The single most important distinction in generator load calculation is between running watts and starting watts — and getting it wrong is the fastest way to undersize a generator.
The power an appliance draws once it is running. A refrigerator draws 700 W, a laptop 60 W, a light bulb 9 W. These are the steady-state loads your generator must support for hours at a time.
The short, sharp surge a motor draws at startup — typically 2–4× the running watts for residential motors, and 3–6× for industrial direct-on-line motors. A 700 W refrigerator can require 1,500–2,200 W to start.
Sum of all running watts + the single largest starting surge. Only one motor starts at a time, so you size for the biggest one — not the sum of every starting surge.
Summing running watts alone always understates the load. A motor draws 2–4× its running watts at startup (industrial DOL motors draw 3–6× full-load current). Your generator must be able to meet that surge for the second or two it lasts, or the motor will stall and the breaker will trip. Always include the largest starting surge in your calculation.
List the continuous power of every appliance and motor. Check nameplates, use the tool's built-in appliance table, or enter watts manually.
Identify the motor with the highest startup requirement — often a pump, compressor, or HVAC unit. Use the tool's appliance data or enter the starting watts directly.
Total load = sum of all running watts + the largest single starting surge. This is the number your generator must be able to deliver — not the sum of every starting surge.
Enter your appliances or watts directly. The calculator adds running loads, accounts for the largest surge, applies a safety margin, and returns your total load in watts, kW, and kVA.
The calculator answers one question: how much power, in total, must the generator deliver? It builds that answer in three steps — list your loads, sum the running load, and add the largest starting surge.
Pick appliances from the preset library — refrigerator, air conditioner, sump pump, power tools — or enter your own equipment. Each entry has running watts and starting watts.
Enter your equipment in watts or kilowatts. Perfect for industrial loads where you already know the nameplate ratings.
Below are typical running and starting watts for the loads that end up on a generator. Use these as a starting point, then check the nameplate on your own equipment — actual values vary by model and voltage.
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator / freezer | 700 | 1,500–2,200 |
| Sump pump (1/2 HP) | 1,050 | 2,150 |
| Furnace fan (1/2 HP) | 875 | 2,350 |
| Well pump (1 HP) | 2,000 | 4,000 |
| Clothes washer | 1,150 | 2,300 |
| Dishwasher | 1,300–1,500 | 1,500–1,800 |
| Window air conditioner | 1,200 | 1,800–3,600 |
| Microwave | 600–1,000 | 0 |
| Coffee maker | 1,000–1,750 | 0 |
| Television | 500 | 0 |
| Space heater | 1,800–2,000 | 0 |
| Circular saw / power tools | 1,200 | 2,400 |
Here is the full method, the same sequence the calculator runs. You can reproduce it on paper or in a spreadsheet in minutes. Follow these six steps to determine the generator capacity you actually need.
Go room by room and write down everything that will run on the generator. Record the running watts from the equipment nameplate or specification sheet. Use the appliance wattage chart above as a starting point.
Multiply running watts by roughly 2–3× for compressor and pump motors, or read the starting amps from the nameplate. This is the surge your generator must handle for the first second or two of startup.
Sum the continuous power requirement of all equipment expected to operate simultaneously. This establishes the site's normal connected running demand — the steady-state load the generator must carry for hours at a time.
Compare the starting watts of every motor-driven load, and keep only the single biggest. Motors do not start together — the generator only needs to absorb one large surge at a time.
Add the largest starting surge to the sum of the running watts. This is your peak load — the maximum power the generator must deliver at any moment, surge included.
Multiply your peak load by 1.2 to 1.25 to cover load growth, voltage drop, and future additions. This final number is the generator capacity you should specify.
Running loads: radio 100 W + fan 200 W + power drill 1,800 W = 2,100 W running. Largest starting surge: the drill draws about 4,500 W (2.5× running). Generator must deliver: 4,500 + 100 + 200 = 4,800 W. Add 20% margin: 4,800 × 1.2 = 5,760 W (≈ 5.8 kW). A 6 kVA generator comfortably carries this workshop.
