Generator Sizing Calculator

Kilowatts (kW) measure real power, the actual work done. Kilovolt-amperes (kVA) measure apparent power, which includes both real power and reactive power drawn by inductive loads like motors. The ratio of kW to kVA is the power factor. Most residential generators are rated at a power factor of 1.0 (unity), so kW equals kVA. Industrial generators driving heavy motor loads use a power factor of 0.8, meaning an 8 kVA generator delivers 6.4 kW of real power. Always check whether your appliances are rated in kW or kVA when sizing.

Electric motors draw significantly more current at startup than during normal running. This is because the motor must overcome inertia and build up magnetic flux before it can run at its rated speed. The starting surge typically lasts half a second to two seconds and can be two to three times the running wattage for single-phase induction motors, and up to six times for older motors without soft-start electronics. If a generator cannot supply this starting surge, the motor may fail to start and the generator may overload or stall.

Continuous load (running watts) is what devices draw once they are up and running. Surge load (starting watts) is the peak draw when a motor or compressor starts. To size a generator: add up all running watts for everything you want to run simultaneously, then add the starting watts only for the single largest motor or compressor (not all motors at once, since they are unlikely to all start at the same moment). The sum determines the minimum generator surge rating.

Fuel consumption varies with generator size and load. A portable gasoline generator rated at 5,000 W running at 50% load consumes roughly 0.5 to 0.6 gallons per hour. At full load, consumption rises to around 0.8 to 1.0 gallons per hour. Diesel generators are more efficient: a 10 kW diesel unit at 50% load typically uses around 0.5 to 0.7 gallons per hour. Propane generators consume roughly 1.2 to 1.5 gallons of propane per hour at full load for a 5,000 W unit. Your generator's manual will have a load-vs-consumption table.

Yes, if you are connecting a generator to your home's wiring. Connecting a generator directly to household circuits without a transfer switch (also called backfeeding) is illegal and extremely dangerous. It can electrocute utility workers by energizing lines they believe are dead. A manual transfer switch (around $300 to $600 installed) disconnects your home from the utility grid before the generator is connected. An automatic transfer switch (ATS) does this automatically when utility power fails. The National Electrical Code (NEC) Article 702 governs optional standby systems.