What will the generator power at the same time?
Set how many of each. Figures are from generator wattage charts, so edit them to match your labels.
More appliances, including AC, heat pump, laundry and water heater
Quick answer. For a fridge, lights, a router and phone charging, plan on a generator with about 1,000 running watts and 3,000 starting watts. Add a furnace fan and you need about 2,000 running and 4,000 starting watts. Essentials with a sump pump, furnace and microwave need about 4,800 running and 6,300 starting watts, and central air pushes the starting need near 9,600 watts. These figures come from this calculator using Lowe's chart values, so check your own labels.
What size generator do I need for common setups?
We ran these setups through the calculator above with its default chart figures and 20 percent headroom on running watts. Your appliances may use less or more, so treat them as a starting point.
| What you want to run | Running watts | Running watts with 20% headroom | Starting watts needed | Biggest motor start |
|---|---|---|---|---|
| Fridge only | 700 | 840 | 2,900 | Fridge |
| Fridge, LED lights, router, two phones | 870 | 1,044 | 3,070 | Fridge |
| Well pump (1/2 HP) only | 1,000 | 1,200 | 3,100 | Well pump |
| Furnace fan, fridge, LED lights, router | 1,695 | 2,034 | 3,995 | Furnace fan |
| Calculator defaults (fridge, sump pump, furnace fan, microwave, TV, lights, router, phones, laptop) | 3,965 | 4,758 | 6,265 | Furnace fan |
| Defaults plus a 1/2 HP well pump | 4,965 | 5,958 | 7,265 | Furnace fan |
| Central AC (24,000 BTU), fridge, LED lights, router | 4,620 | 5,544 | 9,570 | Central AC |
Pick a generator whose running rating covers the headroom column and whose starting or surge rating covers the starting column. Heating is a common reason to size up, and our guide to running a gas furnace on a generator covers blower watts and safe hookup. On a private well, check Franklin Electric's minimum sizes in our guide to what size generator a well pump needs.
Generator wattage chart used by the calculator
These are the default figures behind each row above. Extra starting watts is the brief surge on top of running watts when a motor starts.
| Appliance | Running watts | Extra starting watts | Source |
|---|---|---|---|
| Refrigerator or freezer | 700 | 2,200 | Lowe's chart |
| Sump pump (1/2 HP) | 1,050 | 2,150 | Lowe's chart |
| Well pump (1/2 HP) | 1,000 | 2,100 | Lowe's chart |
| Furnace fan blower (1/2 HP) | 875 | 2,300 | Lowe's chart |
| Window AC (12,000 BTU) | 3,250 | 3,950 | Lowe's chart |
| Central AC (24,000 BTU) | 3,800 | 4,950 | Lowe's chart |
| Heat pump | 4,700 | 4,500 | Lowe's chart |
| Washing machine | 1,150 | 2,250 | Greengear chart |
| Clothes dryer (electric) | 5,400 | 6,750 | Greengear chart |
| Water heater (electric, 40 gal) | 4,500 | 0 | Virginia Cooperative Extension |
| Microwave (1,000 W) | 1,000 | 0 | Lowe's chart |
| Coffee maker | 1,000 | 0 | Greengear chart |
| Space heater | 1,800 | 0 | Lowe's chart |
| TV (flat screen) | 120 | 0 | Virginia Cooperative Extension |
| LED lights (10 bulbs) | 100 | 0 | Virginia Cooperative Extension |
| Wi-Fi router and modem | 20 | 0 | Our estimate |
| Phone charger | 25 | 0 | Lowe's chart |
| Laptop | 50 | 0 | Virginia Cooperative Extension |
How the calculator works
- It adds the running watts of every appliance you set above 0.
- It finds the single appliance with the largest extra starting watts and adds that once. Running total plus that figure is the starting watts your generator needs.
- It adds your headroom (20 percent by default) to the running total and rounds up to a common generator size.
This is the same worksheet method printed on Lowe's and Generac generator charts. For example, a fridge at 700 W, a sump pump at 1,050 W, a belt sander at 1,200 W and an air compressor at 1,000 W add up to 3,950 running watts. The largest extra start is 2,400 W, which brings the starting total to 6,350 watts. This calculator gives the same figures for those inputs.
