BACK PAGE BASICS: Adding Backup to an Existing PV System


While batteryless grid-tied (GT) PV systems are a cost-effective and efficient way to use solar electricity, most cannot provide power for a home if there is a utility outage. This is because all GT inverters have an “anti-islanding” restriction, ensuring that the PV system cannot energize the utility lines for the safety of utility line workers who are working on restoring power. One exception is systems with a secure power supply (SPS) feature available with the SMA Sunny Boy TL-US series inverters, which can provide up to 2,000 watts via a dedicated SPS outlet—not to the grid—but only up to the amount of power from sunlight on the array.

If power outages are not a concern, batteryless GT systems work well. If access to power during a utility outage is important, new equipment and system alteration is needed. Several options are available, and the best fit will depend on the level of backup desired and your budget:

  • A small uninterruptible power supply (UPS) can be an affordable option that can work to power small loads during short outages.
  • Changing to an inverter with the SPS feature will provide a limited amount of daytime power and may be a workable option depending on how your existing array is configured.
  • A backup generator is an option that can offer a range of capabilities and fit a range of budgets; if you don’t mind dealing with generator noise, emissions, maintenance, and buying and storing fuel.
  • Adding batteries and a battery-based inverter to an existing PV system via AC-coupling is a common solution. The battery-based inverter acts like the grid to trick the batteryless GT inverter(s) into staying live during an outage. This usually requires a significant investment—for the battery bank and a battery-based inverter. It also requires connecting the inverters to a backed-up loads AC subpanel, which becomes isolated from the main service panel during an outage for utility worker safety.
  • New hybrid inverters will make the transition from batteryless to a battery-based system easier and more affordable. See “Next Generation Grid-Tied Inverters” in this issue.
  • Depending on your goals during an outage (i.e., heating, cooking), non-electric options, such a wood-burning stove, may be more suitable.

Web Extras

“Backup Power for Grid-Tied Systems” by Zeke Yewdall HP170

“Adding Battery Backup to Your PV System with AC-Coupling” by Justine Sanchez in HP168

Comments (1)

Jim and Elaine Stack's picture

The key is a good Inverter that can use the batteries as a back up and or time of day use to reduce your costs of power.
Also good non toxic long lasting batteries. .
Lithium or Nickle Iron are the best longest lasting batteries. Iron Edison has both in stock and can get you set in short order.

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