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Redwood Alliance's PV Systems

Redwood Alliance’s office building demonstrates two identically sized—but very different—PV systems:

  • Grid-tied with battery backup PV (solar-electric) system. 3.2 kW with an OutBack inverter system.
  • Batteryless grid-tied PV system. 3.2 kW with a Xantrex GT inverter.

Redwood Alliance's two PV systemsThese systems offer a unique opportunity to examine the differences between a grid-tied system that is batteryless and one with battery backup. In addition, the battery-based system equipment is identical to a typical off-grid system, so that possibility is also demonstrated. These systems were featured in Home Power magazine in the first two issues of 2006. Download those articles here:

These PV systems power all 5 of the businesses in the building, with nearly no electric energy needed from the utility each year. The businesses include 2 nonprofits, a tattoo parlor, and an investment firm.

System 1 (battery-based) has its PV modules mounted as an awning, suspended from the second story walkway railing. All the balance of system equipment, including batteries, are readily visible from inside Redwood Alliance’s office space. Note that the balance of system components take up more space than the equipment for most grid-tied systems.

System 2 (batteryless) has its PV modules mounted on the building’s flat roof, the same number and same types of modules as for System 1. The relatively small size of the inverter, compared to the batteries and equipment required for System 1, make it a more attractive prospect for most potential system owners. In addition to the amount of space the balance of system components take up, the other advantages in a batteryless system are lower cost, simpler installation, and higher system efficiency.

Because it does not need to deal with keeping the batteries charged up, the batteryless system is consistently about 10% more efficient than the battery-based system. This means that it pumps 10% more energy into the grid, offsetting that much more of Redwood Alliance’s energy usage.

The battery-based system’s equipment and installation cost nearly $10,000 more dollars for the same sized system—yet produces less usable energy. More equipment is required to manage the batteries, the system is more complex to install, and good, deep-cycle batteries are themselves expensive.

So, why would someone choose a battery-based system over a batteryless one? For off-gridders, the answer is obvious. You must have batteries to store PV system energy for when the sun is not shining, like at night or during cloudy times.

For on-grid folks that can use the utility’s net-billing program to “store” energy, there are a couple of reasons that work together to determine if the extra cost and lost efficiency of a battery-based system is worthwhile. If you live in an area with frequent or extended utility outages, having battery backup can make the difference in whether your home is heated, or whether your fridge or freezer contents spoil.

The other factor is whether or not your loads are critical, even if outages are not frequent or long. For example, some types of medical equipment are critical to the health of some homes’ inhabitants, and must be powered all the time.

Redwood Alliance is fortunate to have such a battery-based system. If the grid goes down, as it has many times since the installation, the switchover to battery-backup is so transparent that they usually do not realize that there was a utility outage.