I have an old Hampton Bay air conditioner. Model # HBQ051A, FL435765 1559. I don't know how many BTU's. Can you answer this?

The same plate or tag you got the model number from probably has the BTU`s
Input BTU`s and output BTU`s,, its somewhere maybe inside the furnace

I am not familiar with Hampton Bay, but I do know that if you can find what is called the tonnage of the unit you can convert. I ton equals 12000 BTU.

It truncated my answer above...

This unit does not have the output posted anywhere. The label simply has a model number and a service phone number.

I have one of these units and a similar one that's rated at 5000BTUs. Given the '51' in the model number and similar size to a 5k unit, my guess is that it's rated at 5100BTUs

All I can find on Hampton Bay air conditioner,says you have a window ac, would have helped if new this . When you remove the front cover,with a flashlight look inside near the top, usually hard to find you should find a tag,probably a glued on paper or film,with the BTU`s written on it

Hope this is of some help

Here is the following from the rating plate on my air conditioner

Hampton Bay HBQ051A-B
Volts:115
Hz:60
Cooling BTU/Hr: 5100
Amps: 5.3
Watts: 565
FER: 9
Refrigerant 22: 15.00 Oz