I think 15.5-13 is about 2.5. Sorry, just kidding.

As far as a percentage of efficiency difference, it's 19.2% more efficient. What that translates to in cost of operation greatly depends on both electric rates and loads on the house.

In a cooling dominant area with high electric rates, that difference is more significant. The higher the electric rate, the bigger dollar impact each point of SEER makes.

One more thing about that difference... 15.5 SEER and 13 SEER are lab settings, based on specific equipment, certain tonnage, and near perfect conditions. Often a 2 ton condensing unit of the same make and model will actually have a different SEER than a 3 ton. If in the lab, the 3 ton tests at 15.2 SEER, the 2 ton at 14.8 SEER, and the 4 ton at 15.5 SEER, then all of the sizes get the rating of 15.5 SEER. It's kind of a joke. It goes right along with the "fossil fools".

For a given EER rating, is it possible to figure out the SEER equilivant? For example a 26 EER equals what in SEER?

 

 

There's no straight forward conversion for that.

For a 26 EER, I'd assume the equivalent SEER would be in the 30+ range, maybe 33 or so. I say that, because an EER of 21 has about a SEER of 28.

I've researched the SEER vs EER issue with limited concrete finding. SEER is figured at a quite benign 80 indoor 82 outdoor if memory serves. EER is what you actually get at your site-specific conditions - Btu / Watt-hour. For window ACs I believe EER is calculated at 95 F, which partially accounts for why darned few are rated much above 11.

I have also heard a rule-of-thumb that SEER is 1/3 more than EER, which matches Clark's note above.

Another issue with high SEER is that is obtained with humongous evaporators which may not dehumidify as required for comfort in humid climates. Installers may cut blower CFM / ton to compensate. Users may respond with lower thermostat settings. Either way, bye-bye high SEER.