Air conditioner running costs: calculation, examples, saving tips

Electricity cost = power input (kW) × running time (h) × electricity price (€/kWh). The power input is on the spec sheet – not to be confused with the cooling capacity. A 12,000 BTU unit cools at 3.5 kW but only draws around 1.1–1.4 kW. The ratio of the two is the EER (Energy Efficiency Ratio): 3.5 kW / 1.3 kW = 2.7.

On top of that, the compressor does not run continuously. Once the target temperature is reached, it cycles or, on inverter units, throttles down. Over the day the average load is usually 50–70%.

Monoblock with 1.3 kW input and 60% load: 0.78 kW × 6 h = 4.7 kWh per day. At 38 cents per kilowatt hour that is 1.78 euros a day and around 107 euros for a summer with 60 hot days. A mobile split with 1.1 kW input and better efficiency comes to about 4 kWh and 90 euros. The difference is real, but smaller than the difference in purchase price – it pays off over several years.

The biggest lever is not the unit but the heat that never gets in: shutters or external blinds down during the day, windows open at night and closed during the day. Next comes the window seal – a monoblock without a sealed window draws warm air back in and runs twice as long. Only then is it worth looking at EER and energy class.

Portable air conditioners carry an EU energy label from A+++ to D. It is based on the EER, the ratio of cooling output to power input under standard conditions (35 °C outside, 27 °C inside). A unit with 3.5 kW of cooling and 1.3 kW input has an EER of 2.7 – class A. Permanently installed split systems are rated by SEER, which reflects a whole season with changing temperatures and therefore produces higher figures (5 to 8). The two values are not directly comparable.

In practice this means: among monoblocks it is worth checking the EER, because the differences are small (2.6 to 3.2). The big leap comes from the design: a mobile split with an outdoor unit achieves much better real-world figures because no warm air flows back in and the condenser works more efficiently outside. With daily use, the extra cost pays off over two to three summers.

Air conditioners and photovoltaics are a perfect match: when the sun is strongest, the need for cooling is highest. On a clear summer day at noon, a balcony solar system (Balkonkraftwerk) with 800 W feed-in power covers a large part of a mobile split's input (around 1.1 kW) plus the flat's base load. If you let the unit pre-cool between 11 a.m. and 4 p.m. and only hold the temperature in the evening, you cut grid costs considerably – with a battery in the balcony system, even into the night.

On paper, an 800 W balcony solar system replaces about 150–200 kWh of grid electricity for cooling over 60 hot days – around 60–75 euros per summer. Over the lifetime of the air conditioner that is more than the price of a basic monoblock. More on this on our partner sites wosolar.de and wospeicher.de.

Units with heating work as heat pumps and, depending on the outdoor temperature, deliver two and a half to three and a half times the electricity used as heat (COP 2.5–3.5). At 38 cents per kilowatt hour of electricity, a kilowatt hour of heat therefore costs 11 to 15 cents – on a par with gas and far below a fan heater, which needs 38 cents per kilowatt hour of heat. Below 5 °C outside the COP drops sharply, and below 0 °C many portable units switch off; for spring and autumn, though, the function is a real money-saver.

All figures here are guide values. How much your unit actually draws is shown by an energy meter plugged in between socket and plug, which costs only a few euros – or a smart Wi-Fi plug with energy monitoring that records the daily profile as well. Measure for a week with your usual settings; then you will know whether switching to a more efficient unit is worth it or whether a better window seal is enough. We compare suitable meters on wostrom.de.

A fan does not cool, it only moves air – but the evaporation effect on your skin makes the room feel three to four degrees cooler, and for 30 to 50 W instead of 1,300 W. At temperatures up to about 28 °C, that is plenty for many people. Above that, in top-floor flats or in muggy air, only real air conditioning that also dehumidifies will help. The most economical combination: set the air conditioner to 25–26 °C and let a fan distribute the cool air. That cuts the compressor's running time, and therefore consumption, far more than one degree lower on the thermostat costs.