SEER and SEER2 savings calculator

SEER is the Seasonal Energy Efficiency Ratio: Btu of cooling delivered over a season divided by watt-hours of electricity used. A 16 SEER unit delivers 16 Btu of cooling for each watt-hour.

Since January 1, 2023, new equipment is rated in SEER2. The test is the same idea, but it runs the blower against more duct static pressure, closer to what real duct systems put on a unit. The same unit scores lower in SEER2; a common rule of thumb is about 4.5% lower, so 14 SEER is about 13.4 SEER2. The federal minimums for split-system air conditioners went from 13 SEER (14 in the Southeast) before 2023 to 13.4 SEER2 nationally and 14.3 SEER2 in the Southeast and Southwest.

The old unit on a job almost always has a SEER sticker and the new one a SEER2 rating. The calculator lets you mark each and puts both on SEER2 with that 0.955 factor before comparing. Skip that step and a 16 SEER2 unit looks like it beats a 16 SEER unit, when they are about the same.

Power used for cooling = Btu of cooling ÷ SEER ÷ 1,000 (kWh). Cooling needed does not change with the new unit, so the saving is a straight ratio:

Share saved = 1 − old SEER ÷ new SEER (both on the same scale)

Dollars saved = old cooling kWh × share saved × price per kWh

The only hard part is the old cooling kWh. There are two ways to get it.

From the power bills (the better way). Spring and fall months show what the house uses with no heat or air conditioning. The extra in the summer months is mostly the AC. Take the average summer month, subtract the average spring month, and multiply by the number of hot months.

From cooling hours. Tons × 12,000 × full-load hours ÷ (SEER × 1,000). Full-load hours are how long the unit would run flat out to do a year's cooling. They range widely: a few hundred in cool coastal places, a few thousand in the hottest parts of the South. Without a local figure, look at the table the calculator shows across 500 to 3,000 hours rather than trusting one national number.

A 3-ton, 10 SEER unit from the 1990s is being replaced with a 16 SEER2 unit. The summer bills average 1,600 kWh for four months; spring and fall average 750 kWh.

The check tells you whether the bills make sense. If the hours look far too high for the area, something else is running in summer, like a pool pump or a second fridge in the garage.

For comparison, one published example has a 3-ton unit going from 9 to 16 SEER at 2,100 hours and 14 cents saves about $514 a year. The calculator's cooling hours mode gives $515 on those inputs, the difference being rounding.

How to find the old unit's rating

The savings depend on the old unit's real rating, so it is worth a minute to find it.

• The outdoor unit's data plate gives the model and serial number. Some list SEER directly; most do not.
• The AHRI directory lists the certified SEER (or SEER2) of a matched outdoor unit and indoor coil or air handler. Search by the outdoor model and the indoor coil model.
• The serial number usually encodes the year it was made. Split air conditioners made from 2015 through 2022 had to meet 13 SEER, or 14 SEER in the southern states. If you cannot find the exact rating, the federal minimum for its year and region is a fair floor; older units were held to lower minimums.

Mismatched systems, such as a new outdoor unit on an old coil, often do not reach either part's rating. If the old system was mismatched, its real efficiency was probably lower than the outdoor unit's sticker.

For a 3-ton system with 1,500 full-load hours at the EIA average of 18.31 cents per kWh, old units in SEER and new units in SEER2:

The biggest savings come from replacing the oldest, least efficient units. Each step up the efficiency ladder saves less than the one before, because there is less power left to save. Going from 18 to 20 SEER2 saves far less than going from 10 to 16.

SEER and SEER2 cover cooling only. A heat pump also has a heating rating, HSPF2, which works the same way for the heating season: Btu of heat delivered per watt-hour. The federal minimum for split heat pumps is 14.3 SEER2 and 7.5 HSPF2.

To estimate heating savings when a new heat pump replaces an old one, use the same ratio idea on the heating kWh: share saved = 1 − old HSPF ÷ new HSPF, with both on the same scale. Winter bills minus shoulder-month bills give the heating kWh, just as summer minus spring gives cooling.

When a heat pump replaces electric resistance heat (baseboard, an electric furnace or strips), the heating saving is much bigger than any SEER step. Resistance heat delivers 3,412 Btu per kWh; a heat pump delivers several times that in mild weather. In homes with resistance heat, the heating side usually pays for the upgrade before the cooling side does.

The bills method is simple, but a few habits make it much more accurate.

• Use a full year. Most utilities show 12 or 13 months of use on the bill or in the online account. Pick the two or three mildest months, when neither heat nor air conditioning runs, as the baseline.
• Count the hot months honestly. In much of the South the AC runs hard for five or six months; in the North, two or three. Count only the months that are clearly above the baseline.
• Watch for other summer loads. A pool pump, a dehumidifier, a second fridge in a hot garage, or kids home from school all raise summer use. If you know one is there, take its share out, or the AC gets credit for it and the savings look bigger than they will be.
• Electric heat changes the baseline. In a home with a heat pump or electric furnace, winter use is high too. Use spring and fall only for the baseline, never the yearly average.
• Check with the full-load hours. The calculator turns the AC's share back into full-load hours. If that number is far from what you would expect for the area, look again at the months you counted.

The price matters as much as the kWh. Many utilities charge more per kWh in summer or at peak hours, when the AC runs most. If the home is on a time-of-use plan, use the summer peak price rather than the bill's average, and the savings will come out higher.

Enter the price of the new system, less any rebate or tax credit, and the calculator divides it by the yearly saving for a simple payback. It uses today's electricity price, so it leaves out rate increases and the repairs an old unit would need. It is a fair way to compare two options, such as a 15.2 and an 18 SEER2 unit, by asking how many years the extra cost takes to come back.

Ratings are lab numbers. In the field:

• An old unit that is low on refrigerant, has dirty coils or a worn compressor uses more than its sticker says, so the real saving is often bigger.
• A new unit on undersized or leaky ducts, or with the wrong charge, delivers less than its rating, so the saving is smaller.
• An oversized new unit short cycles and never reaches its rated efficiency. Size it from a load calculation, not from the old unit.

Before you quote, run the load calculator so the new system is the right size. Then send good, better and best options from Koira with the yearly saving of each in the description, so the homeowner can see what the step up buys.