Free Electrical & HVAC Tool
Generator Sizing Calculator
Pick the loads the home actually runs during an outage. You get total running watts, the peak starting surge from the single largest motor, and the standby generator size to quote — with headroom and altitude derate applied.
Built by SubcontractorHub — the software electrical contractors use to quote, finance, and run every job.
Book a DemoSelect the loads that run during an outage
Heating & Cooling
Central A/C — 3 ton (36k BTU)
3,500 W running · 3x start
Central A/C — 2 ton (24k BTU)
2,400 W running · 3x start
Heat pump — 3 ton
3,800 W running · 3x start
Gas furnace blower (1/2 hp)
800 W running · 2.6x start
Mini-split (inverter, 12k BTU)
1,100 W running · 1.2x start
Electric space heater
1,500 W running · no surge
Water
Well pump (1 hp)
1,000 W running · 3.5x start
Sump pump (1/2 hp)
800 W running · 3x start
Electric water heater
4,500 W running · no surge
Kitchen
Refrigerator / freezer
700 W running · 3x start
Electric range (one element)
2,100 W running · no surge
Microwave
1,000 W running · no surge
Dishwasher
1,400 W running · 1.5x start
Laundry & Misc
Clothes washer
1,150 W running · 2.2x start
Electric dryer
5,400 W running · no surge
Lighting circuit (LED, whole floor)
300 W running · no surge
General outlets / electronics
600 W running · no surge
Garage door opener
700 W running · 2.5x start
EV charger (Level 2, 32A)
7,700 W running · no surge
Total running watts
2,400 W
Peak demand with surge
3,800 W
+1,400 W from Refrigerator / freezer
Generator to quote
7.5 kW
needs 4.8 kW after derate
Wattages shown are typical nameplate averages and vary by manufacturer, motor type and age; inverter-driven equipment draws far less starting current than the fixed-speed figures used here. This calculator adds only the largest single starting load to running watts and does not perform an NEC Article 220 load calculation. All calculations are estimates based on historical information and should be verified by the user with a licensed electrical contractor. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.
Common residential standby sizes
| What it powers | Running load | Typical size | Tier |
|---|---|---|---|
| Fridge, lights, outlets, furnace blower | 2.5-3 kW | 7.5 kW | Essential circuits only |
| Above plus well pump and sump pump | 4-5 kW | 10-12 kW | Essentials with water |
| Above plus 2-ton A/C | 7-8 kW | 14-16 kW | Partial comfort |
| Above plus 3-ton A/C and range | 10-12 kW | 20-22 kW | Most common whole-home size |
| Whole home with electric water heat / EV | 16-20 kW | 26-38 kW | Large or all-electric home |
How to Size a Standby Generator
Sizing is not a total-the-nameplates exercise. Three numbers decide the unit you quote, and the second one is where most undersized installs go wrong.
1. Total the running watts. Add the steady draw of everything the homeowner expects to run at once. Be honest about simultaneity — a family that runs the dryer during an outage is rare, but the refrigerator, furnace blower and well pump all run whether anyone is home or not.
2. Add only the largest starting surge. An induction motor pulls roughly three times its running watts for the first fraction of a second. Because compressors and pumps do not break away at the same instant, you add one surge — the biggest — on top of the running total. Adding every surge together is the most common sizing mistake and produces a quote two to three times too large.
3. Apply headroom and derate. Size so the unit runs near 80% of rated output, then derate about 3% per 1,000 feet of elevation and roughly 1% per 10°F above 77°F. A 20 kW unit at 5,000 feet in a hot climate behaves like a 16 kW unit.
Inverter-driven heat pumps and mini-splits change the math in the homeowner's favor — soft-start behavior means a fraction of the surge a fixed-speed compressor demands. If the home has one, or a hard-start kit can be added to an existing condenser, you can often drop a full size. Check the HVAC sizing calculator for the equipment side and the wire size calculator for the feeder once the unit is chosen.
Frequently Asked Questions
- What size generator do I need for a whole house?
- Most homes with a 3-ton air conditioner, a well or sump pump, refrigeration, lighting and general outlets land between 18 kW and 22 kW, which is why 20 kW and 22 kW are the highest-volume residential standby sizes. An all-electric home with electric water heating, an electric range and an EV charger commonly needs 26 kW to 38 kW. The figure that drives the decision is not total running watts but the peak starting watts — the largest single motor starting while everything else is already running.
- What is the difference between running watts and starting watts?
- Running watts are the steady draw an appliance holds once it is up to speed. Starting watts, also called surge or locked-rotor watts, are the brief spike an induction motor pulls in the first fraction of a second as it breaks away under load. Compressors and well pumps commonly draw three to three and a half times their running watts. A generator sized only to total running watts will brown out or stall the moment a compressor kicks on, so size to running watts plus the single largest surge.
- Do you add every appliance's starting watts together?
- No, and doing so is the most common sizing error. Motors do not all start at the same instant, so the industry practice is to total the running watts of everything that runs simultaneously, then add only the surge of the single largest starting load on top. Adding every surge together produces a generator two to three times larger and far more expensive than the installation needs.
- How much headroom should you leave on a standby generator?
- Plan to run a generator at roughly 70 to 80 percent of its rated output. That margin covers future load additions, derating from altitude and high ambient temperature, and the efficiency loss every unit shows as it ages. Continuously loading a generator above about 80 percent shortens engine life and leaves nothing in reserve for a motor start.
- Does altitude affect generator sizing?
- Yes. Naturally aspirated engines lose roughly 3 percent of rated output per 1,000 feet of elevation above sea level, and about 1 percent for every 10 degrees Fahrenheit above 77. A 20 kW unit installed at 5,000 feet in a hot climate can deliver closer to 16 kW. Confirm the derate table for the specific model before committing to a size on a mountain or desert installation.
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Book a Free DemoGenerator sizing results on this page are planning estimates only. Actual demand varies with motor efficiency, hard-start kits, inverter equipment, simultaneity of use, ambient temperature and site elevation, and a compliant installation requires a load calculation performed to the applicable edition of the National Electrical Code by a licensed professional. All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.