Generator Wattage Calculator
Enter the running watts and starting watts for each appliance you want to power, and this calculator works out the minimum generator size you need. It accounts for motor surge on startup, adjusts for altitude if you are above sea level, and shows a breakdown of your total load. Scroll down for a full appliance wattage reference table covering kitchen, climate, laundry, and power-tool loads.
Formula
Worked example
Refrigerator (700 W running, 1,200 W starting) + Window AC (1,200 W running, 2,600 W starting) + ceiling fan (80 W running, 120 W starting). Total running = 700 + 1,200 + 80 = 1,980 W. Largest surge = 2,600 - 1,200 = 1,400 W. Peak demand = 1,980 + 1,400 = 3,380 W. With 20% headroom at sea level: 3,380 x 1.20 = 4,056 W, so a 4.5 kW or 5 kW generator is the recommended minimum.
How generator sizing works
A generator must supply two types of power at once: running watts and starting watts. Running watts (also called rated watts) are the steady-state power an appliance consumes while it is operating. Starting watts (also called surge watts) are the brief spike of extra power that motors draw for the first one to two seconds when they start. The spike is caused by the inrush current needed to overcome the motor's magnetic inertia and can be two to three times the running draw for compressors, pumps, and air conditioners. If your generator's rated output falls below the peak starting demand, the engine stalls, the circuit trips, or the voltage sags enough to damage sensitive electronics. The calculation therefore adds together all the running watts of every appliance you need to run simultaneously, then adds only the largest single appliance's startup surge, because appliances rarely all start at the exact same instant.
What size generator do you need?
A common mistake is buying a generator sized exactly to your calculated load. Most manufacturers recommend never exceeding 80% of the generator's rated output during normal operation, because running at 100% load for extended periods overheats the alternator, degrades the engine, and leaves no headroom for brief demand spikes. The 20% safety margin built into this calculator accounts for that rule. If you are at high elevation, you need an even larger unit because internal combustion engines lose roughly 3% of their power output for every 1,000 feet above sea level - at 5,000 feet you are already operating at 85% of the sea-level rating before you plug in a single appliance. As a practical guide: a 2 kW inverter generator covers phone charging, lighting, and a small refrigerator; 5 kW handles a refrigerator, several lights, a TV, and a window AC; 10 kW covers most of a typical home excluding electric water heaters or large central AC; 20 kW and above is standby territory for whole-home coverage including electric heat.
Running vs. starting watts - why the difference matters
Any appliance containing an electric motor draws a startup surge: refrigerators, freezers, sump pumps, well pumps, air conditioners, air compressors, washing machines, clothes dryers, and power tools with induction motors all fall in this category. The surge typically lasts under two seconds, but the generator must be capable of supplying it, otherwise it bogs down and either stalls or causes voltage to sag. Resistive loads - heating elements, incandescent and LED lights, laptops, TVs, phone chargers - have no startup surge, so their starting watts equal their running watts. If your generator label shows "peak watts" and "rated watts", compare the rated watts (not peak) to your total running load, and the peak watts to your peak starting demand.
Altitude derating and fuel type
The altitude derating formula is: effective output = rated watts x (1 - 0.03 x elevation in thousands of feet). A generator rated at 6,500 W at sea level produces only about 5,525 W at 5,000 ft - a reduction of nearly 1,000 W. Gasoline and dual-fuel generators are affected equally by altitude because both rely on the same Otto-cycle combustion. Propane generators are similarly affected but often have a slightly higher power loss curve per manufacturer spec. If you are at high elevation, divide your required output by the derating factor to find the sea-level rating you need to buy. The calculator handles this automatically. Diesel generators also derate at altitude but typically have a separate turbocharging correction; check the manufacturer spec sheet for precise values.
