How the calculation works
Generators carry two ratings. Running watts is the power the engine can sustain indefinitely; starting watts is a brief surge capacity, usually held for only a few seconds, that exists to get motors spinning. Sizing means satisfying both numbers at once.
Motors are the whole reason the second number exists. A well pump drawing 1,000 W while running can demand 3,000 W for the instant its rotor breaks free. If the generator cannot supply that surge, the motor stalls, voltage sags, and in the worst case both the motor windings and any electronics on the same circuit suffer.
Altitude and temperature
Combustion engines breathe air, and thin air means less power. A rough rule is 3 to 3.5% loss per 1,000 feet of elevation, which the calculator applies for you. High ambient temperature costs a further few percent. A generator that comfortably covers a cabin at sea level can fall short at a mountain site, and this is one of the most common sizing surprises in off-grid installs.
Worked example
A cabin runs 3,000 W simultaneously, including a 1,000 W well pump with a 3x start surge, at low elevation:
The purchase target is a generator rated about 5,000 W starting and at least 3,300 W running. Note that a unit advertised as "5,000 W" is often 4,000 W running, which would fail this load; always read the running figure rather than the headline.
Common mistakes
- Buying on the headline number. Advertised wattage is starting wattage. The running rating, usually 15 to 25% lower, is the one your loads actually live on.
- Running a generator at full load continuously. Most generators are designed to run happily at 50 to 75% of rated output. Constant full-load operation shortens engine life and burns fuel inefficiently.
- Running one too lightly, too. Conventional generators loaded below about 25% suffer wet stacking, where unburned fuel fouls the exhaust. An oversized generator idling under a small load is its own problem; inverter generators avoid this by varying engine speed.
- Ignoring fuel and runtime. A larger generator burns more fuel per hour even at the same load. If the generator is backup for a solar system, the right size is often smaller than people assume, because its job is to top up a battery bank rather than power a house directly.
Frequently asked questions
What size generator to charge an off-grid battery bank?
Size it to your battery charger rather than your household loads. A 60 A charger on a 48 V bank draws roughly 3,000 W of AC, so a 4,000 W running generator covers it with margin. Charging is a steady resistive-style load with no surge, which is the easiest job a generator can have.
Inverter generator or conventional?
Inverter generators produce cleaner power, run quieter, throttle to match load, and tolerate light loads without wet stacking. Conventional units cost less per watt and suit heavy tool or pump duty where noise and waveform matter less. For a backup that mostly charges batteries and runs electronics, inverter models are usually worth the premium.
Can I run everything at once?
Rarely necessary, and expensive if you insist. Staggering big loads, starting motors one at a time, and letting the battery bank absorb short peaks all reduce the generator size you need. Load management is almost always cheaper than a bigger engine.