Wondering what size generator you need for your house? Our free generator size calculator works it out in seconds: check off the appliances you want to run during a power outage and the tool adds up their running and starting watts, applies a safe 25 percent headroom, and recommends the right generator size in kilowatts. Undersized generators trip breakers and struggle to start motors; oversized ones waste fuel and money. Whether you are planning for storms, camping trips, or whole-house backup, this calculator helps you buy once and buy right — no electrician math required, just an honest list of what your home actually needs to keep running.
How to Use the Generator Size Calculator
- Check the appliances you must run during an outage. Be honest: a refrigerator and lights are essentials; the clothes dryer usually is not.
- Click “Calculate Generator Size.” The calculator totals the running watts and starting (surge) watts of everything you selected.
- Read your recommendation. You will see a generator size in kilowatts that covers the larger of your surge load or your running load plus 25 percent headroom.
- Adjust and recalculate. Drop a big load like the electric dryer and watch the recommended size fall — load management is the cheapest way to afford a smaller generator.
Running Watts vs Starting Watts: The Key Idea
Every appliance has two power numbers. Running watts are what it draws continuously once it is on — a refrigerator sips about 150 watts while running. Starting watts (also called surge watts) are the brief spike a motor draws when it kicks on — that same refrigerator can pull 1,200 watts for a second or two at startup. Any appliance with a motor — fridge, freezer, sump pump, well pump, air conditioner, washing machine — has a surge, typically two to three times its running watts. Lights, TVs, phone chargers, and heating elements do not; their starting watts equal their running watts. Your generator must handle the total running load continuously AND absorb the surges when motors start. That is why a proper generator size calculator always compares both numbers and sizes for the worst moment, not the average one.
To find the numbers on your own appliances, check the nameplate sticker or the owner’s manual; if it lists amps and volts instead, multiply them (amps × volts = watts). Our checklist uses realistic typical values, so it works well even if you do not have the manuals handy.
The Sizing Formula This Calculator Uses
The calculator is deliberately conservative. It adds up the running watts of everything you selected, multiplies that total by 1.25 to build in 25 percent headroom, then compares the result against the total starting watts of your selected appliances — because two motors restarting within the same moment is exactly what happens when power flickers back. It recommends the smallest standard generator size (3.5, 5, 7.5, 10, 12.5, 15, 20, or 26 kW) that covers the larger of those two numbers.
Electricians often use a slightly different worksheet method: total running watts plus only the single largest starting surge, then add a buffer. That method can point to a smaller unit. Our calculator’s approach costs a little more headroom, but it protects you against the unlucky moment when the fridge and the sump pump restart together — and running a generator at 70 to 80 percent of its rating instead of flat-out also keeps it quieter, cooler, and longer-lived.
Typical Appliance Wattage Chart
These are the same typical values built into the calculator above, so the chart and the tool always agree. Your own appliance may differ — the nameplate always wins.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 150 | 1,200 |
| Freezer | 200 | 1,200 |
| Sump pump | 1,000 | 2,000 |
| Well pump | 1,000 | 3,000 |
| Furnace fan (gas heat) | 800 | 2,350 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Central AC (3 ton) | 3,500 | 10,500 |
| Microwave | 1,000 | 1,000 |
| Coffee maker | 1,000 | 1,000 |
| Toaster | 1,200 | 1,200 |
| LED TV | 150 | 150 |
| Computer and monitor | 300 | 300 |
| Lights (one room) | 300 | 300 |
| Washing machine | 800 | 2,400 |
| Clothes dryer (electric) | 5,000 | 5,000 |
| Water heater (electric) | 4,500 | 4,500 |
| Space heater | 1,500 | 1,500 |
| Phone chargers | 50 | 50 |
Worked Examples
Example 1: Essentials only
Refrigerator (150 W run), lights for one room (300 W), phone chargers (50 W) and a TV (150 W) total just 650 running watts, with 1,700 watts of combined starting demand. With headroom, the calculator recommends a 3.5 kW portable or inverter generator — perfect for keeping food cold and staying informed through a storm.
Example 2: Typical family home
Add a freezer (200 W), sump pump (1,000 W), furnace fan (800 W), microwave (1,000 W), and coffee maker (1,000 W) to the essentials. Running watts reach 4,650 W, and combined starting demand climbs to 9,250 W. The calculator recommends a 10 kW unit, which keeps the house livable without babysitting loads.
Example 3: Central air and electric water heating
Take central air conditioning (3,500 W running, 10,500 W starting), an electric water heater (4,500 W), a washing machine (800 W), a refrigerator (150 W), and lights (300 W). The running total is 9,250 W, but the combined starting demand is 18,900 W — and surges decide. The recommendation is a 20 kW unit, solidly in standby-generator territory. Drop the water heater or the AC from the list and watch the recommended size fall instantly, which is why sequencing what runs matters so much.
Example 4: How large a generator to run a house with well and sump pumps
A well pump (1,000 W run, 3,000 W surge) and sump pump (1,000 W run) on top of a fridge, lights, TV, phone chargers, and a microwave give 3,650 running watts and 7,700 starting watts. The calculator recommends 10 kW. Those two hard-starting motors alone force the answer up a full size class — exactly the question this calculator is built to answer honestly.