Where the figures come from
Running and extra starting watts for fridges, pumps, furnace fans, AC and the microwave come from Lowe's portable generator wattage chart. Washer and dryer figures come from Greengear's sizing chart, and electronics from Virginia Cooperative Extension. Chart figures are deliberately conservative, and modern appliances often use less, so check labels where you can. For an appliance not in the list, look it up in our appliance wattage chart. Planning backup with batteries instead? Try the power station runtime calculator.
Frequently asked questions
How do I work out what size generator I need?
Add up the running watts of everything you want on at the same time, then add the single largest extra starting watts figure (usually a fridge, pump or AC motor). That total is the minimum starting watts, and the running total is the minimum running watts. This is the method generator wattage charts from Lowe's and Generac use.
What size generator do I need to run a house?
For essentials (fridge, sump pump, furnace fan, lights, router and a microwave) the defaults in this calculator come to about 4,000 running watts and roughly 6,300 starting watts, which suits a 5,500 W class portable or larger. Central air conditioning, electric water heaters and dryers push you towards a whole home standby generator, often 14 kW to 22 kW or more. An electrician should do a load calculation before you buy a standby unit.
What is the difference between running watts and starting watts?
Running watts are what an appliance draws continuously. Starting watts, also called surge watts, are the brief extra burst a motor needs to start, usually for a few seconds. Generators list both ratings, and you need enough of each.
Why add only the largest starting load and not all of them?
Motors rarely start at exactly the same moment, so wattage charts assume intermittent starting and add only the single biggest extra starting load. If you have several large motors that could start together, such as two AC units, add their extra starting watts yourself or stagger them.
Should I buy a bigger generator than the result?
Yes, a margin helps. Running a generator near its limit for hours is hard on it and leaves no room for extra loads. This tool adds 20 percent headroom to running watts by default before suggesting a common size, and you can change it.
What size generator do I need for a refrigerator?
Using Lowe's chart figures of 700 running watts and 2,200 extra starting watts, a fridge on its own needs about 840 running watts with 20 percent headroom and 2,900 starting watts. Modern ENERGY STAR fridges often use far less, and Honda lists one at 132 to 192 running watts and 1,200 starting watts, so check your label.
What size generator do I need for a furnace and fridge?
With Lowe's figures for a 1/2 HP furnace fan and a fridge, plus LED lights and a router, the calculator gives 1,695 running watts and 3,995 starting watts, which points to about a 3,000 W class generator with at least 4,000 starting watts. Our guide to running a gas furnace on a generator covers blower sizes and safe connection.
What size generator do I need for a well pump?
Lowe's chart lists a 1/2 HP well pump at 1,000 running watts and 2,100 extra starting watts, so the pump alone needs about 3,100 starting watts. Many well pumps run on 240 V, which means the generator needs a 240 V outlet and a transfer switch or interlock to reach the pump circuit.
Will a 5,000 watt generator run my house?
It can run essentials but not everything. The calculator defaults (fridge, sump pump, furnace fan, microwave, TV, lights, router, phones and a laptop) total 3,965 running watts and 6,265 starting watts, so check that any 5,000 W model's starting or surge rating reaches that figure before you buy. Add central air and the starting need passes 9,500 watts.
What is a generator wattage calculator?
It is a worksheet that adds the running watts of the appliances you will use at the same time, adds the largest motor starting surge, and compares the result with generator ratings. This page does that automatically, using figures from published generator wattage charts that you can edit.
Can I connect a generator to my house wiring?
Only through a transfer switch or an approved interlock kit installed by a licensed electrician. Plugging a generator into a wall outlet (backfeeding) can kill utility workers and neighbours. Always run a fuel generator outdoors, far from windows and doors, because of carbon monoxide.
Sources
- Lowe's, Portable generator wattage chart
- Honda Power Equipment, Generator wattage estimation guide
- Greengear, Generator sizing charts
- Virginia Cooperative Extension, Estimating Appliance and Home Electronic Energy Use (BSE-301)
- US CPSC, Carbon Monoxide Information Center
Figures are typical values for planning, not a substitute for your appliance labels or an electrician's load calculation.
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