Generator Sizing Calculator
What size generator do you need? Size backup or job-site power for a home, a commercial building, or a restoration job in seconds — based on real running and starting-watt loads and NEC connected-load figures. Then rent or buy the right unit from us.
To size a whole-house generator, add the running watts of everything you will power at once, add the single largest motor’s starting surge, then allow 20–25% headroom and round up to a standard size. Most homes land between 14 and 26 kW. Restoration Equipment Supply’s calculator below does it in seconds.
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Estimate for planning. We confirm the exact unit, transfer switch, and fuel with you — for rental or purchase, residential or commercial.
How Do You Size a Whole-House Generator?
Size a home standby or backup generator by load, not by square footage: total the running watts, add the biggest starting surge, then add headroom. This works for Generac, Kohler, Briggs & Stratton, Champion and other standby units — compare your result to the manufacturer’s rated output.
List the loads
Everything you want running during an outage: AC, well pump, fridge, furnace blower, sump pump, lights, water heater.
Sum running watts
Add the running (continuous) watts of every load that can run at the same time.
Add the largest surge
Add the biggest single motor’s starting surge (starting watts minus running watts). Only one large motor usually starts at once.
Add headroom, round up
Allow 20–25% reserve over the running load and round up to the next standard generator size.
Running watts vs. starting watts
Running watts keep a device going. Starting watts are the brief surge a motor or compressor needs to spin up — often 2–3× its running watts. A generator must cover the continuous load and the largest start, or the AC compressor will trip it.
Worked example: 3-ton AC home
A home with a 3-ton central AC (4,900 W running, about 14,000 W starting), refrigerator, furnace blower, sump pump, lights and router runs 8,800 W. The AC’s surge adds 9,100 W, for a 17,900 W peak. Running load × 1.25 is 11,000 W, so the peak governs: round 17.9 kW up to an 18 kW standby unit with a 75 A transfer switch. Those are the calculator’s default inputs — uncheck the AC and the answer drops sharply.
kW vs. kVA
Generators are rated in kW (real power) and kVA (apparent power). kW = kVA × power factor. Most standby gensets are rated at a 0.8 power factor, so a 100 kVA unit delivers 80 kW. Residential loads are usually figured directly in watts; commercial loads start in VA from NEC Article 220 and are converted to kW.
Code references
Residential and commercial load calculations follow NFPA 70 (National Electrical Code) Article 220. Optional standby systems — the category most home backup generators fall under — are covered by NEC Article 702, which requires the generator to carry the full load it is connected to unless an automatic load-management system sheds loads. A licensed electrician should confirm the final size against your panel.
| Appliance | Typical running watts | Typical starting watts |
|---|---|---|
| Central AC, 2 ton | 3,800 | 11,000 |
| Central AC, 3 ton | 4,900 | 14,000 |
| Central AC, 4 ton | 5,500 | 17,000 |
| Central AC, 5 ton | 6,500 | 20,000 |
| Well pump, 1 HP | 2,000 | 4,000 |
| Refrigerator / freezer | 700 | 2,200 |
| Furnace blower | 800 | 2,350 |
| Sump pump | 800 | 1,300 |
| Electric water heater | 4,500 | 4,500 |
| Electric dryer | 5,400 | 6,750 |
| Electric range / oven | 3,000 | 3,000 |
| Washing machine | 1,200 | 2,300 |
| Microwave | 1,000 | 1,000 |
| Level 2 EV charger | 7,200 | 7,200 |
| Lights & outlets (whole home) | ~1,500 | ~1,500 |
Typical generator sizes
- Essentials onlyFridge, sump, furnace, lights: 7.5–12 kW portable or small standby
- Whole house, gas heatMost homes with central AC: 14–26 kW standby
- All-electric homeElectric range, dryer, water heater, heat: often 26–48 kW
- Commercial buildingSized from NEC demand load in kVA — use the Commercial tab
Fuel matters too: many standby generators are rated lower on natural gas than on propane, so check the rating for the fuel you will use. Once you know the size, estimate fuel with our generator runtime calculator, or see generator rental options for storm season and job sites.
What size generator do I need to run my whole house?
Most homes with central air and gas heat need a 14–26 kW whole-house generator. Add the running watts of everything you want on, add the largest motor’s starting surge, add 20–25% headroom, and round up. All-electric homes often need more.
Can I use this as a Generac sizing calculator?
Yes. The load method is the same for Generac, Kohler, Briggs & Stratton, Champion and other standby brands. Take the kW result and pick a model whose rated output on your fuel (propane or natural gas) meets or exceeds it.
How do I calculate generator size for a home?
Generator size (W) = total running watts × 1.25, or running watts + largest starting surge — whichever is larger. Divide by 1,000 for kW and round up to the next standard size.
What size backup generator do I need for essentials only?
Refrigerator, furnace blower, sump pump, lights and a router run about 3,900 W with a fridge or furnace start surge on top — typically covered by a 7.5–12 kW portable or small standby unit.
How big a generator do I need for a 3-ton AC?
A 3-ton central AC typically runs around 4,900 W but can surge to about 14,000 W on startup. With a fridge, furnace blower, sump pump and lights, that home needs about an 18 kW generator — or less with a soft-start kit on the compressor.
How do you convert kVA to kW for a generator?
kW = kVA × power factor. Standby generators are typically rated at 0.8 PF, so 50 kVA = 40 kW and 125 kVA = 100 kW.
Updated October 2026 · Reviewed by the Restoration Equipment Supply dispatch team
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The Sizing Method
How do you size a residential generator?
We add up the running (continuous) watts of everything you want to power at once, then add the single largest starting surge — because motors (AC, well pump, fridge) draw 2–3× their running watts for a split second on startup, and only one big motor typically starts at a time. We then add ~25% reserve and round up to the next standard generator size. Whole-home standby units run 14–26 kW for most homes; essentials-only can be covered by a 7.5–12 kW portable.
How do you size a commercial generator?
We estimate the building's connected load as square footage × a VA-per-square-foot figure by occupancy type (based on NEC 220.12 lighting classes plus typical receptacle and HVAC load), apply the NEC 220.42 demand allowance (first 50 kVA at 100%, remainder at 50%), add the largest motor's starting surge and a growth reserve, then convert apparent power (kVA) to real power (kW) at a 0.8 power factor and round up to a standard genset. Loads at or above ~50 kW usually run 3-phase. Always confirm against your actual panel schedule with a licensed electrician.
Running watts vs. starting watts — what's the difference?
Running watts keep a device going; starting watts are the brief surge to spin up a motor or compressor. A fridge might run at 700W but spike to ~2,200W to start. Size the generator so its rated (running) output covers your continuous load and its surge rating covers the largest single startup.
What size generator do I need to run restoration equipment?
Add up the running watts of your air movers (~180W each), LGR dehumidifiers (~700W each), air scrubbers (~350W each), and negative-air machines (~400W each), then add the biggest single starting surge and ~15% reserve. A typical 12-air-mover, 4-dehu drying setup runs comfortably on a mid-size portable. Use the Job-Site tab above — and remember we rent the equipment and the generator together.
Should I rent or buy a generator?
Rent for storm season, a single project, a restoration job, or to bridge until a permanent standby is installed — no capital outlay, we deliver and service it. Buy for permanent home or facility backup. We do both, residential and commercial, and can size, deliver, and connect either way. Get a quote →
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