Sizing an emergency generator for an old AC with a missing label

Question

My daughter is trying to work out the current draw of an old AC with a worn label. She's trying to size an emergency generator that will power critical appliances in the event of a power outage. The AC label is completely worn, so there is no model number or energy specifications.

Some details about the AC: The home inspector and a handyman have independently told her they think it is from the 1990s, but can't be sure. It cools the 2000 sq ft house without problem throughout the hot Phoenix summer. It is on a 220V dedicated circuit with a 50A breaker and a 60A switch. The circuit breaker has never tripped. She has the hourly energy usage for the whole house, and she never goes above 11kW (a 118 degree day, with the AC, other household appliances, a 1hp pool pump and a 3.5 kW EV charger going). Typical mid summer hourly energy use peaks at 7-8kW.

I'm looking for answers for whether it's effective to estimate usage requirements for a generator using the circuit breaker, the hourly usage data, perhaps, taking apart the condenser unit to get to the compressor and putting a clamp ammeter on it, or maybe something else. Please don't suggest she replaces a perfectly running air conditioner, especially not with the suggestion that it is inefficient, as it is impossible to make an informed decision about energy savings without actually answering this question about the amp draw first.

Answer 1

She's trying to size an emergency generator.

Then the size you are looking for is going to be significantly more than the A/C, probably closer to the typical summer day peak.

The home inspector and a handyman have independently told her they think it is from the 1990s, but can't be sure.

Then it is likely due for replacement. That is bad news in terms of an extra big cost. It is good news because new units are much more energy efficient. Meaning both lower energy costs and a lower peak load, which means a smaller generator. So it may be worth considering sooner rather than later.

It is on a 220V dedicated circuit with a 50A breaker and a 60A switch.

Which is really a 240V dedicated circuit. 50A should indicate a maximum of 40A continuous, so that's the upper limit for this part of the generator load. 60A switch is irrelevant - that is a standard "big enough for any A/C system" disconnect switch.

The circuit breaker has never tripped.

Good, because if it did then that would be a sign of a serious problem. In fact, A/C (and other continuous loads) have oversized (125% over the actual continuous load) breakers which means trips should be extremely rare unless there is a component failure.

She has the hourly energy usage for the whole house, and she never goes above 11kW (a 118 degree day, with the AC, other household appliances, a 1hp pool pump and a 3.5 kW EV charger going). Typical mid summer hourly energy use peaks at 7-8kW.

These are useful numbers. If 11 kW is the peak of the peak and that includes 3.5 kW EV charging, subtract the 3.5 kW and you get 7.5 kW, which is right in line with the "typical" peak of 7 - 8 kW. Obviously you wouldn't be charging the EV from the generator, so that puts a new maximum needed of 8 kW. Which is around 33 A, quite a bit below the theoretical max. of the A/C at 40A, and includes other loads as well.

putting a clamp ammeter on it

That actually would give you a real-world number. But I wouldn't take it all apart, just put a clamp on one of the hots feeding the A/C inside the breaker panel.

So it looks like your target is on the order of 8 kW. Probably a good idea to upsize a bit because a generator will typically run better if it isn't being pushed to the limit.

But seriously take a look at a new air conditioner. Could save a lot in electricity.