How the math works
Condensus mathCooling starts from the ENERGY STAR room sizing chart (for example 450 sq ft = 10,000 BTU, 550 sq ft = 12,000 BTU), then applies its adjustments: -10% for a heavily shaded room, +10% for a very sunny one, +600 BTU per person beyond two and +4,000 BTU for a kitchen. Ceilings over 8 ft scale the load up in proportion, and insulation adds or removes a share. Heating = floor area × a BTU-per-square-foot figure for your winters (20 in hot climates up to 60 in very cold ones), adjusted the same way.
Condensus mathA system is listed when its stated cooling capacity is at least your cooling need and no more than the next size up (the larger of +50% or +6,000 BTU), and its stated heating capacity, when the maker gives one, covers your heating need.
LimitsThese are rules of thumb for one room. A heat pump's heating output falls in cold weather, so near its low-temperature limit it delivers less than its rating. For a whole home, or anywhere winters are severe, a licensed installer's Manual J load calculation is the standard.
Questions
What happens if a mini-split is too big?
An oversized system reaches the set temperature quickly and cycles off before it removes much humidity, so rooms can feel clammy, and frequent cycling is harder on the compressor. Inverter systems can turn down, which helps, but only to their minimum speed.
Should I size a mini-split for heating or cooling?
Size for whichever need is larger. In most cold and mixed climates that is heating, so check the heating capacity and the lowest temperature the maker says it can heat at.
How many BTU do I need for 500 square feet?
The ENERGY STAR chart puts 450-550 sq ft at 12,000 BTU for cooling. In a cold climate the same room can need more heating than that, so a 12,000 BTU system with strong low-temperature heating, or the next size up, is common.