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A single-phase solar inverter and a three-phase solar inverter are separated by the utility service they are allowed to feed, not by the modules on the roof. Southwestern Electric Power Company’s 2024 electric service handbook, which is one utility’s rules and not a national code, says residential service on its system is single-phase, three-wire, nominally 120/240 volts, or, where available, a network service at nominally 120/208 volts. Commercial secondary service on that system may be single-phase or three-phase, three-wire or four-wire. The handbook’s instruction is to ask the utility which service will actually be furnished before buying equipment. That instruction is the whole article in one sentence.

What single-phase and three-phase mean

Alternating current reverses direction. A single-phase service has one of those waveforms, delivered on the conductors the utility defines. On the SWEPCO residential service that is a three-wire 120/240-volt arrangement: two hot legs and a neutral, so 120-volt loads and 240-volt loads can both exist in one house. It is not “one hot wire and a neutral” in that handbook, and it should not be described that way for a US split-phase house. A three-phase service uses three alternating voltages, offset by 120 electrical degrees, which is why motors and larger buildings are built for it. Fluke’s power-quality note states the 120-degree offset. The conductor count and the voltage still come from the utility, not from the phrase three-phase.

A solar inverter has to produce the voltage and the number of phases the service has. A single-phase 240-volt inverter does not become a three-phase 480-volt inverter because the array is large. A three-phase inverter does not belong on a house that the utility will only serve as 120/240. Pacific Northwest National Laboratory’s scale note is consistent with this and still not a voltage law: a small array, under about 50 kilowatts in that description, may land on the building service, and a larger one may need a transformer. Their illustration of a step-up, 480 volts toward 15 kilovolts, is an example of “larger,” not a finding that every commercial solar inverter is 480 volts.

A US tax definition is not the service

Section 45X of the US Internal Revenue Code defines inverter product categories for a manufacturing credit. In that statute, a “residential inverter” is suitable for a residence, rated 120 or 240 volt single-phase, and not more than 20 kilowatts AC. A “commercial inverter” in the same subsection is suitable for commercial or utility-scale use, rated 208, 480, 600, or 800 volt three-phase, between 20 and 125 kilowatts AC. A “utility inverter” there is at least 600 volt three-phase and between 125 and 1,000 kilowatts. Those bins are about a tax credit for making inverters. They are not the list of legal voltages in every country, and they are not SWEPCO’s handbook. A 230-volt single-phase inverter that is normal in another market does not fit the statute’s “120 or 240 volt” residential bin, and it may be exactly what that other market’s utility requires. Match the destination service. Do not match a tax category from a different country by accident.

Microinverters add a further split. The same statute’s microinverter bin allows 120 or 240 volt single-phase, or 208 or 480 volt three-phase, at not more than 650 watts AC. Again, that is a manufacturing definition. The physical unit still has to be listed for the voltage at the site. A 220–240 volt microinverter is a product for a 220–240 volt service. It is not a 120/240 split-phase device unless its listing says it is.

How to choose a single-phase or three-phase solar inverter

What you know What follows
The utility letter says single-phase 120/240 The inverter output has to be that service. A three-phase unit is the wrong machine.
The building is commercial and the service is not stated Stop. SWEPCO’s handbook says commercial service may be single-phase or three-phase. Ask before purchasing.
The array is large enough to need a transformer The inverter voltage and the transformer are one design. PNNL’s 480-volt example is an illustration, not your voltage.
The inverter is for an RV or a cabin with no utility Phase is the wrong first question. The inverter has to form the AC the appliances use. See the RV inverter page.

A service change is its own project. If the building may need three-phase later, say so before the inverter is ordered. Swapping a single-phase unit after the array is built means a second interconnection application, not a menu setting. The modules do not care. The utility equipment does.

Grid behavior does not change with phase count. A grid-tied inverter, single-phase or three-phase, must cease energizing an unintentional island within 2 seconds under IEEE 1547-2018. Three-phase does not mean the building stays up in an outage. Backup is a separate design. Motors that are three-phase need a three-phase source. A single-phase inverter plus a phase converter is a different, usually worse, plan than a three-phase service the utility was willing to provide. Ask the utility first, then buy the inverter that listing and that letter both allow.

Frequently asked questions

Is a house always single-phase?

On the SWEPCO system, residential service is single-phase 120/240 volts, or 120/208 volts where they supply a network. Another utility can differ. Another country often uses a single-phase service near 230 volts, which is not 120/240. Read the service letter for the address.

Is three-phase the same as 480 volts?

No. Three-phase describes the number of offset waveforms. 480 volts is one commercial voltage among others. The US tax-code bins also list 208, 600, and 800 volts for certain product categories. The site voltage is whatever the utility will connect.

Can one solar inverter be switched between single-phase and three-phase?

Only if that exact model is listed and designed for both, and the utility will accept the setting. Most units are built for one service. Do not rewire a single-phase inverter onto three poles because the roof is large.

Sources and references

Residential 120/240-volt single-phase service, optional 120/208 network service, and commercial service that may be single-phase or three-phase: SWEPCO electric service handbook, 2024. Three-phase voltages offset by 120 electrical degrees: Fluke, single-phase and three-phase power. Manufacturing categories in the US tax code, which are not installation rules: 26 USC § 45X, inverter definitions, via Cornell LII. Array landing and the 480-volt illustration: Pacific Northwest National Laboratory. Islanding: NREL and Sandia IEEE 1547-2018 primer.