Air conditioner daily energy use:
formula and worked example

What daily energy use tells you about an AC

Air conditioning is frequently the biggest single electrical load in a home during hot months, and the quantity that shows up on the bill is energy in kilowatt-hours, not power in kilowatts. Daily energy use connects the two by multiplying how hard the unit works by how long it works.

With a daily figure you can work out the monthly consumption, compare it with a solar or inverter-backup capacity, and see what happens if the thermostat is moved by a degree or the schedule is trimmed by an hour.

The result is a planning number. The actual figure shifts with outdoor temperature, room insulation and how often the compressor cycles.

The formula and its two inputs

• Average power in kW is the mean electrical draw while the unit is on, including the compressor cycling and the fans. It is not the cooling capacity in tonnes or BTU.
• Hours is the time the unit is switched on over the day, counting the whole span rather than only the minutes the compressor spins.

Keep cooling capacity and electrical power apart. A 1.5 tonne unit delivers about 5 kW of cooling but draws a fraction of that in electricity, which is exactly what the efficiency rating describes.

Worked example: 1.4 kW average for 10 hours

A bedroom unit draws an average of 1.4 kW while it runs and is on for ten hours overnight.

1. 1Average power: 1.4 kW.
2. 2Running time: 10 hours.
3. 3Multiply: 1.4 x 10 = 14 kWh per day.
4. 4Over a 30-day month: 14 x 30 = 420 kWh.

At a flat tariff of 8 per unit the daily cost would be 112 and the monthly 3,360, though the rate itself is not part of this formula.

How to judge 14 kWh a day

Compare the figure against the whole household. A day of heavy air conditioning can exceed the rest of a flat's electricity use put together, so 14 kWh from one unit is a substantial share of a household's daily use.

The average power depends on conditions. On a hot afternoon the compressor runs continuously and the average approaches the full running draw; on a mild night it cycles off for a larger share of each hour. An inverter unit varies its compressor speed and so averages differently from a fixed-speed one.

• To sanity check, read the meter at the start and end of a night with only the AC on, and compare it with the formula.
• Use separate figures for day and night schedules when the conditions differ a lot.

How a higher average draw changes the day

Increasing the average power by 10% from 1.4 kW to 1.54 kW with the same ten hours moves the daily energy from 14 to 15.4 kWh.

The extra 1.4 kWh per day adds up to 42 kWh a month. A change in hours has exactly the same effect, so an hour less per night saves as much as lowering the average power by 10%.

Mistakes that skew AC energy estimates

• Using the rating-plate power for the average, which overstates energy for any unit that cycles.
• Confusing tonnage or BTU with watts, which are different quantities altogether.
• Counting only compressor-on minutes as hours and ignoring fan operation, or the reverse.
• Assuming the same average through the whole season even though the average changes with outside temperature.

Where possible, replace the estimate with a measurement from a clamp meter or the energy readout of a smart plug.

Using the daily figure to plan

Once you have kWh per day, you can size supporting equipment. A battery or inverter backup needs to cover this energy for the hours you want to keep the unit running, and a solar array needs to generate about this amount on top of other loads.

Record the calculated figure with the settings used, so you can see whether later changes to the room or the unit move it.