Calculator
Select what you'd want running during an outage and we'll estimate a starting point for generator size — a licensed electrician still needs to confirm the final numbers.
Generator size depends on the appliances and systems you want to keep running during an outage — not on square footage alone. The calculator sums the running (continuous) wattage of your selected loads, adds the worst-case starting surge of the largest motor, and applies a safety margin of 15–20% before rounding up to the nearest standard size. Treat the result as a starting estimate. A licensed electrician must review your actual electrical service, verify the load calculation, and confirm the generator and transfer switch specification before installation.
A 10 kW generator can support essential circuits in a modest home — lights, refrigerator, sump pump, a furnace blower, and outlets — but typically cannot handle central air conditioning, electric water heating, an electric range, an electric dryer, or multiple large appliances running simultaneously. Whether 10 kW is adequate depends entirely on which loads you need covered. This calculator can help you identify the right size for your specific appliance selection.
Running watts (or running load) are the steady-state power an appliance draws once it has started. Starting watts are the higher, brief surge that motors and compressors — refrigerators, air conditioners, well pumps, sump pumps, and furnace blowers — need to get spinning. A generator must supply the starting surge without stalling or tripping a breaker, which is why starting watts determine the minimum generator capacity, not running watts alone. This calculator estimates the running load of all selected appliances and then adds the starting surge of the single largest motor to find the peak demand.
Actual appliance draw varies by model, age, voltage, and operating condition. A compressor on a warm day draws more than the nameplate average; an aging well pump may draw significantly more than a new one. The safety margin — 15% for essential-circuit coverage and 20% for whole-home coverage in this calculator — helps account for that variation and leaves room for modest, unanticipated loads. It does not replace a professional load calculation, which accounts for your specific equipment, wiring, and service configuration.
Central air conditioning is one of the most demanding generator loads because the compressor requires a large starting surge before settling to its running draw. The actual amount depends on AC tonnage, system efficiency (SEER rating), compressor type, whether soft-start equipment is installed, and what other loads are running simultaneously. This calculator uses typical nameplate estimates grouped by common AC sizes. Your electrician should verify the actual starting and running demand for your specific system before finalizing generator size.
Continuous operation at 100% of rated capacity is not recommended. A generator's rated output is a maximum, not a target. Manufacturers generally recommend sizing so that expected continuous load stays at roughly 80% of capacity or below, preserving headroom for motor starting surges and preventing sustained operation at the thermal limit. This calculator applies a safety margin to produce an estimate with that headroom built in. Final sizing should follow your specific generator manufacturer's specifications and the guidance of your installer.
No. Square footage helps estimate the general lighting and receptacle load — the baseline draw from lights and outlets — but major appliances, HVAC systems, electric heating, well pumps, and other high-wattage equipment typically have a far larger effect on the required generator size than general wiring does. This calculator uses square footage only to estimate the lighting and outlet baseline; the rest of the load comes from the appliances you select.
Often, yes — standard residential standby generators are available in a set of common sizes, and choosing the next available size above the calculated minimum provides practical headroom and is often a cost-effective choice within a product family. However, larger is not automatically better. Significantly oversized generators cost more to purchase, install, and maintain, and may not be the most efficient fit for the connected load. Your installer can identify the most cost-effective model for your specific load profile.
Yes. This calculator is an educational planning tool, not an engineering determination. A licensed electrician or qualified generator installer must verify the actual electrical load, confirm the service entrance configuration, specify the appropriate transfer switch, review the fuel supply and utility coordination requirements, and ensure the installation meets local electrical code. The estimate from this calculator is a useful starting point for that professional conversation.
