A ton of cooling.

One ton of cooling is 12,000 BTU per hour. The name is literal: it is roughly the rate of cooling you would get from a ton of ice melting over a day, from back when that is how buildings were cooled.

So a 3 ton system removes about 36,000 BTU per hour, a 4 ton about 48,000, and so on. Residential equipment comes in half ton steps.

Reading it off your unit

The model number on the outdoor unit data plate almost always contains it. Look for a two digit group that is a multiple of 6: 18, 24, 30, 36, 42, 48, 60. Divide by 12. A 36 in the model number is a 3 ton.

It is not a guarantee across every manufacturer, but it is right far more often than not, and it is a useful thing to know before somebody tells you what you have.

Bigger is not better

This is the part that surprises people. An oversized system cools the air down to the thermostat setpoint quickly and shuts off. That sounds ideal and is not, for two reasons.

It never runs long enough to pull much moisture out, so the house gets cold and clammy rather than comfortable. And short cycling is hard on a compressor, which would rather run for long steady periods than start repeatedly.

An undersized system runs constantly and never quite catches up on the worst afternoons, which is its own problem. Correct sizing comes from a load calculation on the actual house, not from a rule of thumb about square footage.

Two systems

Plenty of larger Valley houses run two or more systems rather than one large one, which is usually the better answer for a two storey house. Each gets checked separately, and additional systems in the same visit are priced lower here because the drive is already paid for.