Choosing a Generator for a Solar Installation

Standard grid tied photovoltaic (PV) solar electric systems do not have any on site storage of energy. If you need more electricity than your solar is producing, you just draw it in from the utility as normal. If you are making more power then you need, you use it to spin your meter backwards while pushing it back onto the utility grid. Essentially, the utility is serving as your battery bank for free.

But if the utility experiences an outage, your “battery” is not available and your solar system shuts down, even if it’s sunny out. If having backup power during a utility outage is important, you will need to decide between either installing a battery backup system as part of your solar electric system or installing a backup generator.

Battery Backup

Advantages

  • Silent operation.
  • No pollution during operation.

Disadvantages

  • Limited capacity. A standard battery backup can provide about 35 amps of power at 120 volts but only 17 amps for 240-volt appliances. This limits what can be put on the battery backup.
  • Limited run-time. Depending on the size of your battery bank and how many amps you are drawing, your batteries may only hold for several hours. To get extended run times, you have to severely limit your usage to only the barest of essentials.
  • Batteries have about a five-year life in backup applications. Replacement costs run around $2,000 for a standard bank, including labor.
  • There are some risks with battery systems that don’t exist with non-battery systems. While it’s an unlikely occurrence, the failure of a charge controller can cause overcharging, which can cause fires or sudden release of toxic battery contents. For this reason, it’s not a good idea to install them inside a residence. If installed outside, cold temperatures can affect performance.
  • The additional cost of battery backup is specifically excluded from most rebate and tax credits.
  • Systems with battery backup are less efficient — the same size solar array makes a little less power per year then a system without batteries.
  • Batteries require some routine maintenance, although this is generally manageable.

Automatic Backup Generator

Advantages

  • Twice as much power for the same installed cost. An 8-kW generator costs about the same as a 4-kW battery backup.
  • Unlimited run-time. These generators run on either natural gas or propane. They will continue to provide power until you run out of gas. The propane tanks are generally sized to provide a minimum of 4 days of run time.
  • No effect on the efficiency of your solar power system.
  • With annual maintenance automatic backup generators can operate for decades.
  • They can be sized to run as little or as much as you want during a utility outage, including everything in your home or business.

Disadvantages

  • Operating noise. A central generator sounds about like a car running in your driveway.
  • Small amounts of air pollution are created when running, although the natural gas or propane fired generators produce significantly less pollution then gasoline or diesel driven units.
  • Annual maintenance fees run approximately $180 after the first year.
  • The cost of a generator is also excluded from rebates and tax credits.