Generator Sizing Calculator

The calculator sums running watts across all simultaneous loads, applies a 20% safety margin, then checks that the generator's surge rating covers the highest startup current in your list.

Surge requirements are evaluated separately — the generator's peak rating must exceed the largest single-appliance startup draw, typically an air conditioner or well pump.

How do I calculate what size generator I need?

List every appliance you plan to run simultaneously. For each, note the running watts and surge watts (startup current). Sum all running watts to get your continuous load. Identify the appliance with the highest surge draw — usually an air conditioner, refrigerator, or well pump — and ensure the generator's surge rating meets or exceeds that peak. Add a 20% safety margin to the running watt total. The calculator automates this process and flags any surge constraints.

What is the difference between running watts and surge watts?

Running watts (also called rated watts) are the continuous power a generator delivers during normal operation. Surge watts (also called starting watts) are the brief burst of power — typically 2–3× running watts for 1–3 seconds — that motor-driven appliances need to start. A generator rated at 3,500W running / 4,375W surge can handle a refrigerator compressor startup without stalling. Always verify the surge rating when motors are in your load list.

What size generator do I need to run an RV air conditioner?

A 13,500 BTU RV air conditioner needs approximately 1,500W running and up to 3,500W to start. A 3,000W running / 3,500W surge generator is the practical minimum for one air conditioner without other loads. To run the AC plus a microwave and refrigerator simultaneously, a 4,000W generator is more appropriate. A soft starter kit on the AC unit reduces the surge current significantly and lets a smaller generator handle the load.

How long will a generator run on a tank of gas?

Fuel consumption depends on load level and tank size. Most portable generators consume 0.5–1.0 gallons per hour at 50% load. A 4,000W generator with a 3.5-gallon tank runs roughly 7–10 hours at half load. At 75–100% load, runtime drops to 4–6 hours. Inverter generators are significantly more fuel-efficient at partial loads — fuel consumption drops proportionally as load decreases, unlike conventional generators that run at fixed RPM.

What is an inverter generator and why does it cost more?

An inverter generator produces AC power, converts it to DC, then inverts it back to a clean sine wave — identical in quality to utility power. This makes it safe for sensitive electronics (laptops, CPAP, modern appliances). Inverter generators also throttle engine speed to match the load, reducing noise and fuel consumption by 20–40% compared to conventional generators at the same output. The additional electronics account for the higher cost. For RV use with any sensitive devices, an inverter generator is worth the investment.