What Size Off-Grid Inverter Does a Solar System Need?

An off-grid inverter needs a continuous rating above everything you will run at the same time, plus a margin of about 20 to 25%, and a surge rating that covers the biggest motor start, such as a fridge or well pump. Then match its DC input to your battery bank voltage, and choose split-phase output if you have 240V loads.

Step 1: Add up what runs at the same time

An inverter does not care about your daily energy use; that is the battery’s job. It cares about the most power drawn at one moment. List every load that could run together on a busy evening: fridge, lights, water pump, microwave, laptop, a power tool. Add their running watts from the nameplate or manual. If a label lists amps instead, multiply volts by amps.

Then add headroom. Renogy’s home inverter guide recommends adding a 20 to 25% margin to the total to cover surges and extra loads. An inverter working near its limit for hours runs hot and may shut down.

Step 2: Check surge for motors and pumps

Motors draw far more power for a moment as they start. Renogy says starting wattage is usually about three times the running wattage, and Growatt puts the surge at two to three times. Fridge and freezer compressors, well pumps, air conditioners and power tools are the usual culprits.

Inverter makers state surge differently, so compare the fine print. Schneider’s datasheet for the Conext XW+ 6848 lists 6,800 watts continuous at 25°C, with overload ratings of 8,500 watts for 30 minutes and 12,000 watts for 60 seconds. Other makers may quote surge for a different length of time, so compare like with like. A short surge rating may not be long enough for a heavy pump to come up to speed.

Step 3: Match the battery voltage

Inverters are built for one battery voltage. Schneider’s Conext XW+ 6848, for example, takes 42 to 60 volts DC from a 48V bank, so a 12V bank needs a 12V inverter. Renogy’s wiring guide says inverters over about 500 watts generally need to be wired directly to the battery bank, with the cable size the inverter’s manual specifies.

Voltage matters because current is power divided by voltage. A 3,000 watt load pulls about 250 amps from a 12V bank but about 63 amps from a 48V bank, before inverter losses. Lower current means thinner, cheaper cable and less heat. That is why Renogy suggests 12V for small setups such as RVs and boats, 24V for medium systems and 48V for large home installations.

Step 4: Choose the output: 120V or split-phase 240V

Most cabin loads run on 120V. Some do not: electric dryers and some deep well pumps need 240V. Franklin Electric’s residential 4-inch well motors, for example, include 230V single-phase models. For those loads, look for an inverter with split-phase 120/240V output, as on the Conext XW+ models, or a pair of compatible units that the maker says can be stacked for split phase. Stacking usually needs a communication link between the units: Victron’s manual for its stacked systems calls for identical units linked by VE.Bus cables. Check the manual before buying two.

Pure sine wave, charger and solar input

For any home or cabin, pick a pure sine wave inverter. Its output matches grid power, which suits electronics and motors. Many off-grid units are inverter-chargers that also charge the batteries from a generator, and some “all-in-one” models add an MPPT solar charge controller. If yours does, check its maximum solar input in watts and volts against your panel array.

When a large power station is simpler

For a small cabin or backup setup, a large portable power station already combines the inverter, battery and solar controller in one box, with no DC wiring to design. Our guide to what a 3000 watt solar generator will run shows how continuous and surge ratings work on those units, and solar generators for a refrigerator covers compressor starts in detail. For whole-home outage planning, see solar generators for home backup.

Before you buy an off-grid inverter

• Write down running watts and starting draw for every load that can run together.
• Add the margin, then check surge against your largest motor.
• Fix the battery voltage first; the inverter follows it.
• Confirm 240V needs before choosing a 120V-only unit.
• Read the manual’s cable, fuse and grounding requirements, and follow local electrical code.

For the battery behind the inverter, see the 12V LiFePO4 100Ah battery guide; for a boxed set of parts, see solar panel kits. More guides on panels, kits and charging are in our solar panels section.