Backup Power Generator

Generator Sizing Guide For Off-Grid Solar Systems

Most off-grid solar systems are paired with a generator. Without utility power as a fallback, off-grid setups need a backup plan in case their solar system can’t produce enough to meet the property’s needs.

Generator Sizing

As a general rule, the generator should be around 2 times the size of the inverter’s continuous output. For example, a 4,000-watt inverter should be paired with an 8,000-watt generator.

This is because the generator needs to charge batteries while still powering the loads (appliances using energy). If the loads total 4,000 watts, and the charger is 60 amps at 48 volts, that totals around 7kW of continuous power:

60a x 48v = 2,880 watts + 4,000 watts = 6,880 watts

8kW would make sense as a minimum generator size to power the loads and have enough power to charge the battery bank as well.

It’s a good idea to round up slightly to give some headroom for the generator, especially at higher elevations where your generator is going to lose some of its power. Engines can lose around 3% of their power for every 1,000’ increase in altitude. Make sure you account for this if your system will be installed at a high elevation.

Can I Use a Larger Generator?

A larger generator can be beneficial if you have large loads like an air conditioner or a welder that will only be used when the generator is running.

Can I Use a Smaller Generator?

Smaller generators will still work, but it typically requires adjusting the settings to limit the battery charger’s output. Most battery chargers allow you to adjust the AC input amperage and charge rate.

You will need to match the generator’s voltage with the inverter. For example, 120Vac generators should be paired with 120Vac inverter/chargers, while 120/240Vac generators need an inverter that outputs 120/240Vac.

In the rare case that you are using a three-phase generator or inverter, the same rules apply: the generator’s voltage would need to match the inverter.

Generator Fuel Type (Natural Gas, Propane, Diesel)

Most standby home generators work on natural gas, propane or diesel. Diesel generators tend to be much more fuel efficient and longer lasting, but the initial cost can be 2-3 times more than a natural gas or propane alternative.

The Kohler 12kW & 14kW generators we sell can work with either natural gas or propane. Off-grid customers typically use propane instead of natural gas, but these models can be configured to work with either fuel type.

Other Considerations

Warranty

2-wire start

Off-grid generators paired with solar power systems need 2-wire start capability to allow the automatic generator start (AGS) function to work. When your batteries drop below a certain voltage, the AGS kicks in to turn on the generator and recharge your battery bank.

Note that 2-wire start is different than electric start. Some generators will have a button for electric start/stop, but they can’t be controlled by a 2-wire signal, which means the inverter can’t communicate with the generator to trigger the AGS mechanism.

The Kohler 12RES and 14RESA are both capable of two-wire start.

1800 RPM vs. 3600 RPM

Most generators operate at one of two engine speeds: 1800 or 3600 RPM.

The difference is based on engine design and the alternator being used. 1800 RPM generators are generally considered superior because they are more fuel-efficient, but they cost quite a bit more up front. 3600 RPM generators tend to be cheaper but less efficient.

Conclusion

When searching for a generator to support your off-grid solar system, keep a few things in mind:

• Generator output should be 2x your inverter’s output
• Match generator voltage to inverter voltage
• Generator must be warrantied for off-grid use
• 2-wire start is mandatory to work automatically with solar system
• 3600 RPM generators are more cost-effective, while 1800 RPM generators cost more up front but last longer and are more efficient