Generator Size Calculator
A generator size calculator adds running watts to the largest starting surge, then × 1.2: 4,000 W running plus a 2,000 W surge calls for a 7.2 kW generator.
What size generator do I need?
Add the running watts of everything you want on at the same time, plus the largest starting surge, then multiply by 1.2. For 4,000 W running and a 2,000 W surge, that gives a 7,200 W (7.2 kW) generator.
This generator size calculator finds peak demand by adding your total running watts to the single largest starting surge, then adds 20% headroom: recommended watts = (running watts + largest surge) × 1.2. With 4,000 W running and a 2,000 W surge, peak demand is 6,000 W and the recommended generator is 7,200 W, or 7.2 kW.
Enter the total running watts of everything that will be on at once and the largest single starting surge. The result shows the recommended size in kW, the peak demand, and the watts with headroom.
How the formula works
Think of the generator as a water tap and your appliances as glasses being filled. Running watts are the glasses filling steadily. A motor starting is someone dumping in an extra splash for a second. The tap must handle the steady flow plus the biggest splash.
- Running total = sum of running watts for everything on at once.
- Peak demand = running total + largest starting surge.
- Recommended size = peak demand × 1.2.
- kW = watts ÷ 1,000.
Worked examples
- Default: 4,000 W running, 2,000 W largest surge
- Peak: 4,000 + 2,000 = 6,000 W
- With headroom: 6,000 × 1.2 = 7,200 W
- Size: 7.20 kW
- Essentials during an outage
Example nameplate figures; read your own labels.
| Load | Running watts | Extra starting watts |
|---|---|---|
| Refrigerator | 700 | 1,000 |
| Sump pump | 800 | 1,300 |
| Furnace blower | 700 | 800 |
| Lights and phone chargers | 300 | 0 |
| Total | 2,500 | Largest: 1,300 |
- Peak: 2,500 + 1,300 = 3,800 W
- With headroom: 3,800 × 1.2 = 4,560 W
- Size: 4.56 kW
How to find the watts
- Nameplate: most appliances list watts, or volts and amps, on a label.
- Convert amps: watts = volts × amps. A 120 V device drawing 6 A uses 120 × 6 = 720 W. The watts to amps calculator works either way.
- 240 V loads: well pumps, ranges, and central AC often run on 240 V. A 240 V, 10 A load is 2,400 W.
- Motor surge: look for “starting watts,” “LRA” (locked-rotor amps), or ask the maker.
Generator sizes and what they suit
| Recommended size | Typical use |
|---|---|
| Under 3 kW | Lights, fridge, phone charging |
| 3–6 kW | Essentials: fridge, sump pump, furnace fan, some outlets |
| 6–10 kW | Essentials plus a well pump or window AC |
| 10 kW and up | Larger portions of a house, often a standby unit |
These are broad bands. Your own load list decides the size.
Common mistakes
- Counting things that will not run together. You probably will not run the dryer and the oven at the same time during an outage. Plan which loads share time.
- Forgetting 240 V loads. A well pump can be the largest surge in the house.
- Buying by peak watts on the box. Many generators advertise starting or peak watts. Match the calculator result to rated running watts.
- Backfeeding through an outlet. It can energize utility lines and endanger line workers. Use a transfer switch or interlock installed by an electrician.
Related calculators
For a standby unit, the whole house generator cost calculator prices the install. To estimate running cost, see the electricity cost calculator. Considering solar backup instead? Start with the solar panel calculator.
Generator hookups are electrical work governed by code. Have a licensed electrician install the transfer switch or interlock and confirm circuit and wire sizing.
Frequently asked questions
How do I size a backup generator?
List everything you want running at the same time and add their running watts. Add the single biggest starting surge on top for peak demand, then multiply by 1.2 for 20% headroom. 4,000 W running plus a 2,000 W surge is 6,000 W peak, and 6,000 × 1.2 = 7,200 W, so look for at least 7.2 kW of rated running output.
Why do you add only the largest starting surge?
Motors pull a short burst of extra power as they start, but rarely all in the same second. Adding every surge would oversize the generator. Adding only the biggest one covers the worst likely moment: everything running, and the largest motor kicking on.
Should I enter the full starting watts or just the extra?
Enter the extra. Many appliance charts list additional starting watts, the surge above running watts, and that is what belongs in the surge box. If a label gives total starting watts, subtract that appliance's running watts first, because they are already in your running total. Entering the full figure double-counts and slightly oversizes the result.
What does the 20% headroom factor do?
It keeps the generator from running at its limit. A unit working near 100% of its rating for hours runs hot, uses more fuel, and leaves no margin for a load you forgot. Multiplying by 1.2 means a 6,000 W peak becomes a 7,200 W target.
What is the difference between running watts and starting watts?
Running watts is the steady draw while a device is on. Starting watts is the brief spike as a motor gets moving, in things like refrigerators, well pumps, sump pumps, and air conditioners. Heaters, toasters, and light bulbs have no motor, so their starting and running watts are the same.
What size generator do I need to run my house?
Add the running watts of the loads you want on at once, then add the largest starting surge. Multiply the total by 1.2 for headroom, so 4,000 W running plus a 2,000 W surge gives a 7,200 W generator. Most homes plan only essentials for an outage, since running the whole house takes a much larger unit.
Estimates only. Verify quantities with your supplier before purchasing.