Running watts, starting watts, and which one sizes the machine
Anything with a motor in it draws far more current at the instant it starts than it does once it is turning. The rotor is stationary, there is no back-EMF opposing the supply, and the winding looks close to a short circuit for a fraction of a second. That inrush settles within a second or two as the motor comes up to speed. Depending on the motor type and whether it starts against a load, the surge typically runs somewhere between two and six times the running figure, and a compressor starting against head pressure sits at the unpleasant end of that range.
A generator has to satisfy two conditions simultaneously. It has to sustain the total running load indefinitely, which is thermal and is limited by the engine and the alternator windings. And it has to survive the instantaneous peak without the voltage collapsing far enough to stall everything else, which is limited by the alternator's ability to deliver current briefly and by how quickly the engine governor responds. Manufacturers publish both figures, and the surge figure is a short-duration rating measured in seconds, not something you can lean on.
The peak that matters is not the sum of every starting watt on the list. It is everything already running, plus the additional surge of whichever single item starts at the worst moment. That is the calculation this page does by default. Unchecking the stagger box adds every surge together instead, which is the pessimistic case and is worth looking at if you have several thermostatically controlled loads that could plausibly cut in together after an outage.
Typical figures, and why you should not trust them
| Item | Running W | Starting W |
|---|---|---|
| Refrigerator or freezer | 500-800 | 1,500-2,500 |
| Sump pump, 1/3 to 1/2 hp | 600-1,000 | 1,500-3,000 |
| Well pump, 1/2 hp | 900-1,100 | 2,000-3,500 |
| Furnace blower | 600-900 | 1,500-2,500 |
| Window air conditioner, 10k BTU | 1,000-1,200 | 2,000-3,500 |
| Microwave, 1,000 W output | 1,300-1,600 | same |
| Space heater | 1,500 | same |
| LED lighting, whole house | 100-300 | same |
| Table saw, 1.5 hp | 1,400 | 3,500-4,500 |
These are orientation figures for filling the box quickly, not specifications. The spread within a category is enormous: two refrigerators of the same size can differ by 40 percent, and an inverter-driven compressor behaves nothing like a conventional one on starting. The nameplate on the actual appliance beats any table, and where the nameplate gives amps rather than watts, multiply by the voltage. If you want the number that is genuinely true for your appliance, a clamp meter on the supply cord during a start cycle will tell you, and it frequently disagrees with the table by a wide margin in both directions.
The corrections people skip
Altitude is the big one. A naturally aspirated engine loses roughly 3 percent of its output per 1,000 feet of elevation because there is less oxygen per stroke, so a 7,000 watt generator in Denver behaves like a 5,900 watt one. High ambient temperature costs more on top of that. If the site is high, size for the sea-level rating that leaves you enough after the derate, which is what this page does when you enter an altitude.
Power factor is the quieter one. Generator ratings are usually given in watts for portable units and in kVA for larger ones, and the two are only the same for a resistive load. A load list dominated by motors has a power factor well below one, and the alternator is limited by current, which follows the kVA. A unit rated 5,000 W at a power factor of 1 may deliver noticeably less real power into a motor-heavy load. Where a machine is rated in kVA, multiplying by 0.8 gives a reasonable working watts figure for a mixed load.
Fuel type matters for output as well as for logistics. The same engine typically produces a few percent less on propane than on gasoline and less again on natural gas, so a tri-fuel unit's headline number is the gasoline one and the propane figure is in the fine print.
Fuel, runtime and the part that decides whether you were right
Sizing generously has a running cost. Engines are least efficient lightly loaded, and a unit loafing at 20 percent of its rating burns a surprising fraction of what it burns at 60 percent, so an oversized generator does not deliver proportionally longer runtime on a tank. Most portables land somewhere around half a gallon per hour at a quarter load and a gallon or more at three-quarters, and the published runtime figures are almost always quoted at 25 or 50 percent load rather than at anything realistic. Work out your own: divide the tank capacity by the consumption at the load fraction you actually calculated above, then ask how many of those refills you can physically supply during a three-day outage, at night, in whatever weather caused the outage. That number, more often than the watt total, is what determines whether the machine turns out to be the right one. Sizing for a load you can sustain for the duration you actually need beats sizing for the load list you wrote optimistically on a calm afternoon.
Questions people ask
What size generator do I need to run a house?
It depends entirely on what you mean by run. Keeping a refrigerator, a furnace blower, a sump pump and some lighting alive through an outage is commonly in the 5,000 to 7,500 watt range once the starting surges are accounted for. Adding a well pump usually pushes it up. Running central air conditioning, an electric water heater or an electric range changes the problem completely and lands in standby-unit territory, because those are multi-kilowatt continuous loads with no way around them. The productive exercise is the one this page does: list what you genuinely need, not what you own, and size for that. Most people discover the list is shorter and the generator smaller than they assumed, right up until they add the air conditioning.
Why is my generator struggling to start one appliance?
Almost always the starting surge rather than the running load. A refrigerator that draws 700 watts while running may demand 2,200 for a second or two at start-up, and if the generator is already carrying most of its running capacity there is nothing left to supply that. The symptom is a voltage sag, the engine bogging audibly and either the appliance failing to start or the generator tripping its breaker. Two things help: start the largest motor first while the machine is otherwise unloaded, and where the equipment permits it, fit a soft starter on the offending load. A soft starter can cut inrush dramatically and is often far cheaper than the next generator size up.
Can I plug a generator into a wall outlet to power the house?
No. This is called backfeeding and it is the one thing on this page that is unambiguously dangerous rather than merely unwise. Energising the house wiring through a receptacle also energises the service drop back through the transformer, where it becomes a lethal high voltage on lines that utility crews have every reason to believe are dead. It also bypasses the overcurrent protection on that circuit entirely. A generator that connects to building wiring needs a transfer switch or a listed interlock kit that makes it physically impossible for the utility supply and the generator to be connected at the same time, and installing one is permit and licensed-electrician work in most jurisdictions. If you want to avoid all of that, run appliances on extension cords directly from the generator.
Do I need an inverter generator for electronics?
For sensitive electronics it is worth the money. A conventional generator produces its output directly from an alternator whose frequency and waveform follow engine speed, so both wander as the load changes, and the total harmonic distortion is often in the range where switching supplies and motor controls become unhappy. An inverter unit rectifies the output and synthesises a clean waveform electronically, holding frequency and distortion tight regardless of engine speed, which also lets the engine throttle down under light load and makes the machine markedly quieter and more economical. The trade-off is cost per watt and available size range. For a load list of lights, pumps and a fridge, a conventional unit is fine. For anything with a sensitive control board, or if you want the machine to be tolerable to stand next to, an inverter is the better buy.