Explainers

Heat pump size classes: how efficiency and cold-weather output change with size

By Emma Wilson · Published October 27, 2026 · Data certified as of October 27, 2026

Small mini-splits lead the efficiency tables: the 6,000 Btu/h class has a median HSPF2 of 14.6, against 10 in the 18,000 Btu/h class. Among split systems the 2-ton class keeps the most heat at 5°F (median 82%) and the 3.5-ton class the least (52%).

How this site sorts by size

Size classes on this site are bands of certified cooling capacity. A split system lands in the band of its highest certified cooling capacity, so a nominal 3-ton outdoor unit certified at about 33,000–36,000 Btu/h sits in the 3-ton class. Mini- and multi-splits use bands in Btu/h, from 6,000 to 60,000. The bands follow clusters in the data and are listed on the method page. They describe the certified product; they are not a guide to what size a building needs, and this site gives no sizing advice.

Single-package units form one class of their own; there are too few of them per size to rank them by size. The largest class is the 18,000 Btu/h mini-split class with 311 rated products.

Efficiency falls with size, then levels out

Among mini- and multi-splits the smallest units list the highest seasonal ratings. The 6,000 Btu/h class has a median HSPF2 of 14.6 and a median SEER2 of 25.5. From the 18,000 Btu/h class upward the median HSPF2 stays between 9.2 and 10.2. Split systems show a smaller range, from 8.1 to 9.3 across their classes.

Cold-weather output does not follow size

The share of 47°F capacity kept at 5°F follows a different pattern. Among split systems the 2-ton class keeps a median of 82%, and the 3.5-ton class only 52%. The 3.5-ton class is also the one with the fewest variable-speed units, 18% against 75% in the 2-ton class. Among mini- and multi-splits every class is variable-speed, and the medians range from 75% to 110%.

SEER2 across the classes

Cooling ratings follow the heating pattern. Among mini- and multi-splits the median SEER2 falls from 25.5 in the 6,000 Btu/h class to 20 in the 48,000 Btu/h class. Among split systems the medians stay between 15.6 and 18, with the 2-ton and 3-ton classes at the top. The single-package class has the lowest median of all, 15.5.

The share of products with the EPA cold-climate designation also varies by class, from under half to over nine in ten; the cold-climate explainer charts it class by class.

Where most of the products sit

Most rated products fall into a handful of classes. The 6 classes with more than 200 rated products are 2-ton split, 3-ton split, 9,000 Btu/h mini, 12,000 Btu/h mini, 18,000 Btu/h mini, and 24,000 Btu/h mini; together they hold 57% of the list. The finder starts in the 3-ton split-system class, one of them, and every other class is one click away.

COP at 5°F by class

COP at 5°F is the steadiest measure across sizes. The class medians lie between 1.95 and 2.40, and 11 of the 20 classes have a median of exactly 2.00. The 6,000 Btu/h mini-split class has the highest median, 2.40. The spread inside each class is far wider than the spread between class medians, so COP is ranked inside classes like the other measures.

How full the classes are

The classes differ a lot in size. The 18,000 Btu/h mini-split class holds 311 rated products, the 6,000 Btu/h mini-split class 27. Every rank on this site is printed with the size of its class: "3rd of 147", never just "3rd".

Why ranks stay inside a class

The small classes list higher seasonal ratings, so a ranking of all products together would mostly rank sizes. Every product page on this site therefore places a unit only against its own class, and the finder starts from a size class. A product with an ordinary HSPF2 for its type can still lead its own class, and those leaders are named on each size-class page.

Output at 5°F is the one measure where size dominates completely, so it is ranked inside classes only and never across the whole list. The share kept at 5°F and COP at 5°F do not depend on size in the same way and are compared across classes in the chart on the finder page.

All figures refer to the list certified as of October 27, 2026.