Diesel Generator Backup for Solar Systems | THD < 3% & Dry Contact Start

Many solar customers want a diesel generator as a backup for low-sun weeks, extended outages, or higher-than-normal loads. A common technical question is whether the generator meets a solar inverter’s harmonic distortion requirements. This article explains the basics in plain language and shows how diesel backup typically connects to a solar + battery system.

What is harmonic distortion (THD) and why does it matter for solar?

Solar inverters care about power quality. If a generator produces “dirty” power (higher harmonics), the inverter may show faults, charge batteries inconsistently, or refuse generator input. That is why many inverter manufacturers recommend using a generator with low total harmonic distortion (THD).

Our diesel generator harmonic distortion is < 3% THD, which is a common requirement for inverter-based solar and battery systems.

Basic setup: diesel generator as backup to solar + batteries

In most solar backup setups, the generator does not replace the solar system. The inverter remains the “brain” of the system. Solar power runs loads and charges batteries when available. When batteries drop too low, the inverter calls for generator power.

A typical workflow looks like this:

• Solar powers loads and charges batteries through the inverter
• The inverter monitors battery state of charge
• When the battery reaches a low threshold, the inverter signals the generator to start
• The generator supplies power and/or recharges the battery bank
• When batteries recover, the inverter stops the generator automatically

Do solar systems need an ATS?

Many solar and inverter-based systems do not use a traditional automatic transfer switch (ATS) the same way a standard standby generator installation does. Instead, the inverter controls when generator power is used and how it is blended with battery power.

In other words, the inverter often handles the “switching” logic internally, so an ATS is frequently not required for solar backup generator setups. Always confirm how your inverter manages generator input before finalizing equipment.

Automatic start: dry contact (2-wire) control

Most hybrid and off-grid inverters support an automatic generator start using a dry contact signal (often called 2-wire start). This allows the solar system to start and stop the generator without manual intervention.

When the inverter detects low battery (or another condition you configure), it closes the dry contact circuit to start the generator. When the system recovers, the inverter opens the circuit and the generator shuts down.

What else should you confirm before purchasing?

Generator selection for solar backup depends on your loads, voltage and phase needs, inverter requirements, and how quickly you want batteries recharged. If you have an inverter model in mind, the main compatibility items are typically THD, voltage regulation, frequency stability, and the generator start input method.

Next steps

Browse our commercial and industrial diesel generators to compare options for solar backup use cases.