What is the difference between EER and SEER?

EER is a single-point efficiency measured at one fixed condition (typically 35°C / 95°F outdoors). SEER is a seasonal average across a range of temperatures, so it is usually a higher number than EER for the same unit. EER best predicts hot-day power draw; SEER best predicts whole-season energy use.

What is a good SEER rating?

For a modern mini-split, SEER 16–20 is solid and 20+ is excellent. Window and portable units are lower: a window unit around CEER 11–12 is good, and most portables sit near EER 8–10. Higher is better, but the payback on a premium SEER depends on how many hours you actually run it.

How do I convert EER to watts?

Watts = BTU ÷ EER. A 12,000 BTU unit with an EER of 10 draws about 1,200 W; the same unit at EER 12 draws about 1,000 W — roughly 17% less power and 17% less running cost for identical cooling.

SEER2 is the updated US test standard (effective 2023) that uses higher external static pressure to better reflect real ducted installs. SEER2 numbers run about 4–5% lower than the old SEER for the same equipment, so don't compare a SEER2 figure directly against an older SEER one.

SEER vs EER: Air Conditioner Efficiency Ratings Explained

Published: May 24, 2026

Every air conditioner sold today carries an efficiency rating — EER, CEER, SEER or SEER2. They all answer the same question with different precision: how much cooling do you get per unit of electricity? That ratio is exactly what decides your running cost, which is why CoolingMetrics uses it to rank units by cost per hour to run.

The one ratio that matters

Efficiency is cooling output divided by power input:

EER = cooling capacity (BTU/hr) ÷ power input (W)

A 10,000 BTU unit that draws 1,000 W has an EER of 10. The same 10,000 BTU from only 830 W is an EER of 12 — more cooling per watt, so a lower bill for the identical job.

Because power is what you pay for, you can flip the ratio to get watts directly:

Watts ≈ BTU ÷ EER

This is the estimate CoolingMetrics uses when a manufacturer states BTU but not power draw — and it’s why the efficiency number, not the BTU, is the real lever on running cost.

EER vs CEER vs SEER vs SEER2

Rating	What it measures	Best for
EER	Efficiency at one fixed hot condition (35°C / 95°F)	Predicting peak-day power draw
CEER	Like EER but also counts standby power (US window/portable)	Comparing window & portable units
SEER	Seasonal average across a temperature range	Whole-season energy use (mini-splits)
SEER2	SEER under the 2023 US test (higher static pressure)	Current US ducted/mini-split units

The key trap: these numbers are not interchangeable. SEER is a seasonal average that includes mild days when the unit sips power, so it’s almost always a higher number than the same unit’s EER. Comparing a mini-split’s SEER 19 against a portable’s EER 9 makes the portable look far worse than the gap really is — though the portable usually is less efficient.

What counts as a good number

Mini-split / ductless: SEER 16–20 is good, 20+ is excellent, premium inverters reach 30+.
Window units: CEER 11–12 is good; ENERGY STAR models start around there.
Portables: most land at EER 8–10. They’re the least efficient type because the single-hose designs pull conditioned air out of the room.

Higher is always cheaper to run for the same BTU — but a premium efficiency rating only pays back if you run the unit enough hours. Use the numbers below to see how much.

How efficiency translates to money

Take a 12,000 BTU unit and the UK average 24.5p/kWh, run 8 hours/day:

EER / SEER	Power draw	Cost per hour	Cost per month (8h/day)
8	1,500 W	36.8p	~£88
10	1,200 W	29.4p	~£71
12	1,000 W	24.5p	~£59
18	667 W	16.3p	~£39

Same cooling, same room — the only thing that changed is the efficiency rating, and it nearly halves the bill from EER 8 to SEER 18. That’s the entire argument for reading the efficiency number before the price tag.

The bottom line

EER and SEER aren’t marketing fluff — they’re the direct multiplier on your electricity bill. When a listing states one, CoolingMetrics shows it; when it doesn’t, we estimate the power draw from BTU and the typical efficiency for that AC type (and flag it as estimated). Either way, the comparison ranks by the number that follows from efficiency: cost per hour to run.

Next: see how much it costs to run an air conditioner, work out your BTU by room size, or jump to air conditioners ranked by cost to run.