Understanding three phase vs single phase
Why this is a service question, not a preference question
Unlike most equipment choices in this category, the single vs three-phase decision usually isn't really a decision at all: it's determined by what electrical service already exists at the building. Most US residential homes run single-phase service, while larger commercial buildings, some multi-family properties, and many industrial sites run three-phase. The inverter you buy has to match that existing service, so the practical first step is confirming which one you have, not comparing options as if they were interchangeable. For a commercial building already confirmed as three-phase, the three phase commercial system sizing tool covers the broader system design that follows from here.
Why three-phase current comes out lower for the same power
Three-phase power splits the same real power delivery across three conductors instead of the two used in single-phase, and the standard formula includes a square root of 3 factor in the denominator that reduces current per conductor. At the same voltage, that works out to three-phase current running about 42% lower than single-phase for identical power output, which is a meaningful difference when you're also sizing wire with the wire gauge sizing calculator. The background published by the Department of Energy on inverters and grid services covers this general background on service types.
Why lower current doesn't automatically mean lower total cost
Lower current per conductor can allow a smaller wire gauge, which is a real cost advantage. But three-phase wiring needs three current-carrying conductors instead of two, so there's more total copper in the run even at a smaller individual gauge. Whether three-phase wiring ends up cheaper or more expensive overall depends on the specific run length and gauges involved, not a blanket rule in either direction.
Why voltage choice matters within three-phase options too
Common commercial three-phase service voltages include 208V and 480V, and the voltage you're actually working with changes the current calculation meaningfully, since current is inversely proportional to voltage for a given power. A 480V three-phase circuit draws roughly half the current of an otherwise identical 240V circuit at the same power, worth checking against the voltage drop calculator if the run is long, rather than assuming a single "three-phase" number applies universally.
What this tool doesn't cover
This calculator compares theoretical current at a stated power level; it doesn't tell you which service type your specific building has; that's a question for your utility, your electrician, or your service panel's labeling. It also doesn't account for the inverter's own internal specifications or the disconnect switch sizing tool requirements, both of which still need to be checked against the phase-appropriate current once you've confirmed your service type. Once your service type and equipment are settled, the cost per kWh produced calculator can help translate the resulting system into a rough cost-per-kWh figure, and SEIA's introductory guide to residential solar covers the plain-language basics if any of this is unfamiliar.