and you’ll end up paying for capacity you can’t use and perhaps a less efficient setup.

However, when you get the right size standby generator for your home, you get a reliable source of backup power that can provide security and comfort during emergencies. Finding the ideal capacity generator for your home ensures you get the maximum value for your investment.

Calculate what size generator you need for a home

To figure out the right size generator for your house, you’ll need to know how much power it will need to supply. This involves taking an inventory of your home’s energy usage and considering a few other factors.

1. List essential appliances and devices

Start by listing every device you will want to use during a power outage, beginning with the essentials. This list typically includes:

• Refrigerator and freezer

• Lights

• Medical equipment

• Furnace or heat pump

• Central air conditioner or window unit

• Water heater

• Sump pump

• Well pump

• Wi-Fi router

• Home security system

• Electric range and oven

• Microwave

• Home security system

• Chargers for phones, tablets, and laptops

• Home computer

You may see yourself being conservative about your energy usage when utilities are down, but it’s still a good idea to consider maximum usage. This will allow you to consider price differences between a comprehensive whole-home backup generator and one that supplies only partial backup power. 

2. Calculate total wattage requirements

Once you have your list of appliances, the next step is to calculate the total energy usage of those appliances. You can find average wattages for standard appliances online. We’ve listed some sample figures in the table below. 

3. Consider surge vs. running power needs

You’ll notice that there are two different figures for each appliance listed in the table above. The first is running watts, which measures the sustained power draw an appliance uses after startup. The second is surge, or starting, watts. This measures the initial burst of power some appliances draw when they start up.

Both figures are important. Your generator must be able to handle the initial surge of heavy-duty appliances like AC units, or they could fail to start. However, you should also consider that the majority of your generator usage will only need to cover running watts. In reality, you’ll probably power up some but not all appliances and systems at once when you switch to backup power. This is why a 20% buffer is enough to cover starting surges.

4. Calculate generator size needs

To calculate the home generator size you need, add up the total running watts of your essential appliances, add a buffer of at least 20% to cover starting surges (or multiply by 1.2), and divide the answer by 1,000 to convert watts to kilowatts.

Written out, this formula looks like:

Minimum Generator Size (kW) = (Total Running Watts x 1.2) / 1,000

Using our example home, the total running wattage of all appliances listed in the table adds up to 10,815 watts. Multiplying by 1.2 produces 12,978 W. To find kW, divide by 1,000, which produces 13 kW rounded. Therefore, the minimum generator capacity we should shop for in this example is 13 kW. You could also get a 14 kW or 15 kW system, but beware of over-sizing your generator.

Generator size recommendations by home size

While it’s a good idea to get a precise estimate before you purchase a generator, we thought it would be helpful to provide some general size categories. In this section, we take a look at a few popular generator models for different sizes of homes.

Tiny home generator size

For a tiny home or small apartment, a generator with a capacity of 2,000 to 4,000 watts (2–4 kW) can usually cover basic needs in a power outage. This size is ideal for running essentials like a compact refrigerator, lights, phone chargers, a laptop, and possibly a small fan or space heater, but not larger appliances. 

Small home generator size

For a small home or apartment, a generator with a capacity of 3,000 to 7,000 watts (3 to 7 kW) is usually enough to power essential items during an outage. This size can typically run a refrigerator, some lights, a sump pump, and small electronics, but won’t support high-demand appliances like central air conditioning or electric water heaters.

Medium home generator size

Medium homes with two to three bedrooms and more appliances typically need between 7,000 to 15,000 watts (7 to 15 kW) of capacity. Most standby generators in this range support features like an automatic transfer switch, smart home integration, and whole-home backup.

Large home generator size

Large homes and luxury residences with heavy-usage appliances like dual HVAC systems tend to need generators with capacities between 15,000 and 22,000 watts (15 to 22 kW). The best solutions for these homes are large natural gas or propane generators with remote monitoring support.

Quick reference table for generator sizing

In the table below, we list the standard energy usage of these different home size categories along with a few popular generator options for each one. 

Other factors that affect generator sizing

There are some other factors that may affect the size generator you need for your house. These include:

• Climate: Homes in areas that experience extreme temperatures may require heavier energy use to cool or heat them sufficiently

• Number of people: Each additional person in your household adds to your energy usage with the devices and appliances they use

• Backup goals: Whether you want to only back up your most essential appliances or supply backup power for your entire home affects your capacity needs

• Electric vehicle: If you need to be able to charge an EV, you’ll need to include those power needs in your energy usage calculations

• Expansion plans: If you are considering expanding your floor plan in the near future, you should invest in a generator that takes those additional needs into consideration

Other generator considerations

Capacity is the first and most important aspect of your generator to consider, but it isn’t the only one. There are other things that should factor into your decision, including:

• Fuel type: Consider which type of fuel you want to use (natural gas, propane, or diesel) based on efficiency and existing utility lines

• Features: Some generators have advanced safety and performance features like load management and monitoring technology that you may want

• Additional equipment: You’ll also need compatible equipment like a transfer switch to connect your generator to your home electrical system

• Storage: When your generator isn’t in use, you will need a safe, dry place to store it. Consider your home’s storage capacity

Bottom line on sizing a home generator

Determining the ideal size generator for your home takes a little work, but the time and effort you spend will pay off. Once you zero in on the right capacity, you can ensure you get the backup support you need without paying for capacity you don’t. In other words, a generator is a major investment that can provide significant value for you and your home. It’s worth doing your homework.

FAQs on home generator capacity

Below are a few frequently asked questions about sizing a generator for home backup power.

What size generator will run a house?

The size generator you need to run a house depends on the size of the house and the energy usage. In general, you will need at least a 3 kW generator for the smallest of tiny homes and up to a 22 kW generator for large homes.

How long will 5 gallons of gas last in a generator?

Five gallons of gas will last between six and 16 hours in a generator, depending on energy usage. The higher the energy usage, the less time the fuel will last.

Will a 5,000-watt generator run a whole house?

A 5,000-watt generator can potentially power an entire small home or apartment. However, this depends on how much energy the home uses on average.

Can I run my whole house on a 12,000-watt generator?

You can run most small and medium homes on a 12,000-watt (12 kW) generator. This size generator should be more than adequate for most homes up to three bedrooms, depending on energy usage.