BTU is not watts
The confusion that starts most of the wrong answers in this category.
BTU per hour is a cooling capacity — how much heat the unit can move. Watts is electrical consumption — how much power it takes to do it. They are related but not interchangeable, and the ratio between them is the unit's efficiency.
A 5,000 BTU window unit does not consume 5,000 watts. It consumes somewhere in the region of 450 to 600, because a heat pump moves several times more heat energy than the electrical energy it uses. That is the whole point of refrigeration.
Which means you cannot convert BTU to watts with a fixed factor. Two units of identical BTU rating and different efficiency draw different amounts, and the only reliable source is the nameplate on your own machine.
Published figures
| Unit | Running watts | Starting watts | Ratio |
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| Window AC, 5,000 BTU | 450-600 | 900-1,200 | 2x |
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| Window AC, 10,000 BTU | 900-1,200 | 1,800-2,400 | 2x |
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| Central AC, 2 ton | 2,000-2,500 | 3,500-4,500 | ~1.8x |
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| Central AC, 3 ton | 3,000-3,500 | 5,000-6,000 | ~1.7x |
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| Central AC, 4 ton | 4,000-5,000 | 6,500-8,000 | ~1.6x |
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| RV roof AC, 13,500 BTU | 1,200-1,700 | 3,000-3,500 | ~2.3x |
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All figures are Champion Power Equipment's published ranges. Ratios are calculated from the midpoints. A "ton" of cooling is 12,000 BTU per hour, so a 3 ton system is 36,000 BTU — the units are conventional in the trade rather than intuitive.
Duty cycle: the number that decides watt-hours
Like a refrigerator, an air conditioner cycles. It runs until the thermostat is satisfied and then stops, so its average consumption across a day is well below its running figure.
Unlike a refrigerator, the duty cycle varies enormously — with outdoor temperature, insulation, sun exposure, how many people are in the house and what the thermostat is set to. On a mild day it might run a quarter of the time; on a 100°F afternoon with a system sized tightly, close to continuously.
Which is why this page gives you starting and running watts and does not give you a daily kilowatt-hour figure. Any number we published would be wrong for most readers, and your own nameplate plus your own thermostat behavior is the only honest route to one.
Why the starting surge is so large
A compressor is a motor starting under load — it has to begin compressing refrigerant against system pressure from a standing start.
Two consequences worth knowing:
- Restarting soon after shutdown is hardest. If the system has not equalized, the compressor starts against full head pressure. This is why a unit can start fine ten times and trip on the eleventh, and why short-cycling is bad for the equipment as well as for your generator.
- A three-minute delay helps. Many systems have a built-in anti-short-cycle timer for exactly this reason. If yours does not, giving it a few minutes between stop and start makes a real difference.
The mechanism, in full.
Two ways to make a surge fit
If the running figure fits your machine but the starting figure does not — which is common — there are two real fixes.
1. A soft starter. Micro-Air publishes a 65-75% reduction in starting current against the compressor's locked-rotor amperage. On a 3 ton system needing 5,000-6,000 starting watts, that is the difference between needing a 9,500W machine and a smaller one.
It fits to the air conditioner, is a common RV and marine retrofit, and for a residential system is work for an HVAC technician.
2. Load management. Standby generator installations often include a module that holds the air conditioner off while other large loads run, then lets it start. Everything works; it just does not all work at once. Sizing a standby unit covers what that saves.
Inverter air conditioners behave differently
The one place the arithmetic on this page is genuinely improving.
A conventional air conditioner runs its compressor at full speed and cycles on and off. An inverter-driven unit varies compressor speed continuously, so it starts gently rather than slamming on, and settles to a partial load once the space is at temperature.
Both effects help backup power: a much lower starting surge, and an average draw below the nameplate. If you are replacing an air conditioner and expect to run it on backup power at some point, that is a specification worth paying for.
What this means for equipment
| You want to run | What that takes |
|---|
| A 5,000 BTU window unit for a few hours | A 2kWh battery with 2,000W+ of inverter |
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| A 10,000 BTU window unit | Marginal on a battery; comfortable on a 3,500W generator |
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| An RV roof unit | A 3,500W-class inverter generator, or two smaller ones in parallel |
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| Central air, 2-3 ton | A 7,500-9,500W generator, or a smaller one plus a soft starter |
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| Central air on a battery | Not viable at any portable size |
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Derived from the published wattage ranges above with margin for the surge. Your system's own nameplate is the figure to size against, and an older system draws more than a current one of the same capacity.
Cheaper than powering it
Worth saying because air conditioning is the load that makes backup power expensive.
- Cool one room, not the house. A single small window unit in a bedroom is a fraction of the load and most of the benefit.
- Pre-cool before the outage if you have warning. Thermal mass is free storage.
- Fans instead of cooling. A 20-40W fan makes a warm room tolerable for a fiftieth of the energy. This is the highest-value substitution available.
- Close blinds on the sun side. Free, and it reduces the load you would otherwise be paying to remove.
Can a power station run an air conditioner works the battery case through in detail.
