Home Generator Size Calculator
Generator kW class from appliance running watts and motor surge.
What it does
Generator sizing fails at motor start, not at steady run. A fridge sipping 700 W demands about 2,100 W for the seconds its compressor kicks, and a sump pump wants 2,400. Add only running watts and the first simultaneous start trips the breaker; add every surge and you buy twice the machine.
The tool above follows the worksheet method. Enter running watts per appliance, and it totals the run load, adds only the single largest motor surge, and applies 25 % headroom to a standard kW class. Essentials at 2,550 W running peak at 4,150 W and land in the 5.5 kW portable class.
Extension sources back the inputs. Mississippi State states the 3x motor start rule with a sum-the-loads method, and Virginia Tech tables the typical watts behind each default: fridge 725, furnace fan 750, well pumps to 1,100, water heaters to 5,500. Watts equal volts times amps throughout.
Safety decides placement before purchase. Ready.gov and FEMA require outdoors operation 20 ft or more from openings, CO detectors on every level, heavy-duty cords, and a transfer switch by a qualified electrician. Backfeeding a panel endangers line crews and stays illegal everywhere.
How to use
- Enter running watts per appliance, 0 for absent ones.
- Keep the big motors honest: fridge, sump, furnace, well, AC.
- Read running watts first, then starting peak.
- Read the kW class with 25 % headroom built in.
- Stagger motor starts on outage day.
- Run outdoors with a transfer switch, never backfeed.
Formula
- watts
- Steady watts per appliance, 0 to 20,000 each.
- surge
- About 3x running watts for each motor load.
- 1.25
- Headroom factor for real world starts.
The 3x motor rule comes from Mississippi State and Virginia Tech Extension. Only the largest surge adds, per the Generac worksheet method.
Worked example
Example: essentials at 2,550 W running
Defaults: essential loads without well pump or central AC.
- Inputs: fridge 700, sump 800, furnace 750, lights 300 W; no well or AC.
- Running: 700 + 800 + 750 + 300 = 2,550 W.
- Largest motor: sump 800 x 3 = 2,400 W starting.
- Peak: 2,550 minus 800 plus 2,400 = 4,150 W.
- Headroom: 4,150 x 1.25 = 5.19 kW. Tool shows 2,550 W, 4,150 W, 5.2 kW: buy 5.5 kW class.
Reference table
| Appliance | Running | Starting |
|---|---|---|
| Refrigerator | 700 | 2,100 |
| Sump pump 1/3 HP | 800 | 2,400 |
| Furnace fan | 750 | 2,250 |
| Well pump 1/2 HP | 1,000 | 3,000 |
| Central AC 3-ton | 3,500 | 10,500 |
| Microwave | 1,000 | 1,000 |
| Lights + TV | 300 | 300 |
Limits
- Planning math only. Nameplate watts beat defaults.
- Single largest surge assumed; simultaneous starts differ.
- Ignores fuel type, altitude derate, and power quality.
- Transfer and code work need an electrician.
Common mistakes
- Sizing on running watts alone.
- Adding every surge instead of the largest.
- Forgetting the well pump on rural panels.
- Running indoors or near windows.
- Backfeeding without a transfer switch.
Try it yourself
Quick checks. Answers sit tucked under each question.
Add a 1,000 W well pump?
Running 3,550 W. Largest surge well 3,000. Peak 3,550 minus 1,000 plus 3,000 = 5,550 W. 6.9 kW: buy 7.5 kW.
Add a 3,500 W central AC too?
Running 7,050 W. Largest surge AC 10,500. Peak 7,050 minus 3,500 plus 10,500 = 14,050 W. 17.6 kW: buy 18 kW.
Questions
How do I total running watts?
Add running watts of everything running at once. Here 700 + 800 + 750 + 300 = 2,550 W.
What surge do motors need?
About 3 times running watts for motors. A 800 W sump needs 2,400 W to start.
How is starting peak figured?
Running total minus the largest motor plus its surge. Here 2,550 minus 800 plus 2,400 = 4,150 W.
What size generator do I buy?
Multiply peak by 1.25 and buy the next standard class: 5.5, 7.5, 10, 14, 18, 22, or 30 kW.
Where do I run it safely?
Outdoors only, 20 ft or more from windows, doors, and garages, with CO detectors inside and a transfer switch.
Why only the largest surge?
Only the single largest surge counts, since motors rarely start together. Stagger starts to keep it true.
Sources
- Mississippi State Extension: Selecting Small Portable Generators (3x rule)
- Virginia Tech Extension: Estimating Appliance Energy Use (watt tables)
- Ready.gov: Power Outages (generator safety, 20 ft rule)