Estimate the generator capacity (watts) you need based on your total running load and the highest starting surge among your appliances.
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This calculator estimates the minimum generator capacity needed to run your appliances by adding your total continuous running wattage to the extra surge wattage required when a motor-driven appliance starts up. Motors in refrigerators, air conditioners, well pumps, and similar equipment briefly draw significantly more power at the moment of startup than during normal operation, and a generator must be sized to handle that momentary surge on top of everything else already running, not just the steady-state total. A 20% safety margin is added on top of the base calculation, which is standard practice to avoid running a generator at its absolute maximum capacity, since consistently operating at 100% load can shorten a generator's lifespan and leaves no room for adding another appliance later. Always check the specific startup current listed on an appliance's nameplate when precision matters, since generic estimates can vary meaningfully between similar-looking devices.
The most common mistake in sizing a generator isn't underestimating how much power your appliances use while running — it's forgetting that many of them briefly demand far more power for a fraction of a second when they first switch on. Getting this distinction right is the difference between a generator that comfortably powers your home and one that trips or stalls the moment the refrigerator compressor kicks on.
Running wattage (sometimes called rated wattage) is the steady amount of power an appliance draws once it's operating normally, and this is the number most people intuitively think of when estimating their power needs. Adding up running wattage across everything you want to power simultaneously gives a reasonable baseline, but it's only half the calculation for any circuit containing an electric motor.
Starting wattage, also called surge or peak wattage, is the brief spike in power draw that occurs in the instant an electric motor begins spinning from a standstill. Motors need considerably more torque to overcome initial inertia than to maintain speed once already spinning, and that extra torque demand translates directly into extra electrical draw — often two to three times the motor's running wattage, though only for a fraction of a second before it settles down to normal operation.
The practical consequence is that a generator sized only for the sum of everyone's running wattage will work fine right up until a motor-driven appliance tries to start, at which point the generator can experience a voltage sag, trip its overload protection, or in some cases stall out entirely — usually at the least convenient moment, like a refrigerator compressor cycling on in the middle of the night.
Because appliances rarely all start at precisely the same instant in normal household use, generator sizing conventionally only accounts for the single largest starting surge among your appliances, added on top of the combined running wattage of everything else. This avoids wildly oversizing (and overpaying for) a generator to cover a worst-case scenario where every motor somehow starts simultaneously, while still providing enough headroom for the realistic case of one motor starting while everything else is already running steadily.
Check the appliance's nameplate, user manual, or manufacturer's website; motor-driven appliances often list both a running (rated) wattage and a higher starting (surge) wattage.
No — only the single largest starting surge matters in most cases, since appliances rarely all start at the exact same instant; add the surge figure from your biggest motor-driven appliance only.
Running a generator continuously at 100% of its rated capacity can shorten its lifespan and leaves no headroom for adding another appliance later, so a margin is standard practice recommended by most generator manufacturers.