Generator sizing calculator
Add what you need to keep running and get the generator size. Motor starting surges are counted properly, one at a time, so you are not sold twice the generator you need.
Inputs
Result
Where the watts go
Running watts are what each load draws continuously. The last column is the extra a motor needs at the instant it starts, over and above its running draw. Only the largest of those counts, which is the row marked below.
| Appliance | Qty | Running W | Extra to start |
|---|
The mistake that sells people twice the generator
Nearly every generator calculator adds up the starting watts of everything on your list. That is wrong, and it is wrong in the direction that costs you money.
Motors do not all start at the same instant. Your fridge compressor kicks in, finishes, and some minutes later the well pump starts. The worst moment your generator will ever see is everything running steadily while the single hardest-starting motor kicks in.
Two identical fridges do not need two surges of headroom; they need one. The results above show what the summed-surge method would have told you, and the gap is often 50% or more. On a home standby unit that difference is thousands of dollars and a bigger gas line.
Running watts and starting watts are two different ratings
A generator is advertised with both, something like 7,500 running and 9,500 starting. Your running load has to fit inside the first and your peak inside the second, and a unit can easily pass one test and fail the other. Check them separately.
Do not plan on using all of it
A generator should carry a continuous load of about 80% of its rating. Running flat out leaves nothing for a motor start, makes the set hot and noisy, and shortens its life. The sizing above already builds that margin in, which is why the recommended size looks larger than your running total.
The opposite case matters too, especially on diesel. Running a diesel set at under about a third of its rating for long periods causes wet stacking, where unburnt fuel fouls the exhaust. Oversizing is not a free safety margin.
Altitude and heat both cost you output
Thin air means less oxygen and less power. In Denver on a 95 °F afternoon a generator delivers around 86% of its sea-level nameplate, so a set that was exactly right at sea level is now undersized. These are typical rules of thumb; where you have the manufacturer's derate curve, use theirs instead.
Questions
What size generator do I need to run a house?
It depends entirely on what you need running at once, not the size of the house. A fridge, furnace blower, well pump and some lights comes to about 2,800 running watts and a 4,800 watt peak, which needs a 4 kW set — sized by the well pump's start rather than the running load. Adding central air conditioning or an electric water heater pushes you toward 14 to 20 kW, because those are large continuous loads.
Do I add up all the starting watts?
No, and this is the single most common error. Motors start at different times, so you need your total running watts plus the extra demanded by the one hardest-starting motor. Summing every surge typically overstates the requirement by half or more.
What is the difference between running and starting watts?
Running watts are the continuous draw. Starting watts are the brief surge a motor pulls as it spins up, often two to three times its running figure. Resistive loads such as heaters, kettles and incandescent lights have no surge at all; anything with a motor or compressor does.
Can I run my generator at full capacity?
Not continuously. Aim for about 80% of the rating, which leaves headroom for motor starts and keeps the set from overheating. On a diesel set, avoid the other extreme as well: long periods below about a third load cause wet stacking.
Does altitude affect generator size?
Yes, and people forget it. Reckon on losing about 3% of output per 1,000 feet above 1,000 feet, plus roughly 1% per 10 °F above 77 °F. At 5,000 feet on a hot day you have lost something like 14% of the nameplate, so the generator has to be correspondingly larger.
What is the difference between kW and kVA?
kW is real power, kVA is apparent power, and they are related by the power factor. Generators are commonly rated at 0.8 power factor, so a 20 kW set is quoted as 25 kVA. For a house of ordinary appliances, size in watts and treat kVA as the spec-sheet translation.
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