The essentials in 30 seconds
- Pure sine wave, no question on a sailboat. A modified-sine inverter (stepped wave) poorly powers — or even overheats — induction motors (refrigerator compressors, some tools) and sensitive electronics. Pure sine wave powers everything like shore power.
- Sizing = continuous power AND startup surge. Motor-driven devices (compressors, tools, pumps) draw a surge at startup; resistive loads (coffee makers, kettles) have no surge but high continuous draw. Both matter.
- Workshop rule: go large. An inverter sized to the exact load will cut out on overload at the first extra device. Allow margin on continuous power and check the datasheet for surge ratings.
- Idle power drains. An inverter left on continuously draws several watts; over a season at anchor, that’s real amp-hours. An accessible ON/OFF switch is not optional.
- For intensive use with frequent harbor calls, the inverter-charger combo (Victron MultiPlus) replaces two boxes with one, with automatic shore/battery switching.
Contents
The DC/AC inverter bridges your 12/24 V bank and the 120 V world. It converts the battery’s DC to a clean AC sine wave to power coffee makers, computers, chargers, and small appliances far from shore. Well-sized, it’s forgotten; undersized, it’s the most temperamental piece of gear aboard. For the battery bank that powers it, see our lithium bank sizing method for sailboats.
Why pure sine wave, not modified sine
Two technologies exist: modified sine (stepped square wave) and pure sine (smooth sine wave identical to shore power).
- Modified sine: cheaper, but the stepped wave stresses some equipment — induction motors (compressors, fans, some tools) overheat and wear, power-supply electronics buzz or malfunction.
- Pure sine: clean power, everything works like at home. It’s the de facto standard in marine use — the price gap no longer justifies the risk on powered devices.
Our workshop stance: in 2026, we no longer install modified-sine inverters on boats, period.
Sizing: the workshop method
- List the 120 V devices used simultaneously — not the sum of everything aboard, but the realistic worst case (e.g., coffee maker + laptop charging).
- Separate resistive and motor loads. Resistive loads (coffee maker, kettle, hot plate) draw their rated power with no surge. Motor or compressor loads draw a startup surge that can be several times their rated power for a brief moment.
- Pick continuous power with margin on the realistic total — an inverter sized to the exact load will go into safety mode at the first surprise.
- Check the datasheet for surge ratings. Each model lists its peak power and duration (e.g., a Phoenix 12/1600 advertises a 3,000 W peak); that’s what must handle motor startups.
For light use — laptops, chargers, small tools — a Phoenix Smart 12/500 is enough. The workshop reference for a 35–45 ft sailboat remains the Phoenix Smart 12/1600. Today’s prices are on the product pages.
Victron Phoenix range
The reference pure-sine range in pleasure boating, available in 12 V and 24 V, from 250 VA to 3,000 VA. The “Smart” models add Bluetooth (monitoring and settings via the app). Our pages: Phoenix Smart 12/500, Phoenix Smart 12/1600, Phoenix Smart 12/2000, Phoenix Smart 24/1600 — and the rest of the range in the inverter collection.
Mastervolt Mass Sine range
The direct competitor, with a specific selling point: native MasterBus/CZone integration. On recent units delivered with CZone (many Beneteau, Jeanneau, Bavaria models), staying with Mastervolt lets you control and monitor the inverter from the existing digital display — that’s the deciding factor, more than the spec sheet. See the Mastervolt collection.
The inverter-charger combo
For intensive use with regular harbor calls, the Victron MultiPlus combo packs inverter and shore charger into one box: at the dock it charges, at anchor it inverts, with automatic switching (commutation in tens of milliseconds — datasheet). If your project already includes replacing the shore charger, the combo saves space and wiring; our shore charger guide helps decide. Model pages: MultiPlus 24/1600/40 and the range in the Victron collection.
Range selection guide
| Need | Power range | Typical family | Watch-out |
|---|---|---|---|
| Laptops, chargers, small tools | 250–800 VA | Phoenix 12/250 to 12/800 | Idle draw looms large on small loads: switch off when not in use |
| Coffee maker, occasional appliances | 1,200–2,000 VA | Phoenix Smart 12/1600 or 12/2000 | Resistive loads in parallel: add real power draws |
| Continuous use, motor-driven devices | 2,000–3,000 VA | Phoenix 12/3000, Mass Sine | Check datasheet peak ratings against compressor startups |
| Intensive use + regular harbor calls | combo | MultiPlus | Size the charger too (charge current vs bank) |
Exact continuous and peak powers: Victron/Mastervolt datasheets and product pages — we don’t freeze those values here.