Your appliances are rated in watts, but generators are sold in kVA — and the two are not the same thing. Converting between them correctly is the difference between a load that fits and one that silently overloads the alternator.
kVA is apparent power — the total power a generator must supply, including the current that magnetizes motors. Generators are sold in kVA because their alternators are sized by current, not just the real power delivered.
kW is real usable power — the power that actually does work, turning motors, heating elements, and lighting lamps. It is what your appliances consume, and what you pay for on your utility bill.
Power factor is how much of the current is doing useful work versus just magnetizing motors and transformers. A PF of 0.8 means 80% of the current is doing work — 20% is reactive, contributing nothing but heat.
kVA = kW ÷ Power Factor. A 100 kVA generator at 0.8 PF delivers 80 kW of real power. If your load is heavily motor-driven — compressors, pumps, crushers — its PF can dip below 0.8.
Watts = Volts × Amps for resistive loads. For motor loads, multiply by the power factor: Watts = Volts × Amps × PF. This is the basic formula for residential and small commercial systems.
Watts = Volts × Amps × √3 × PF. The √3 factor (1.732) accounts for the phase relationship in three-phase systems. This is why a 400 V, 100 A load is 69 kW, not 40 kW — a common calculation error.
The calculator reports your load in both kW and kVA because sizing in watts alone can mislead you on a motor-heavy installation. Here is how to apply the conversion to your project.
Generators are rated in kVA because the alternator's thermal limit is current-based. A 100 kW load at 0.9 PF requires 111 kVA; at 0.7 PF it requires 143 kVA — the same kW load, very different generator.
Motors, VFDs, and transformers typically have PF between 0.7 and 0.85. Resistive loads (heaters, lighting) have PF ≈ 1.0. Use the actual PF of your load — not the generator's 0.8 rating.
A 50 kW motor at 0.8 PF requires 62.5 kVA. At 0.7 PF, the same motor requires 71.4 kVA — a 14% larger generator. This is why PF correction capacitors can reduce generator size.
I = (kW × 1000) ÷ (V × √3 × PF). A 50 kW, 400 V, 0.8 PF load draws 50,000 ÷ (400 × 1.732 × 0.8) = 90 A. This is the number you use for cable sizing and breaker selection.
A 50 kW load at 0.7 PF looks the same as a 50 kW load at 0.9 PF on your kW meter. But the alternator must supply 71 kVA vs 56 kVA — that difference can push you into the next generator size entirely.
Final generator selection should always be made in kVA, not kW. Use the calculator's output in both units, and confirm the power factor of your actual load before ordering. The generator's kVA rating — not its kW rating — determines whether the alternator can handle your starting surge and continuous load.
Our range runs on Cummins, Perkins, Weichai, and Yuchai engines with Stamford alternators, in single- and three-phase, for everything from home backup to 24/7 industrial prime power.
A: List every load and its running watts, find each motor’s starting watts, sum the running load, then add the single largest starting surge and a 20–25% margin. Convert the result to kVA by dividing by the power factor.
A: Running watts are the continuous power an appliance draws once running. Starting watts are the short surge a motor draws at the instant it starts, typically 2–4× its running load. The generator must meet the surge, not just the running load.
A: Add the running watts of everything that will run together, then add the largest single motor’s starting watts on top. Apply a 20–25% margin, then convert to kVA using a 0.8 power factor. Feed that total into a generator sizing calculator.
A: Not for every load. You add the running watts of everything, plus the starting surge of only the single largest motor — because two motors rarely start at the same instant. Summing every surge overstates the load and buys a generator you do not need.
A: A diversity factor is the ratio of a building’s actual peak demand to its total connected load. Because not every machine runs at once, real demand is often 60–90% of the connected total. Applying it avoids oversizing on large commercial installations.
A: Spread single-phase loads evenly across the three phases so no phase carries more than its share. An imbalance of more than a few percent causes voltage differences, overheating, and premature tripping — even when the generator is not fully loaded.
A: Take your total load in kW (running watts plus the largest starting surge, with margin), divide by a 0.8 power factor to get kVA, then match it to a generator rated at or above that figure. Our sizing calculator does the matching from your load.
A: A generator handles its rated kVA at a 0.8 power factor continuously, plus a short-term surge margin for motor starting. Exceed the continuous rating and it overheats and trips; exceed the surge margin and motors stall on start.
