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I am under the impression that, as a general rule, 110v heating and cooling options are inherently not very efficient. So I was surprised to find a 110v mini split shopping around with a fairly decent SEER rating (22). I'm fine hiring an electrician to put in a 220v circuit for me if I need to, but if this lets me save that money without significant downsides, why not right?

So for long term energy efficiency, is a 110v mini split an inherently bad idea?

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    What brand and model? In terms of efficiency the only difference may be how efficient the “inverter “ or variable frequency drive is. Where none of the answers below touch on is balancing your power use a 240v load is balanced but a 120v load is only on 1 leg of your power feed. The difference is very small power wise but without the brand and model everything is just a guess.
    – Ed Beal
    Aug 13 at 22:23
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    Power wise equal btu's and equal seer rating should use the same power. Depending on balance of load on half the transformer winding a 120v may produce an efficiency problem with the transformer, but I've never seen a study of the ability or inability of the residential meter to compensate for that. It is also possible the 120v may require larger wire, a 240v 15A unit potentially could be a larger capacity unit than a 120v 20A unit. Aug 13 at 23:42
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    Power source makes no difference to efficiency, per se
    – Ecnerwal
    Aug 14 at 14:19
  • @EdBeal and other asked for specifics, but I avoided naming brands as that seems to inevitably run afoul of the ban on shopping advice. My question really was the bigger picture question anyway, so I feel it was answered well and am grateful to everyone that answered.
    – grovberg
    Aug 21 at 14:50
  • Well I am glad you feel good and have accepted an answer, going with an imbalanced load can cause other problems and as a licensed electrician with a universal hvac licenses also I can tell you there are things missing and the reason I made comment, I find many highly upvoted answers that n some cases are illegal, others dangerous and many lack true knowledge, just saying. Thanks for choosing an answer
    – Ed Beal
    Aug 22 at 23:56
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The issue is not, generally speaking, efficiency in the sense of SEER or similar ratings. Using 240V instead of 120V allows for more efficient use of wire (12 AWG wire can handle 20A @ 120V or 20A @ 240V), and in some cases more efficient motors. Even higher voltage makes it economical to send electricity long distances, but that is really a different issue.

So the big question becomes, how much power do you need. If a device only needs something on the order of 1,500W, a 120V circuit is good enough and has the advantage that we (in the US) have them "everywhere". If a device needs more than 2,000W, a 240V circuit almost always makes sense. For example, a 5,000W appliance will use 21A when running, a 30A breaker and usually 10 AWG copper wire. The same appliance on 120V will use 42A when running, a 50A breaker and much more expensive 6 AWG copper wire (or possibly larger but cheaper aluminum).

But generally speaking, you don't need to worry about 120V vs. 240V too much, unless you are in a (rare, but they exist) 120V-only building. In most cases, start with:

  • Capacity - how much heating or cooling, based on climate and the size of the area being heated/cooled
  • Efficiency - measured in SEER or in estimated kWh-used
  • Cost - note that in many cases, the cost of the unit is small compared to the lifetime cost of energy to run it

and you will likely find yourself in a small system (120V) or a big system (240V) or in-between where you get to choose depending on other factors.

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The efficiency has nothing to do with the voltage. Most 240V units will be larger than most 120V ones (though there is some overlap) and if you buy one that is too large or too small for your cooling needs, that would be inefficient. If you can find a good deal on a machine that's the right size for your room/situation and it's 120V, that's fine. If you already have a suitable dedicated 120V circuit at the location, all the better but if you don't, it will cost about the same to install the new circuit regardless of voltage.

Unless your breaker panel is full or nearly full ... then you might be better off with a 120V unit!

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    "The efficiency has nothing to do with the voltage." Not true, motors and power electronics like inverters are generally more efficient at a higher voltage and lower current.
    – BrtH
    Aug 15 at 12:42
  • The question and the answer are about purchasing, not designing, an air conditioner. Two A/Cs with the same SEER rating have the same efficiency. The voltage is irrelevant. I will grant you that in other contexts, “in life”, voltage is a factor in the design of efficient electrical systems. Fortunately we have the luxury in SE of confining ourselves to the question at hand.
    – jay613
    Aug 16 at 11:03
  • Also, while yes high current can lead to low efficiency, in practicality a store-bought 120V window A/C will be limited to about 18A on a 20A circuit. So the inefficiency related to current is limited. It is also measured and defined by the SEER rating. OTOH an oversized 240V machine can be very inefficient, even with a great SEER rating and "low current".
    – jay613
    Aug 16 at 11:09
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The loss is due to the difference in current that the wiring has to carry

All else being equal*, a 240V appliance will be more efficient than a 120V appliance, because:

Power loss in wiring = (current)^2 x (resistance)

To deliver the same amount of power to the appliance, 1/2 the current is needed in a 240V appliance, because:

Power to the appliance = (voltage) x (current)

Because the current is reduced by a factor of 1/2, the power loss in the wiring will be reduced by a factor of 1/4.

But how much difference does this make?

According to the U.S. Energy Information Agency's RECS study, the average U.S. household uses about 2000 kWh for air conditioning per year.

If we assume that the a/c runs 100 days out of the year for an average of 10 hours a day (1000 hours total), that means the average demand is 2000 W when it's running, so about 17 amps for a 120V a/c, or 8 amps for a 240V a/c.

Let's assume it's wired with 20' of 12 AWG wire in both cases, which has a resistance of 0.032 ohms (source).

Thus, total power dissipation in the wire is 17^2 x 0.032 = ~ 9 W for the 120V a/c. Over 1,000 hours that's 9 kWh, or $1.30 of electricity at the average U.S. residential rate. For the 240V a/c, dissipation is about 2 kWh, giving you a savings of $1.00 a year.


*Obviously, all else is not equal, the details of which some of the other answers get into. You'd be hard pressed to find two a/c units where the only difference was the operating voltage.

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The efficiency rates the cooling capacity against the electrical input power.

The power is not based on just the voltage but the product of voltage, current and phase angle.

So as long as the cooling capacity meets or exceeds the needs then choose either voltage based on other parameters, such as supply available ie 110 or 230 etc.

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A 220V appliances are generally more efficient, both because of the less power lost over the wires and because the modern power electronics are more efficient at 220V than at 110V for anything over, say, 0.5kW. See e.g. here.

The difference because of these factors should be less than 5 or 7%. If your wiring is quite long (say, 50m / 150ft cable path between the breaker panel and the appliance) it can grow as high as 12-15%. Longer cable paths are impractical for other reasons anyway.

Then again, few years ago I was in a position to use a second-hand 100V (Japanese) small split AC in a pretty much 220V EU country (don't ask about the code compliance). It was considerably more efficient even with ~5% transformer loss, compared to the brand new units available here.

The markets were different enough (esp. in regard to the kWh prices) and what was top efficiency here was considered obsolete in Japan.


p.s being the type of person I am, I disassembled the external unit and found out that it has a voltage doubling rectifier at the input. I rewired it to run on 220V directly and got rid of the transformer - it had an audible hum.

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