Use this whole house generator sizing calculator to choose the loads that must run during an outage, replace planning values with nameplate watts, and see the complete calculation behind the recommended standby-generator tier.
Interactive load worksheet
Model 2026.08 · Reviewed 2026-08-05. Planning result only—not a contractor quote, electrical design, equipment-selection approval, or code decision. Verify actual nameplates, fuel-specific output, transfer equipment, load management, conductors, and local requirements with a qualified professional.
Size the outage plan, not the house label. The same service rating or floor area can produce a very different answer when central air, pumps, electric heat, water heating, cooking, or EV charging change.
A 200-amp service rating is not a generator-size input. It is the home's service capacity—not a statement that every connected load must run during an outage. Select simultaneous loads, include the largest additional motor start, and then review transfer and load-management scope.
Floor area alone still does not choose the tier. The published load plans span essentials through a broader central-air scenario; a 2,000- or 3,000-square-foot home with the same selected loads produces the same worksheet result.
In the published broader plan, 9,700 W of selected running load plus 7,000 W of additional compressor start and the 20% buffer requires 20,100 W before tier rounding. Replace the placeholder with condenser and soft-start data.
The honest answer is a load plan, not one universal number. These three examples keep the house size fixed and change only what must operate together. A 3,000 sq ft house uses the same method; floor area never replaces the load inventory. All rows use the calculator's formula and 20% buffer.
| Outage plan | Selected running | Largest extra start | With 20% buffer | Planning tier |
|---|---|---|---|---|
| Essentials onlyLights, outlets, refrigeration, internet, security, and a gas-furnace blower. | 3,650 W | 2,350 W | 7,200 W | 10 kW |
| Managed comfortEssentials plus a sump pump and microwave, with large loads used selectively. | 6,200 W | 2,350 W | 10,300 W | 14 kW |
| Broader whole-home planManaged comfort plus a central-air planning load, before any electric heat or EV charging. | 9,700 W | 7,000 W | 20,100 W | 22 kW |
Central air, wells, electric heat, water heating, ranges, dryers, and EV charging can move the result quickly. Load management may keep some large appliances out of the simultaneous total. Fuel type and site conditions can also change a generator's usable output.
These are editable worksheet starting points, not universal appliance ratings. Use the planning basis to distinguish manufacturer examples from bounded internal allowances.
| Planning load | Running | Extra start | Planning basis |
|---|---|---|---|
| Lighting & general outletsA planning allowance for LED lighting, device charging, and selected receptacles. | 2,000 W | 0 W | Internal planning allowance; replace it with a measured or panel-derived value when available. |
| Refrigerator / freezerOne typical refrigerator-freezer compressor load. | 700 W | 2,200 W | Generac wattage worksheet planning value. |
| Internet & securityRouter, modem, alarm panel, and small communications equipment. | 150 W | 0 W | Internal planning allowance for small continuous electronic loads. |
| Gas furnace blowerA 1/2 HP furnace fan; electric resistance heat is a separate, much larger load. | 800 W | 2,350 W | Generac buyer worksheet planning value. |
| Central AC planning loadA broad 3-ton-class placeholder. Use the condenser nameplate or installer data for a real project. | 3,500 W | 7,000 W | Internal bounded placeholder; actual compressor LRA, soft-start equipment, and fuel rating can materially change the result. |
| MicrowaveOne countertop or over-range microwave used during an outage. | 1,500 W | 0 W | Generac wattage worksheet planning value. |
| 1/2 HP sump pumpA motor load whose short startup demand can control the required generator size. | 1,050 W | 2,200 W | Generac buyer worksheet planning value. |
| 1 HP well pumpA broad planning value for one residential well pump. | 1,000 W | 3,000 W | Internal planning allowance; use the motor nameplate for a real project. |
| Electric water heaterA common resistive tank water-heater element planning load. | 4,500 W | 0 W | Internal planning allowance; verify the equipment nameplate. |
| Electric range / ovenA managed-use allowance rather than the appliance's full connected rating. | 8,000 W | 0 W | Internal planning allowance; cooking diversity and local sizing methods vary. |
| Electric clothes dryerOne electric dryer operating during an outage. | 5,000 W | 0 W | Internal planning allowance; verify the equipment nameplate. |
| Level 2 EV chargerA 30-amp-class charging allowance that is often shed during an outage. | 7,200 W | 0 W | Internal planning allowance; use the configured charger amperage for a real project. |
Count the loads expected to operate at the same time.
