A Victron split-phase home backup system uses two identical MultiPlus-II 120V inverter-chargers, one on each leg, to make the 120/240V that North American homes use. Together they can run 240V loads like a well pump during an outage, and switch over from the grid in under 20 milliseconds. This guide explains how the setup works, how to size it and where the limits are.
Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.
How Victron Split-Phase Home Backup Works
Victron’s MultiPlus-II 120V datasheet states that two units of the same model can be connected for a 240V split-phase output. Each unit makes 120V; together they form L1 and L2 with a shared neutral. Both share one battery bank and are linked by VE.Bus, and a Cerbo GX handles monitoring and settings.
| Two units | Combined continuous at 25°C (estimate) | Combined peak (estimate) | Charging (combined) | Zero-load (each) |
|---|---|---|---|---|
| 2 × 48/3000/35 | 4,800W | 11,000W | 70A | 11W |
| 2 × 48/5000/70 | 8,000W | 18,000W | 140A | 15W |
Per-unit figures from the Victron MultiPlus-II 120V datasheet; combined values are simple sums.
Split Phase vs MultiPlus-II 2x120V
The MultiPlus-II 2x120V is a single unit that passes 120/240V through from the grid, but inverts only 120V. 240V loads stop when the grid fails. Two standard MultiPlus-II units in split phase invert 240V, so they suit homes with 240V well pumps or mini-splits on the backup panel.
Sizing (Estimate)
- Loads: add up the critical-loads panel, such as fridge, lights, internet, furnace blower and well pump. Check that the starting surge stays within the peak rating.
- Leg balance: each unit carries only its own leg. A 4,000W load on L1 alone needs a 5000VA unit on that leg, whatever the other leg is doing.
- Battery: 10kWh covers about 20 hours at an average 500W (estimate, before losses).
Features That Matter for Backup
- Under 20ms transfer, so computers keep running.
- PowerControl and PowerAssist let the batteries top up a small generator during peaks.
- ESS mode with the external current sensor (100A) for grid-connected self-consumption.
- Expansion: up to 6 units in parallel per phase.
What to Ask Your Electrician
- A critical-loads subpanel fed from the MultiPlus-II outputs.
- Neutral and ground bonding that meets code for a separately switched system.
- Utility rules for any grid-connected system, especially if solar feeds back.
Victron’s MultiPlus-II product page has the full datasheet and manual downloads.
Example: A Critical-Loads Panel on Two 48/5000s
| Circuit | Leg | Running watts (example) |
|---|---|---|
| Fridge | L1 | 200W |
| Chest freezer | L2 | 150W |
| Lights and internet | L1 | 300W |
| Furnace blower | L2 | 600W |
| 240V well pump | L1 + L2 | 1,500W (750W per leg) |
| Microwave (occasional) | L1 | 1,200W |
| Total per leg at peak | L1 about 2,450W, L2 about 1,500W |
Example values. Both legs stay well under each 48/5000’s 4,000W continuous rating, and the well pump’s starting surge fits within the 9,000W per-unit peak. Putting the microwave on the lighter leg would balance the load further.
Balancing the Legs
Because each MultiPlus-II carries only its own leg, try to spread the big 120V loads between L1 and L2. A quick way to check is the Cerbo GX overview, which shows each phase’s load. If one leg is consistently heavier, your electrician can move a breaker to the other side of the critical-loads panel.
Battery Sizing for Longer Outages
| Average load | Hours of backup | Usable energy (estimate) | 48V lithium bank |
|---|---|---|---|
| 500W | 24 | About 13kWh | About 270Ah |
| 1,000W | 24 | About 26kWh | About 540Ah |
| 1,500W | 12 | About 20kWh | About 400Ah |
Estimates including about 10% inverter losses and idle consumption.
Adding a Generator
With a generator connected to AC-in, the pair charges the battery at up to 140A combined (two 48/5000/70s) while powering the house. PowerControl caps the generator’s current, and a Cerbo GX can start and stop a compatible generator automatically. For split phase, the generator needs a 120/240V output.
Adding Solar
SmartSolar MPPTs on the 48V battery work regardless of grid status. A grid-tie PV inverter on the AC output also works, and the MultiPlus-II adjusts its frequency to control the PV inverter when the battery is full, keeping solar running during an outage within Victron’s Factor 1.0 rule.
Commissioning Steps
- Install both units, the battery bank, fusing and the Lynx busbar.
- Connect the VE.Bus cables between the units and the Cerbo GX.
- Use VEConfigure to set up the two units for split phase.
- Set the battery charge parameters and AC input current limit.
- Test a grid failure by switching off the main supply and checking the transfer.
Monitoring the Backup System
The Cerbo GX shows both legs, battery state of charge and grid status on one screen, and VRM sends an alert when the grid fails or the battery runs low. Historical graphs show how much each outage drew from the battery, which helps you fine-tune your critical-loads list over time.
FAQ
Can I mix a 3000 and a 5000 for split phase?
No. Victron specifies two units of the same model.
Does this need programming?
Victron’s datasheet says settings are changed with VEConfigure (computer plus MK3-USB interface), or remotely through a GX device. Will Prowse’s video shows his beginner approach.
Can I add solar?
Yes, with SmartSolar MPPTs on the DC side or a grid-tie inverter on the AC side.
Do both units need the same firmware?
Victron specifies identical models; keeping their firmware matched is good practice, and the GX device or VictronConnect can update them.
Can the system run 240V loads on battery?
Yes. Unlike the MultiPlus-II 2x120V, two units in split phase invert 120/240V, so 240V loads keep running in an outage.
Sources: Victron MultiPlus-II 120V datasheet (victronenergy.com, accessed Oct 2026); DIY Solar Power with Will Prowse, “DIY Victron 120/240V Split Phase System for Beginners! No Programming Required”.
Related: EcoFlow Delta 3 Max for Home Backup, EcoFlow Delta 3 Max Sump Pump, Heater and AC, EcoFlow Delta 3 Max vs Generator, plus the Victron Energy Guide