ASK THE EXPERTS: Using PV Energy During Outages

Intermediate
An SMA Sunny Boy inverter with connected Secure Power Supply backup outlet.

I have a grid-tied PV system and get credit from the electric company for surplus energy my system produces. However, this summer we have had many utility outages, leaving my house without electricity—even though there’s a power source on my rooftop.

The whole community here on the reservation uses grid-tied PV electricity, so outages affect everyone. Outages are getting costly for us, as we’ve lost lots of refrigerated and frozen food when there’s no power.

Should we get a generator? Can I hook it up to my system? If so, what do I need?

Dan Wood • Lake Havasu, California

A batteryless grid-tied solar-electric system shuts off when the grid goes out. The first reason is that you don’t want to be energizing the utility lines coming into your house; this prevents danger to line-workers and keeps your PV system from trying to run the entire neighborhood. The second reason is that the loads in your house rarely match the solar input on your PV array. If there is more sun than loads in the house, it’s fairly easy to control. But if the loads demand more power than the available sun, where does the extra come from? When the grid is up, it comes from the grid, but if the grid is down, the only reliable way to do this is to add batteries to provide the extra power.

The most common option to get backup power from a grid-tied PV system is to use a battery-based grid-tied inverter. These inverters can connect both to the PV system and batteries, and to the grid. If the grid goes down, the inverter automatically isolates some or all loads from the grid, and continues to power those loads from a combination of batteries and the PV array. There are many possibilities for adding a battery-based inverter to an existing PV system:

  • Adding an AC-coupled battery-based inverter and keeping the original grid-tied inverter;
  • Replacing the grid-tied inverter with a high-voltage charge controller and adding a battery-based inverter;
  • Reconfiguring the array to use a standard charge controller and adding a battery-based inverter; or
  • Using a high-voltage battery bank with an inverter, such as the SolarEdge StorEdge inverter.

A cheaper option for limited backup is a grid-tied inverter from SMA America that has a “secure power supply” feature. These inverters do not connect to batteries and can only provide electricity when the sun’s shining on the array. In sunny conditions, they can provide up to 1,500 or 2,000 watts, depending on the model. You simply plug your load into a special outlet when the grid goes down. However, you’ll need to be present to do this, it’s not an automated solution.

Another option for backup power is just to add a gasoline-powered generator that you manually start and plug your desired appliances into when the grid goes down. You can also have a propane- or natural gas-powered generator that automatically starts a few seconds after the grid goes down, and a built-in transfer switch to attach your loads (often the entire house) to the generator. None of these generator options interface with a standard batteryless grid-tied PV—it still shuts off when the grid goes down.

Some grid-tied battery-based inverters have options for generator input. The inverter not only controls transferring from the grid, but also can turn the generator on and off as needed.

In all cases, it’s a good idea to reduce your requirements for grid power, since it will make system upgrades less expensive. In hot, dry areas, evaporative coolers can replace traditional air conditioning and provide more efficient cooling. Designing houses to reject solar heat gain in the summer can also help reduce cooling loads, but this can be hard to apply to existing housing. For refrigeration, a fairly new (5 years or younger) Energy Star-qualified fridge and freezer can cut the cost of backup power compared to an older fridge. A super-efficient DC fridge can operate directly from batteries and cut energy use further.

Zeke Yewdall • Mile Hi Solar

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