Types of Generators
- Portable generators (3–10 kW): gasoline or dual-fuel, wheeled, stored in a garage. Run them outdoors only, and connect with heavy-duty cords or a transfer switch.
- Inverter generators (2–7 kW): quieter, cleaner power for electronics, great for camping and light home backup. Can often be paralleled in pairs.
- Standby generators (10–26 kW): permanently installed, start automatically, run on natural gas or propane. The true whole-house option, at whole-house prices.
Sizing Tips That Save Money
- Keep 20–25 percent headroom. Running a generator at full tilt constantly shortens its life and wastes fuel. Our calculator builds this in.
- Stagger motor starts. Do not start the AC, well pump, and sump pump in the same second — let each settle before switching on the next.
- Cut the biggest loads first. Electric dryers, ovens, and water heaters eat kilowatts. Skipping one often drops you a whole size class.
- Mind the propane derating. Dual-fuel generators make roughly 10 percent fewer watts on propane than on gasoline. If propane is your main fuel, size against the propane rating.
- Account for altitude. Small engines lose about 3 percent of output for every 1,000 feet of elevation. At 5,000 feet you have already lost roughly 15 percent before plugging anything in.
- Consider a soft starter for your AC. An inexpensive soft-start kit can cut a central AC compressor’s starting surge dramatically, sometimes letting a smaller generator handle it.
- Never run a generator indoors or in a garage. Carbon monoxide kills quickly and silently — outdoors, at least 20 feet from windows, every time.
Portable vs Standby: Which Fits Your Home?
Portable generators cost far less upfront, but you must wheel one outside, fuel it, start it manually, and run cords or flip a transfer switch in the dark. Standby generators cost more and need professional installation, yet they start automatically within seconds of an outage and run on natural gas or propane. The break-even question is outage frequency: several outages a year — or medical equipment, sump pumps, or a home office that cannot go dark — justifies standby. For rare, short outages, a correctly sized portable is the smarter buy.
Fuel Choices Explained
- Gasoline: cheapest upfront and available everywhere, but it goes stale in months and stations may be closed during the emergency you bought the generator for.
- Propane: stores indefinitely and burns cleaner; dual-fuel models switch fuels with a dial. Remember the roughly 10 percent output drop and size for it.
- Natural gas: standby-only, but effectively unlimited — no refueling ever, as long as the gas utility holds up.
- Diesel: common in large standby and commercial units; fuel-efficient and long-lived, but noisier and pricier.
Whatever fuel you choose, keep at least 24 hours of supply for your calculated load.
Related Calculators
Frequently Asked Questions
What size generator do I need to run a whole house?
Most whole-house setups need between 10 kW and 20 kW. A typical home running essentials plus central air conditioning lands around 15 to 20 kW. Use the calculator above with your exact appliance list for a precise figure — homes with electric dryers and water heaters need more than homes with gas appliances.
How many watts does a house use during an outage?
Essentials alone (refrigerator, lights, phone chargers) use under 1,000 running watts. A comfortable load with pumps, furnace fan, and kitchen appliances runs 4,000 to 6,000 watts. Nearly everything at once can exceed 13,000 watts. The combined surge when motors start matters as much as the running total.
Can a 7.5 kW generator run central air?
Usually not, unless your AC is small and you manage other loads carefully. A 3-ton central AC starts at around 10,500 surge watts by itself, which overwhelms a 7.5 kW unit. A soft-start kit on the AC can cut that surge significantly and make smaller generators viable.
What is the difference between running and starting watts?
Running watts are the continuous power an appliance draws while operating. Starting watts are the brief higher surge a motor needs to start spinning — often two to three times the running watts for a second or two. Your generator must satisfy both numbers.
Is a bigger generator always better?
No. Oversized generators cost more, burn more fuel at light loads, and can develop carbon buildup from running unloaded (“wet stacking” in diesels). Size for your real loads plus about 25 percent headroom, not for your worst imaginable day.
Do I need a transfer switch?
For connecting a portable generator to your home’s circuits, yes — a transfer switch or interlock kit installed by an electrician is the safe, code-compliant way. Never plug a generator into a wall outlet (“backfeeding”); it can electrocute line workers. Running appliances on extension cords is fine without one.
How large a generator do I need to run a house with a well pump?
Well pumps start hard: a typical 1 HP pump draws about 1,000 running watts but surges to 3,000 watts. With typical home loads on top, most well-pump homes land in the 7.5 to 12.5 kW range. Check the well pump box in the calculator above to see your exact total.
How do I convert amps to watts?
Multiply amps by volts: watts = amps × volts. On a standard 120-volt outlet, a 10-amp device uses 1,200 watts. On a 240-volt circuit, that same 10 amps is 2,400 watts. Check your appliance’s nameplate for its amp rating, multiply by the outlet voltage, and enter the result into the calculator.
What happens if my generator is too small?
An undersized generator will trip its circuit breaker when the load peaks, overheat the engine, deliver unstable voltage, or stall when a motor tries to start. The voltage sag alone can damage sensitive electronics like TVs, computers, and furnace controls. Size for both your running total and your surge, not just one.