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A generator as backup in an off-grid system: how the ATS works

· 6 min read ·

Understand what a generator is for in an off-grid system, what the ATS prevents, the start-up sequence and how to size it in the calculator.

Why have a generator in an off-grid system

An off-grid system is sized for the month with the least sun, but a few days of rain in a row can still empty the bank. The generator comes in as a reserve: instead of oversizing panels and batteries for the worst day of the year, you use it on the few days when sun is missing.

Only panels and battery

Quiet and fuel-free

  • No noise, no fuel, little maintenance
  • To cover days in a row without sun, it needs many more panels and batteries

With a backup generator

Covers the bad days

  • Smaller bank and panels
  • Recharges the bank in a few hours
  • Fuel, noise and maintenance
  • Needs a transfer switch

What the ATS does

The ATS (automatic transfer switch) chooses the source of the energy that reaches the loads: the inverter or the generator. The essential point is that the two sources are never connected at the same time to the same circuit, which can damage equipment and cause accidents.

Manual switch

simplest

Someone changes the switch position. Cheap and safe if it is interlocked, but it depends on somebody being there.

Automatic ATS

more practical

Detects the available source and transfers by itself. With the internal interlock, the change-over is safe.

Inverter AC input

alternative

Many off-grid inverters have a generator input: the inverter itself does the transfer and uses the generator to charge the bank.

The start-up sequence

  1. 1
    1. The bank reaches the minimum level

    The inverter or the management system detects that the battery is low or that the sun is not enough.

  2. 2
    2. Start signal

    A contact on the inverter tells the generator to start, if it has automatic start. In simple systems, someone starts the generator.

  3. 3
    3. The generator settles

    It warms up and stabilizes voltage and frequency. Only then do the ATS or the inverter accept its energy.

  4. 4
    4. The transfer

    The ATS connects the generator to the loads, and the AC input of the inverter/charger uses the same energy to recharge the bank. The ATS alone does not charge the battery.

  5. 5
    5. End of the recharge

    With the bank charged, the inverter releases the generator, which shuts down. The loads go back to the panels and the battery.

Size the generator

The generator must supply, at the same time, the home’s loads and the bank’s recharge power, with a safety margin. Use the generator’s continuous power (not the peak) and check whether the figure is in kW or kVA: the calculator uses kW, and the recharge power is the charger’s AC input.

Interactive

Size the backup generator

Enter the loads that run together with the generator and the power used to recharge the bank. The calculator shows the required power, the standard size and the recharge time.

5 kWrequired power
5 kWstandard size
1.9 hto recharge the bank (90% efficiency)

Illustrative standard sizes. Generators lose power with altitude and heat, and run best at 50% to 80% load. Check the generator manual. The recharge power is the charger’s AC input; the 90% efficiency is already in the calculation. At 0 W there is no recharge to calculate.

Good practice and precautions

  • Ventilation: a combustion generator releases carbon monoxide. Always use it outdoors, away from windows and doors
  • Suitable load: generators run best at 50% to 80% load, and suffer at very low load
  • Interlock: only connect the generator to the loads’ circuit through an ATS or an interlocked switch (running the generator in parallel with the inverter needs equipment designed for it)
  • Earthing and neutral: the generator’s neutral and earth connection follows the manufacturer’s instructions and the local standard; do not improvise
  • Maintenance and fuel: test the generator periodically and store the fuel in a safe place

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The ATS switch chooses between the inverter and the generator ahead of the board, so the two sources are never connected together. The generator also feeds the AC input of the inverter/charger, which recharges the bank.

Try it: Replace the loads with your own and redo the generator power calculation with a 25% margin. Which standard size does it fall into?

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