How do I determine if adding an air conditioner will overload a circuit?

Question

I'm going to take advantage of some Memorial Day sales and pull the trigger on a portable AC unit for my "basement-ish" (bottom of windowsill is mid chest, rather than head height) apartment. Before I do this, however, I've been researching online, and I want to make sure that I don't buy something that's going to overload the electric circuit it'll be connected to.

For the bedroom, the breaker indicates 15 Amps. The circuit will additionally have a desktop computer, two LCD displays, and a couple of lights. How can I tell if a new AC unit plus the existing items is going to cause tripped breakers or worse?

Answer 1

The only way to know what load a device draws is to put the device in to service and measure the load. The rating on the back is interesting, but many factors affect actual loads. An easy way to measure real loads is with a Kill-A-Watt.

Expect breakers to trip at 80% of rating, under continuous load. So 12A in your case.

Answer 2

write down the power rating of all those appliances (in watts) divide by the voltage and add them together

this will give you the total amps used on the circuit,

Answer 3

Get the BTU rating of the AC and divide by the SEER to get watts. If SEER is not available, 10 is a good rule of thumb. So a 5000 BTU AC would use 500 watts. Divide by line voltage (110) to get amps. In this case, 4.5 amps.