Air Conditioner Electricity Cost

Air conditioning is usually the single biggest line on a summer electricity bill. A 3-ton central unit (36,000 BTU) rated SEER 14, running 8 hours a day, costs about $99 a month at the current US average of roughly $0.16/kWh.

SEER vs EER, and why they are not interchangeable

EER (Energy Efficiency Ratio) is a snapshot: BTU per hour divided by watts at one fixed test condition, 95°F outside and 80°F inside at 50% humidity. Divide the BTU rating by EER and you get the actual power the unit draws on a hot day.

SEER (Seasonal Energy Efficiency Ratio) is an average over a whole simulated cooling season, including all the mild days and the cycling. It is always the higher of the two numbers for the same machine, typically by 2 to 3 points, because most of the season is easier than the EER test condition.

That difference matters for how you read the answer. Enter a SEER figure and the watts shown below are a seasonal average, which is the right basis for a monthly bill but understates what the unit pulls at 2pm in August. Enter an EER figure and the watts are the real peak draw, but the monthly cost then assumes every hour is a design-condition hour, which overstates the bill. Neither is wrong; they answer slightly different questions.

One more wrinkle if you are reading a nameplate bought since 2023: US ratings switched to SEER2 and EER2, measured against higher external static pressure. A SEER2 number is roughly 4.5% lower than the old SEER for the same unit, so SEER2 13.4 is about SEER 14.

The formula

Watts = BTU_per_hour / EER (or SEER)

kWh per day = Watts / 1,000 x runtime_hours

Monthly cost = kWh per day x 30 x electricity_rate

Interpreting BTU ratings

AC capacity is rated in BTU/hour. Common sizes: 5,000 BTU (small window unit, ~150 sq ft), 12,000 BTU (1 ton, ~550 sq ft), 24,000 BTU (2 ton, ~1,000 sq ft), 36,000 BTU (3 ton, ~1,500 sq ft). Larger is not always better — an oversized unit short-cycles, runs inefficiently, and removes less humidity.

Minimum efficiency has ratcheted up over the years. New split systems must now meet roughly SEER 14 in northern states and SEER 15 in the south (SEER2 13.4 and 14.3 in the current wording). High-efficiency units reach SEER 20 to 25. Going from SEER 14 to SEER 18 cuts electricity use by 22%, since 1/14 against 1/18 is a 22.2% reduction, and that saving compounds over the fifteen-odd years the unit will run.

Runtime estimate

A well-sized AC in a typical climate runs 8-12 hours on a hot summer day. On peak days, runtime can reach 16-20 hours. For annual cost, sum the monthly usage across the cooling season rather than multiplying 12 months of peak-day runtime.