Single-phase or three-phase inverters for an electric boat — a detailed selection guide

"Inverter" is one of the most overloaded words in marine electrics. On an electric boat it can mean three different things:

1. The motor controller, which turns battery DC into three-phase AC for the propulsion motor
2. The hotel inverter (often a combined inverter/charger), which turns battery DC into AC for the galley, outlets and air-conditioning
3. The shore-power side, which sets whether you plug into a single-phase, split-phase or three-phase pedestal

The single- vs three-phase question has a different answer for each. For the motor, three-phase is a given. For the hotel system, single-phase is usually right on a boat that size. The shore supply is decided by the marinas you use.

→ Size the propulsion side first, then plan the hotel system around it →

What the phases mean

A single-phase supply has one line and a neutral. The voltage swings sinusoidally at 50 or 60 Hz, and the power delivered pulses, falling to zero twice per cycle.

A three-phase supply has three lines, each offset from the next by 120°. The three phase powers add up to a constant total, with no zero-crossings. That's why motors, which want smooth torque, are almost always three-phase.

Split-phase is the North American 120/240 V system: two 120 V lines 180° apart around a centre-tapped neutral. It gives 120 V line to neutral and 240 V line to line. It isn't three-phase, and a three-phase load won't run on it.

The motor: three-phase

Marine propulsion motors are three-phase permanent-magnet or induction machines, driven by a three-phase controller. Every controller in our controller comparison and every packaged drive in our motor comparison is three-phase. There's no single-phase decision to make on the propulsion side.

What you do have to get right is the phase current. Don't calculate it as power ÷ (DC voltage × √3): the line-to-line voltage a controller can produce is only 0.61–0.71 × the DC voltage, depending on its modulation. The calculator works phase current out from the DC current at the pack's minimum voltage. The DC vs phase current guide explains how, and the motor and controller guide compares the figures with what real controllers can carry.

The hotel system: usually single-phase

The hotel inverter only needs to match the appliances on board and the shore supply.

What shore power gives you

Choosing the hotel topology

• European boat, European appliances: single-phase 230 V is the default. Almost every appliance on a recreational boat is single-phase.
• North American boat with 240 V appliances: split-phase 120/240 V. An induction range, dryer or water heater built for 240 V needs both legs.
• Three-phase 400 V makes sense when you have three-phase appliances on board, or a continuous AC load large enough that single-phase conductors get unwieldy, as on a large yacht. Single-phase appliances still run on it, each between one line and neutral.

The current for a given load shows why three-phase helps large systems. For 12 kW:

That's about 6:1 between 120 V single-phase and 400 V three-phase, the same arithmetic that makes three-phase standard in industry. Size the actual conductors from your electrical standard and installation conditions.

Sizing a hotel inverter

Continuous rating

Add up the loads that run together, and size for that. Check the rating at your installation temperature: Victron's MultiPlus-II 48/5000, for example, delivers 4,000 W continuously at 25 °C but 3,700 W at 40 °C.

Surge rating

Motors and compressors draw several times their running current when they start. The MultiPlus-II 48/5000's peak rating is 9,000 W. If two large compressors could start together, stagger them or fit soft-starters rather than buying a bigger inverter.

Three-phase ratings

A three-phase array is rated per phase as well as in total. Three MultiPlus-II 48/5000s make 15,000 VA in total but only 5,000 VA per phase, and no single phase may carry more than one unit's rating. Victron builds three-phase systems from identical units, one or more per phase, with the same number on each phase. The configuration guide covers the rules.

Waveform

Use a pure sine wave inverter. Modified sine wave output is unsuitable for many appliances, especially motor loads and electronics.

Frequency

European boats run at 50 Hz and North American boats at 60 Hz. If you cruise between regions, choose a unit whose frequency can be changed: the MultiPlus-II is 50 Hz as delivered and "can be adjusted to 60 Hz". In a three-phase system all three phases run at the same frequency.

DC input voltage

The inverter's DC input must match the battery it runs from. Victron's MultiPlus-II and Quattro-II come in 12, 24 and 48 V versions, with nothing for 96 V. On a 96 V propulsion boat, the usual answer is a separate 48 V (or lower) hotel bank, charged from the propulsion bank through a DC-DC converter. Alternatively, ask the battery or motor maker which inverters they support at 96 V.

Three worked layouts

A 50 ft European cruising sailboat

A 96 V propulsion system, European 230 V appliances and a continuous AC load of about 3.5 kW.

A three-phase hotel system would add cost with no benefit, because nothing on board is three-phase.

A 60 ft North American powerboat

Full electric galley with a 240 V induction range and dryer, two air-conditioning units and a watermaker. Continuous AC load of about 8 kW.

Three-phase is wrong here: no appliance needs it, and the shore supply is split-phase.

A 70 ft European motoryacht

Electric galley, two air-conditioning compressors, water heater, washer and dryer. Continuous AC load of about 14 kW.

Check the per-phase load against one unit's rating. If 14 kW is the true continuous load, three 48/5000s (3 × 4,000 W) are too small, so choose a larger model on each phase.

Sources

Checked on 7 November 2026:

• Victron Energy — MultiPlus-II datasheet: 48 V models, continuous output at 25 °C and 40 °C, peak power, output frequency adjustable to 60 Hz, three-phase operation
• Victron Energy — Quattro-II datasheet: available battery voltages
• Victron Energy — Parallel, split- and three-phase VE.Bus systems: identical units, equal units per phase
• Texas Instruments — Space-Vector PWM With TMS320C24x/F24x (SPRA524): maximum line-to-line output voltage
• Our comparisons of motor controllers and electric inboard motors, with each maker's documents listed there

We are not affiliated with Victron Energy.