Short answer: the BTU number on a mini-split’s box is its heating output at a mild 47°F. At 5°F, MRCOOL’s own performance tables show its DIY 4th Gen units delivering only about 52-66% of that number. If you live where 5°F nights are normal, size for the 5°F capacity, not the nameplate, or plan on backup heat.
Why the rated number drops
A heat pump moves heat from outdoor air into your house. The colder the outdoor air, the less heat there is to move and the harder the compressor works, so both output (BTU/h) and efficiency (COP) fall as the temperature drops. Manufacturers that publish full performance tables let you see exactly how much.
MRCOOL DIY 4th Gen: published capacity at 5°F
MRCOOL publishes a heating performance table for each 4th Gen DIY E Star size, with total capacity and power input at outdoor temperatures from −13°F to 57°F (MRCOOL performance specifications, checked Oct 19, 2026). Here are the 5°F outdoor / 70°F indoor rows:
| Model | Rated heating (BTU/h) | Capacity at 5°F | Share of rating | Power input at 5°F | COP at 5°F (our calc.) |
|---|---|---|---|---|---|
| DIY 12K (115V) | 12,000 | 6,180 BTU/h | 52% | 0.92 kW | 1.97 |
| DIY 18K (230V) | 18,000 | 11,790 BTU/h | 66% | 1.52 kW | 2.27 |
| DIY 24K (230V) | 25,000 | 14,130 BTU/h | 57% | 2.41 kW | 1.72 |
| DIY 36K (230V) | 36,000 | 20,080 BTU/h | 56% | 3.28 kW | 1.79 |
Rated heating figures are from MRCOOL’s DIY E Star brochure. COP = capacity ÷ (input kW × 3,412). A COP of 2 still means about twice the heat per kWh of an electric baseboard heater (COP 1).
The 18K stands out: it keeps about two-thirds of its rating at 5°F, and its 11,790 BTU/h nearly matches the 12K’s full nameplate. At −13°F, the same table shows the 12K at 3,750 BTU/h and the 18K at 6,320 BTU/h (70°F indoor), so even “operates down to −13°F” means a small fraction of rated heat.
What other brands publish
Not every brand puts a 5°F capacity on its product page:
| Model | Voltage | Heat BTU/h | SEER2 | HSPF2 (IV / V) | Heat at 5°F | Min heat temp | Install | Price |
|---|---|---|---|---|---|---|---|---|
| MRCOOL DIY 4th Gen E Star 12K DIY-12-HP-WMAH-115C25 / DIY-12-HP-C-115C25 | 115V | 12,000 | 22.5 | 9 / 7 | 6,180 BTU/h | -13°F | DIY quick-connect | $1,775 MRCOOL DIY Direct list price; shown sold out when checked |
| MRCOOL DIY 4th Gen E Star 18K DIY-18-HP-WMAH-230C25 / DIY-18-HP-C-230C25 | 208-230V | 18,000 | 22.7 | 9.8 / 7.2 | 11,790 BTU/h | -13°F | DIY quick-connect | see store |
| MRCOOL DIY 4th Gen E Star 24K DIY-24-HP-WMAH-230C25 / DIY-24-HP-C-230C25 | 208-230V | 25,000 | 20.5 | 9.2 / 7.1 | 14,130 BTU/h | -13°F | DIY quick-connect | see store |
| MRCOOL DIY 4th Gen E Star 36K DIY-36-HP-WMAH-230C25 / DIY-36-HP-C-230C25 | 208-230V | 36,000 | 18.3 | 8 / 6.4 | 20,080 BTU/h | -13°F | DIY quick-connect | see store |
| Pioneer Diamante Pro 12K (115V) WYT012ALSI20RL (WT012ALSI20HLF / YN012ALSI20RPF) | 115V | 12,000 | 20 | 9.5 / 8.2 | n/p (COP 1.8) | -4°F | Pro install required | $928 Pioneer official store sale price (regular $1,329) |
| Senville AURA SENA-12HF SENA-12HF | 208/230V | 13,000 | 25.4 | 13.7 / 9.6 | n/p | -22°F | Pro install required | $1,399.99 Senville official store, 16 ft kit |
- Pioneer Diamante Pro 12K publishes a COP of 1.8 at 5°F but not the capacity on the product page. It’s rated to operate down to −4°F (Pioneer).
- Senville AURA SENA-12HF is rated to heat down to −22°F and lists an HSPF2 Region V of 9.6, the best of the three brands here, but its product page doesn’t give a 5°F capacity (Senville).
If a seller can’t show you a low-temperature capacity table, ask for the submittal or engineering data before you buy. For cold-climate shopping, the NEEP cold-climate air-source heat pump list is another place to compare published 5°F data across brands.
How to use this when sizing
- Estimate your space’s heating need with our BTU sizing calculator, which flags it when a kit’s published 5°F output falls short.
- Compare that need with the 5°F capacity, not the nameplate, if your winters reach 5°F.
- If it falls short, you have three options: size up (in MRCOOL’s lineup, the 18K delivers almost twice the 12K’s heat at 5°F), split the space into two zones, or keep your existing heat as backup for the coldest days.
Example: a 500 sq ft room in Chicago (zone 5) with our rule of thumb needs about 20,000 BTU/h on a design night. A 12K delivers about 6,180 BTU/h at 5°F, and an 18K about 11,790. Neither covers it alone. The 24K’s 14,130 BTU/h gets closer, but improving insulation and air sealing may be the cheapest fix of all.
What it means for your bill
Lower COP in deep cold means higher running costs on those days, but HSPF2 already averages cold and mild days over a season. Use the Region V HSPF2 in our heating cost calculator for a cold-climate estimate. MRCOOL’s 12K is rated at 7.0 HSPF2 in Region V (MRCOOL brochure).