Use the largest additional motor-start requirement, not every startup value combined.
The default 20% buffer can be changed; it is a budgeting assumption, not an electrical-code rule.
The worksheet chooses the first standard planning tier that is not below the rounded requirement.
Σ running W+largest extra start W+buffer
Use the formula-driven workbook during equipment inventory or contractor conversations. Yellow cells are editable; formulas calculate selected running load, the largest additional starting load, buffer watts, and the next planning tier.
Download the Excel worksheetSizing Worksheet20 editable rows + live summary
Assumptions & SourcesTier lookup + evidence boundaries
Formula-driven · 11 KB · Reviewed 2026-08-05
Manufacturer material supports the worksheet sequence and selected example loads. Values labeled as internal planning allowances are bounded placeholders and should be replaced with equipment data for a real project.
The running-watts plus single-largest additional-starting-watts worksheet method and selected household planning defaults.
Adding the largest starting requirement to simultaneous running loads and retaining planning headroom.
A 20% planning buffer and the importance of selecting the household loads that must operate during an outage.
This result is not a contractor quote, electrical design, or equipment-selection approval. Final sizing must account for actual nameplates, motor-start characteristics, fuel-specific ratings, transfer equipment, conductors, load management, site altitude and temperature, and applicable local requirements.
Use the answers to plan the worksheet—not to replace a project-specific load calculation.
Start with the loads that must operate together, add the largest single additional motor-start requirement, apply visible headroom, and round up to a supported tier. The published examples on this page produce 10 kW for essentials, 14 kW for managed comfort, and 22 kW for a broader plan with central air; they are load-plan examples rather than universal house-size answers.
A 200-amp service rating is not a generator-size input. It describes the home's service capacity, not the smaller group of outage loads that must operate together. Inventory those loads, include motor starting demand, then review transfer equipment and load management for the actual project.
Square footage does not determine generator size by itself. In this planning model, selected essentials produce a 10 kW tier, managed comfort loads produce 14 kW, and a broader plan with central air produces 22 kW. Actual equipment, fuel rating, and simultaneous loads can change the answer.
A 3,000-square-foot house does not have one dependable generator size. Floor area can suggest that more loads may exist, but the calculation still depends on which heating, cooling, water, pump, cooking, and other loads must run together. Use the same load worksheet and replace planning values with equipment data.
Central air must be evaluated with the other simultaneous loads and the compressor's starting demand. The page's bounded 3-ton-class placeholder contributes 3,500 running watts and 7,000 additional starting watts; in the published broader whole-home example, the complete load plan reaches a 22 kW planning tier. Actual condenser data, soft-start equipment, fuel rating, and load management can change the result.
It adds all selected running watts, then adds only the largest single additional starting requirement. It applies the selected planning buffer and rounds up to the next supported standby-generator tier.
Not in this worksheet method. It assumes the largest motor startup is the controlling additional load rather than every motor starting simultaneously. A project with unusual sequencing or multiple simultaneous starts needs project-specific review.
Yes. The downloadable workbook contains editable load rows, formulas for running and starting watts, an adjustable buffer, a standard-tier lookup, and a separate assumptions-and-sources sheet.
No. It is an early planning worksheet for comparing outage priorities and budget scenarios. A qualified local professional must verify equipment nameplates, fuel-specific output, transfer equipment, conductors, load management, and applicable requirements.