What Size Generator Do I Need? Running vs Starting Watts

Running watts vs starting watts

Every motor-driven appliance has two wattage figures. Running watts is what it draws steadily once up to speed. Starting watts (or surge) is the brief spike — typically two to three times the running watts — that an induction motor demands in the first fraction of a second as it overcomes inertia. A refrigerator might run at 700 W but surge to 2,100 W on startup; a well pump might run at 1,000 W and surge to 3,000 W. Resistive loads such as lights, heaters and kettles have no surge: their starting and running watts are equal.

Sizing a generator that ignores surge is the classic error. The generator can be perfectly adequate for the steady load yet stall or trip the moment the well pump kicks in. You must size for the worst instant, not the average.

The sizing method

The correct approach adds all the running watts of loads that run simultaneously, then adds only the single largest surge (because two large motors almost never start in the same instant). In formula form:

running total = Σ running watts

peak = running total + (largest surge − that item’s running watts)

The reason for subtracting the item’s own running watts is that it is already counted in the running total; only its extra surge needs to be added. Finally apply a margin (about 20%) so the generator is not run flat-out: recommended kW ≈ peak / 1,000 × 1.2.

Worked example: fridge, lights and a well pump

Suppose you must run, at the same time, a refrigerator (700 W running, 2,100 W surge), lights (200 W, no surge) and a well pump (1,000 W running, 3,000 W surge).

1. Running total: 700 + 200 + 1,000 = 1,900 W.
2. Largest surge is the well pump: its extra surge is 3,000 − 1,000 = 2,000 W.
3. Peak: 1,900 + 2,000 = 3,900 W.

So you need a generator that delivers at least 1,900 running watts continuously and can surge to about 3,900 watts. With a 20% margin that points to roughly a 4.7 kW unit. The Generator Size calculator performs exactly this running-plus-largest-surge calculation.

Finding each appliance’s watts

Read the nameplate where you can. Where only volts and amps are given, watts = volts × amps: a 120 V appliance drawing 6 A is 720 W. For motor surge, multiply running watts by 2 to 3 depending on motor type. The Appliance Wattage calculator converts volts and amps to running and starting watts, and the appliance wattage table lists typical values for common loads as a starting point.

Generator types and what the size means

• Recreational inverter generators (1–2 kW) cover a few small loads and electronics quietly.
• Mid-size portables (3–5 kW) cover a fridge, lights, a sump or well pump and small tools — the range our example lands in.
• Large portables and standbys (7–20+ kW) can run an entire home including central air and electric appliances.

Why motors surge and resistive loads do not

The surge that dominates generator sizing comes entirely from motors. When an induction motor first energizes it is not yet turning, so it behaves almost like a short circuit and pulls a large inrush current until it spins up, which is why its starting watts run two to three times its running figure. Anything with a motor behaves this way: refrigerators, freezers, well pumps, air conditioners, power tools and furnace blowers. Purely resistive loads have no such spike because they have no inertia to overcome; a heater, a kettle, an incandescent lamp or a toaster draws the same watts the instant it switches on as it does a minute later. Separating your loads into motor-driven and resistive is the first step in any honest generator estimate, because only the motors contribute surge.

Why only the single largest surge counts

It might seem safest to add up the starting watts of every motor, but that drastically over-sizes the generator and wastes money on capacity you will never use. In practice motors do not all start in the same instant; a refrigerator and a well pump cycle on their own schedules and the odds of two large motors hitting their inrush in the same fraction of a second are slim. The accepted method therefore adds all the running watts, which do coincide, plus the surge of only the single largest motor, since that is the worst instant the generator must survive. This is why the example adds the well pump surge but not the refrigerator surge, and it keeps the recommended size realistic rather than wildly conservative.

Choosing which loads to run

During an outage you rarely need everything at once, and choosing your loads deliberately can shrink the generator you need. A common strategy is to identify the essentials, refrigeration, a few lights, a well or sump pump, a furnace blower, and perhaps a circuit for charging phones and laptops, and to size for those running together rather than for the whole house. Large discretionary loads such as electric ranges, water heaters and central air conditioning are what push a generator into the large standby class, so if you can do without them or run them one at a time, a mid-size portable will cover the rest. Being intentional about what runs simultaneously is often the cheapest way to keep the generator size, fuel use and noise manageable.

Inverter generators and clean power

Beyond raw wattage, the quality of the power matters for modern electronics. Inverter generators produce a clean sine wave and adjust engine speed to the load, which makes them quieter, more fuel-efficient at light load and safe for sensitive devices such as laptops, televisions and variable-speed equipment. Conventional generators run the engine at a fixed speed and produce a rougher waveform that some electronics dislike. For a build that mixes a few motors with a lot of electronics, an inverter generator sized to the running-plus-largest-surge figure usually gives the best balance of clean power, efficiency and noise, at a somewhat higher purchase price than a basic open-frame unit of the same wattage.

Don’t forget fuel and runtime

Sizing for watts is only half the picture; runtime depends on tank size and consumption. Runtime h = tank gallons / consumption in gallons per hour. A 7-gallon tank burning 0.75 gal/h at load runs 7 / 0.75 = 9.3 hours. The Generator Fuel / Runtime calculator covers this; you enter your own fuel figures, so there are no prices to keep current. Size generously, prioritize the loads you truly need during an outage, and never run a generator indoors.