Installation: technical rules
- DC wiring sized for real current. A 1,600 W inverter on 12 V draws over 130 A at full load: heavy cable (around 35 mm² on a short run — follow the model manual), minimal length. A cable that’s too thin drops voltage and the inverter cuts on undervoltage.
- Proper fuse on the battery feed, type and rating per the model manual (class T or MEGA as required). This fuse protects the cable, not the inverter.
- Accessible ON/OFF switch. Idle power drains continuously; a switch at the helm turns wasted amp-hours into range.
- Ventilation. Efficiency isn’t perfect — the difference becomes heat. Ventilated compartment, never a stuffed closed locker.
- 230 V safety aboard. The inverter’s AC output ties into the boat’s 120 V system with its own protections and grounding, per the pleasure-craft electrical-installation standard (ISO 13297). We systematically have this validated — permanent 120 V aboard is not a DIY domain.
Three workshop cases
Case 1 — Bavaria 41, family cruising
2024 installation: Phoenix Smart 12/1600, accessible switch from the cabin, fuse sized per the manual. Typical use: morning coffee at anchor, devices charging while sailing. Measured draw over 24 h: 30–40 Ah, idle included. Customer feedback: “Coffee at anchor changes the quality of the cruise.”
Case 2 — Lagoon 42, offshore program
Replaced the existing shore charger with a MultiPlus combo (inverter + charger in one box): automatic shore/battery switching, continuous 120 V at anchor, space saved in the technical locker. Six hours of shop time, DC wiring redone to spec.
Case 3 — 8 m sailboat, coastal solo sailing
Minimal need — laptop and phones. Phoenix Smart 12/500, fuse and switch, two hours to install. Customer feedback: “For my use, it’s exactly the right power — not oversized, not frustrating.”
Special case: a 120 V refrigerator on an inverter?
First check the need: most marine refrigerators (Vitrifrigo, Dometic…) run natively on 12/24 V with Danfoss/Secop compressors — in that case, no inverter at all, it’s more efficient. If your fridge is truly 120 V (domestic appliance retrofitted aboard), three points: the compressor startup surge sets the inverter’s peak rating, the 24 h draw weighs heavily on the bank, and the inverter must stay on continuously — idle included. This is typically where you recalculate the whole energy budget before buying anything: the method is here.
FAQ
12 V or 24 V inverter?
The one that matches your bank voltage — an inverter plugs straight into the bank, and adding a 12 → 24 V converter would be inefficient. Most pleasure sailboats are 12 V; larger vessels switch to 24 V.
Can I use the inverter with the engine running?
Yes — it’s even a good idea, the alternator offsets the load. On a small bank, starting a big load briefly dips voltage: running the inverter and the load with the engine running avoids the sag.
How much power does an inverter draw in idle?
Several watts continuously depending on the model (exact value on the datasheet; ECO modes reduce it). Over a season at anchor, an inverter left on represents real amp-hours — hence the accessible switch.
Can I power an air conditioner with an inverter?
Technically possible, rarely sensible: an air conditioner draws continuously and drains a bank in hours. See our guide air conditioning or ventilation in harbor — the answer lies in the energy budget, not the inverter.
Inverter-charger combo or separate units?
Combo (MultiPlus) if you use both functions intensively and the shore charger is being replaced anyway. Separate units if the existing charger is fine, or to keep maintenance independent — a combo failure takes both functions offline.
Further reading
- Sizing a lithium bank for a sailboat — workshop method
- Shore charger: sizing for a lithium bank
- Marine lithium BMS comparison 2026
- DC/AC inverter catalog · Victron Energy · Mastervolt
About the author
Pol Conin, Digital Manager at Skysat. Software engineer and tech entrepreneur, Pol joined to bring Skysat into the digital age. See his author page for the full list of articles.
Transparency: Skysat distributes Victron Energy and Mastervolt and sells the products mentioned; the links point to our catalog. This does not change sizing rules or cases where the right answer is the smallest model.
