Split phase or single phase, and what it costs to get that wrong

Cheaper 48V inverters output a single 120 volt leg. North American houses are wired for 120/240 split phase, and the loads people build off grid systems for tend to live on the 240 volt side.

Well pumps, electric dryers, water heaters, most air conditioners, and every welder in the garage need 240. A single phase inverter runs none of them regardless of how many watts it is rated for.

Three ways out, in order of cost

Buy a split phase inverter to start with. Units like the EG4 6000XP and the Growatt SPF 6000T produce 120/240 from one box, and the price difference over a single phase unit is smaller than any of the workarounds.

Add an autotransformer to an existing single phase inverter. It works, it adds standby losses, and it adds a component that must be sized for the largest 240 volt load you intend to start.

Run two inverters in a stacked configuration. Some models support it and many do not, so this is only a plan if the manual says so by name.

The starting current problem underneath it

Split phase alone does not guarantee a well pump will start. Motors draw several times their running current for the first second, and the inverter has to supply that surge while the battery supplies the current behind it. A 6000 watt inverter with a 12000 watt surge rating pulls roughly 250 amps from a 48 volt bank during that second.

If the pack is rated for 100 amps, the BMS is what stops the surge, and the pump does not start. That is why surge is one of the four checks on every pair page here, and why the answer to a hard starting motor is often another battery rather than another inverter.

Related pages on this site

• Growatt SPF 5000 ES, single phase output
• Growatt SPF 6000T, split phase from one box
• EG4 6000XP, split phase and off grid only