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 120v mini split shopping around with a fairly decent SEER rating (22). I'm fine hiring an electrician to put in a 240v 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?
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:
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.
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!
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.
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.
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.
Most U.S. utilities do not charge residential customers for poor power factor so there is little incentive for manufacturers of appliances to improve it. E.U. has PF regulations on all electrical appliances depending on their power consumption level.
California has begun charging residential customers for poor power factor. All new smart power meters being installed have the capability to measure and report power factor via VA consumption in addition to true watts of consumption. It is just a matter of time before all utilities will charge residential customers for poor power factor.
When a large part of electric consumption in an area is due to air conditioners running it is a significant extra power loss in utility line transformers and cables that is presently not paid for by residential customers in the U.S.
Most U.S. central air conditioners have PF between 0.75 and 0.90 depending on air conditioner loading conditions. The run capacitor to start winding gives some improvement to PF when its value is correct.
Mini-splits power factor can be very good or very poor. 230vac units in E.U. are very good PF as they are required by regulations to have PF correction circuitry in their three-phase inverter power supply to make the conversion from AC input to high voltage DC to run inverter. Since 120 vac mini-splits are pretty much a U.S. market product, they often do not have the power factor correction circuitry. 120vac mini-splits can be in the 0.60 to 0.65 range for power factor with their simple rectifier-filter AC to HV DC conversion.
120vac variable speed compressor refrigerators in U.S. have the same issue with poor power factor. Most all of them have PF in the 0.60 to 0.65 range.
Biggest impact presently, in U.S., is anyone running a mini-split with poor power factor on a generator or battery-based AC inverter in an RV or for home backup power. Poor power factor load causes more power loss in generator or battery powered AC inverter.
From the answers in this thread, it sounds like the improved efficiency of 240vac mini split heat pumps vs 120vac is partly related to decreased wire loss with 240v, maybe a few dollars per year. More important is generally lower efficiency of the actual 120vac units because of US energy policy and lack of efficiency incentives compared to EU, Japan, etc. Hopefully that will change in 2023 with the High Efficiency Electric Home Rebate Act (HEEHRA). Starting in 2 weeks, many individuals will have added incentives to choose high-SEER units, so in theory more 120vac options with high SEER should become available. The higher initial cost for units with higher efficiency will be partly covered by rebates.
If you compare units that offer 120v and 240v of the same unit, the 240V units have about .5 more SEER.
That said, the very highest efficiency units that exist (Mitsubishi 6000 BTU and Fujitsu 9000 BTU for example) all happen to be 240V only. That is probably because these oversized/underrated units are capable of producing 14,000 BTUs in heating mode which can use a bit of power, so they stay with 240V.
