What size heat pump do I need?

What a load calculation actually measures

It adds up heat loss through every surface: wall and attic insulation levels, window count and glazing type, air leakage, and floor construction over a crawlspace or slab. Orientation counts too, because west-facing glass and a shaded hillside are not the same load. Two 1,600 square foot houses on the same street can differ by thousands of BTU per hour if one has been re-sided and re-windowed, which is why the shortcut fails.

Why oversizing is a real problem, not a safety margin

An oversized system satisfies the thermostat fast, shuts off, and repeats. That short cycling wastes the efficient part of the run, leaves temperatures uneven room to room, and does almost no dehumidification in a marine summer. Inverter-driven equipment tolerates it better than a single-stage compressor, but it still costs you comfort and it costs you money at purchase.

The 19°F target, and where to put the balance point

Sizing here is a decision about what happens near 19°F. We can select equipment that carries the entire load at design condition, or select smaller equipment that runs at high efficiency most of the winter and hands off to backup on the few coldest nights. Both are legitimate. Which one is right depends on what backup you already own and what you are willing to spend up front.

What we need from you to size it

Bring the build year, any insulation or window work you know about, the panel size, and last winter’s PSE bills if you have them. Bills are useful because measured consumption is a sanity check on a calculated load. If your PSE rebate depends on the ductless system being at least three-quarters of a ton in heating mode, the load calculation is what proves the selection is honest rather than reverse-engineered from a rebate table.