Common Appliance Wattage Reference
| Appliance | Category | Running Watts | Starting Watts |
|---|---|---|---|
| Refrigerator | Kitchen | 700 | 1200 |
| Chest freezer | Kitchen | 500 | 900 |
| Microwave oven | Kitchen | 1000 | 1000 |
| Coffee maker | Kitchen | 1000 | 1000 |
| Dishwasher | Kitchen | 1200 | 1800 |
| Electric range (8 in burner) | Kitchen | 2100 | 2100 |
| Window AC (10,000 BTU) | Climate | 1200 | 2600 |
| Central AC (3 ton) | Climate | 3800 | 6000 |
| Electric furnace | Climate | 5000 | 7500 |
| Ceiling fan | Climate | 80 | 120 |
| Space heater (1,500 W) | Climate | 1500 | 1500 |
| LED bulb (10 W) | Lighting & Electronics | 10 | 10 |
| LED TV (50 in) | Lighting & Electronics | 100 | 100 |
| Desktop computer | Lighting & Electronics | 300 | 300 |
| Wi-Fi router | Lighting & Electronics | 10 | 10 |
| Washing machine | Laundry & Water | 1200 | 2300 |
| Clothes dryer (electric) | Laundry & Water | 5000 | 6750 |
| Water heater (electric) | Laundry & Water | 4000 | 4000 |
| Sump pump (1/3 HP) | Laundry & Water | 800 | 1300 |
| Well pump (1/2 HP) | Laundry & Water | 1000 | 2000 |
| Circular saw (7-1/4 in) | Tools | 1400 | 2300 |
| Air compressor (1 HP) | Tools | 1600 | 2800 |
| Electric drill (1/2 in) | Tools | 500 | 900 |
| Table saw (10 in) | Tools | 1800 | 3000 |
| Welder (140 A) | Tools | 4000 | 4000 |
Typical running and starting watt values. Starting watts for appliances without motors equal running watts. Source: Honda Power Equipment sizing guide and U.S. Department of Energy.
Frequently asked questions
What is the difference between running watts and starting watts?
Running watts (rated watts) are the continuous power an appliance needs while it is operating. Starting watts (surge watts) are the brief spike of extra power that motor-driven appliances draw for one to two seconds when they start up. Compressors, pumps, and air conditioners typically surge to two or three times their running draw on startup. Resistive loads like lights and heaters have no surge, so their starting watts equal their running watts.
How do I find my appliance wattage?
Check the nameplate label on the back or bottom of the appliance, usually stamped near the power cord. It lists watts (W), amps (A), or both. If only amps are shown, multiply by your voltage (120 V in North America) to get watts. The scroll-down reference table in this page lists typical values for the most common household appliances and power tools.
Should I add all appliances' starting watts together?
No. Appliances rarely all start at the exact same moment. The standard method is to add the running watts for every appliance, then add only the single largest startup surge once. That covers the worst-case scenario of that one appliance starting while everything else is already running.
How does elevation affect generator output?
Generators lose about 3% of their rated output for every 1,000 feet above sea level, because thinner air delivers less oxygen to the combustion cycle. At 5,000 feet, a 6,500-watt generator produces only about 5,525 watts. Enter your elevation in this calculator to have the derating applied automatically to the recommended size.
What is the 80% load rule for generators?
Generator manufacturers recommend running the unit at no more than 80% of its rated capacity during sustained operation. Constant 100% loading overheats the alternator windings, stresses the engine, and shortens its service life. The 20% safety headroom the calculator applies by default reflects this guideline. If budget is tight, you can lower it to 10%, but 20% is the widely cited best practice.
What generator size do I need for a whole house?
A typical American home with central air conditioning, a refrigerator, lights, and basic electronics needs between 8 kW and 12 kW for partial coverage, or 14 kW to 20 kW for whole-home coverage including electric water heating and large AC. Enter the actual appliances you want to power in this calculator for a precise answer specific to your home.
Can I run a refrigerator and an air conditioner on the same generator?
Yes, provided the generator is sized correctly. The key is accounting for the startup surge of both motor-driven appliances. A standard refrigerator surges to about 1,200 W on startup, and a 10,000-BTU window AC surges to about 2,600 W. If the AC starts while the refrigerator is already running, you need a generator that can supply the refrigerator's running watts (700 W) plus the AC's starting surge (2,600 W), for a peak demand of 3,300 W, before any other loads.