What Size Generator Do I Need? A Simple Sizing Guide

The short answer: add up the running watts of everything you want on at the same time, add the single largest starting (surge) wattage on that list, then add a 10–25% buffer. For most homes that means a few thousand watts to cover essentials like the fridge, lights, Wi-Fi and a sump pump, and a much larger system if you want central air, an electric range or the whole house. This guide walks you through it step by step, with a worksheet you can copy.

Step 1: Decide what “backed up” means for you

Before you think about watts, decide what you actually need to keep running. Most people fall into one of three groups:

• Essentials only: refrigerator, a few lights, phone and laptop charging, internet, maybe a CPAP or other medical device. This is the smallest and cheapest system.
• Essentials plus comfort: add a sump pump or well pump, a furnace blower in winter or a window air conditioner in summer, a microwave or coffee maker.
• Whole home: everything keeps working, including central air, an electric water heater, laundry and cooking. This is where permanently installed standby generators and whole-home battery systems come in.

Write your list down. Be honest about what you would run at the same time. You do not run the microwave, the well pump and the space heater all at once very often, and sizing for “everything at once” is the most common reason people overspend.

Step 2: Find the running watts and starting watts of each item

Every appliance has two numbers that matter:

• Running watts (also called rated or continuous watts): what it draws while it is working.
• Starting watts (also called surge or peak watts): the short burst a motor needs to get going. Refrigerators, pumps, air conditioners and furnace blowers can briefly pull two to three times their running watts when they start.

The best source is the data plate or sticker on the appliance itself, or its manual. If it only lists volts and amps, multiply them: watts = volts × amps. A 120-volt device drawing 6 amps uses about 720 watts.

The ranges below are rough figures seen on typical household appliances. Yours may be quite different, so treat this as a starting point and check your own labels.

Step 3: Do the math (worksheet)

1. Add up the running watts of everything on your “at the same time” list.
2. Find the one item with the biggest starting surge and add its extra starting watts (starting watts minus running watts). You only add the largest one, because motors rarely start at the exact same second.
3. Add a buffer of 10–25%. Running any power source flat-out all day is hard on it, and you will probably add something to the list later.

Example (essentials plus a sump pump):

• Refrigerator 700 W + lights 100 W + router 25 W + TV 150 W + chargers 100 W + sump pump 1,000 W = 2,075 running watts
• Largest surge: the sump pump, which might need about 2,000 extra watts to start
• 2,075 + 2,000 = 4,075 W, plus a 20% buffer ≈ 4,900 W

So this household should look for something rated around 5,000 watts of output that can handle that pump’s starting surge.

Step 4: Check energy (how long), not just power (how much)

Watts tell you how much you can run at once. If you are looking at a battery system such as a portable power station or a home battery backup system, you also need to know how long it will last. That is measured in watt-hours (Wh) or kilowatt-hours (kWh).

Watt-hours = watts × hours. A 100-watt load running for 10 hours uses 1,000 Wh (1 kWh). A typical modern refrigerator uses very roughly 1–2 kWh per day because its compressor cycles on and off. Add up the daily use of your essentials and compare it to the battery’s usable capacity. Remember that real-world run time is lower than the math suggests because of inverter losses, temperature and battery age, so plan for some margin.

For fuel generators, run time depends on the tank size and how hard the generator is working. Manufacturers usually publish run time at a quarter or half load; check those figures for any model you consider.

Step 5: Match your number to a type of backup power

Once you know your wattage and how many hours you need, the right category usually becomes obvious:

• Under about 3,000 W, indoor use, quiet: a portable power station works well for essentials, apartments and medical devices. No fuel, no fumes. Browse options for condos and apartments.
• Multi-day outages without fuel: a solar generator (a power station paired with solar panels) can recharge during the day.
• Heavy loads like well pumps or a few big circuits: an inverter or dual-fuel generator gives you a lot of watts for the money, connected through a transfer switch or interlock kit.
• Whole home, automatic: a permanently installed standby generator or a whole-home battery system, sized by an electrician with a proper load calculation.

Not sure which way to go? Our guide to standby vs portable vs solar generators compares them side by side.

Common sizing mistakes to avoid

• Ignoring starting watts. A unit that runs your fridge fine can still trip when the fridge and the sump pump try to start together.
• Adding every surge. You only add the largest one. Adding them all makes you buy far more than you need.
• Forgetting 240-volt loads. Well pumps, central air and electric water heaters often run on 240 volts. Make sure the unit and your transfer setup support 240 V if you need them.
• Using extension cords for the whole house. Cords are fine for a few appliances. For circuits in your panel you need a transfer switch or interlock installed by an electrician. Never backfeed a home through a dryer outlet.
• Sizing for the rare day. If the big load is something you can live without for a day or two, leave it off the list and save money.

Whole-house sizing: let an electrician do the load calculation

If you want the whole home covered, a quick list is not enough. An electrician will look at your panel, service size (for example 100, 150 or 200 amps), the big motor loads and how your family actually uses power, then size the system using a formal load calculation. They can also tell you whether a load-management module, which sheds big loads like the AC for a moment while others start, would let you use a smaller unit. If you are budgeting for that route, our guide on how much a whole-house generator costs breaks down the parts of the price.

FAQ

What size generator do I need to run a house?

For essentials only (fridge, lights, internet, a few outlets), many homes need roughly 3,000–5,000 watts. Adding a well pump, sump pump or window AC pushes that higher. Running the whole house, including central air and electric appliances, usually means a permanently installed system sized by an electrician. Use the worksheet above with your own appliance labels to get your number.

How many watts does it take to run a refrigerator?

Most refrigerators draw somewhere between 100 and 800 running watts and need two to three times that for a moment when the compressor starts. Check the sticker inside the door or on the back for your model.

Is it better to oversize a generator?

A buffer of 10–25% above your calculated need is sensible. Going far beyond that costs more up front and, with fuel generators, can mean burning more fuel than you need for light loads.

Do I need a transfer switch?

If you want to power circuits in your electrical panel rather than plugging appliances into cords, yes. A transfer switch or interlock kit keeps power from flowing back into utility lines, which protects line workers and your equipment. It must be installed by a licensed electrician.

Can a portable power station run a whole house?

Not usually. Larger power stations can run essentials and some can connect to a home panel through a transfer switch, but whole-home loads like central air and electric water heaters normally need a whole-home battery system or a standby generator.

Have your list of appliances and want a second opinion on sizing? Email us and we will help you work through the numbers.

Sources and further reading

• Virginia Cooperative Extension: estimating appliance and electronics energy use (how to read wattage labels and turn watts into watt-hours).
• Ready.gov: power outage preparedness (generator safety and keeping food and medical devices going).
• NFPA: NFPA 70, the National Electrical Code (the code your electrician follows for transfer switches and panel connections).

This guide is general information, not electrical advice. Anything connected to your home’s wiring should be installed by a licensed electrician and checked against local code.