Terms like SEER and tonnage get thrown around a lot in the HVAC world.It is a common term used in the HVAC industry is an air conditioner or heat pump’s SEER rating.

But what exactly is SEER and why is it important? SEER stands for Seasonal Energy Efficiency Ratio. It’s a measurement of an air conditioner’s cooling capacity to power input, or simply, the ratio of cooling produced (in BTUs) divided by the amount of electricity used (in watts). The higher the SEER rating means the greater the unit’s efficiency.

Older air conditioning systems have a SEER rating of 10 or under. Today’s more efficient air conditioning systems have SEER ratings as high as 23. The United States now requires that residential air conditioning systems manufactured after 2005 have a minimum SEER rating of 13 (window units are exempt from this law, so their SEERs are still around 10). The SEER rating is usually shown on a yellow and black EnergyGuide sticker attached to the outside unit of the air conditioner.

A SEER ratio is calculated over an entire cooling season using a constant indoor temperature and a variety of outdoor temperatures ranging from 60 degrees to 100 plus. This is how it simulates a typical season.

Keep in mind that SEER ratio is a maximum efficiency rating, like the miles per gallon for your car. Say your car gets 28 miles per gallon on the highway. But if you’re stuck in city traffic it’s a lot less efficient. The same goes for your air conditioner. If your SEER ratio is 21, that’s the maximum efficiency and it could be lower depending on conditions.

The higher the unit's SEER rating the more energy efficient it is. In the U.S., the SEER is the ratio of cooling in British thermal unit (BTU) to the energy consumed in watt-hours. The coefficient of performance (COP), a more universal unit-less measure of efficiency, is discussed in the following section.

For example, consider a 5000 BTU/h (1465 watts cooling capacity) air-conditioning unit, with a SEER of 10 BTU/W·h, operating for a total of 1000 hours during an annual cooling season (e.g., 8 hours per day for 125 days).

The annual total cooling output would be:5000 BTU/h × 8 h/day × 125 days/year = 5,000,000 BTU/year

With a SEER of 10 BTU/W·h, the annual electrical energy usage would be about:5,000,000 BTU/year / 10 BTU/(W·h) = 500,000 W·h/year

The average power usage may also be calculated more simply by:Average power = (BTU/h) / (SEER) = 5000 / 10 = 500 W

If your electricity cost is 20¢/kW·h, then your cost per operating hour is:

0.5 kW * 20¢/kW·h = 10¢/h

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