Beating the Achilles Heel of Grid-Tied Solar Electric Systems

A new inverter from SMA allows us to draw some daytime power from our PV system when the grid is down, even without batteries

The 18 kW PV array on our barn is a group-net-metered system with some of the output going to other houses. Click to enlarge.Photo Credit: Alex Wilson

One of the biggest complaints I hear about most solar-electric (photovoltaic, PV) systems is that when the grid goes down you can’t use any of the power that’s produced. Consumers have spent thousands of dollars on a PV system, and during an extended power outage during a bright, sunny day when the PV modules are certainly generating electricity, they are disappointed that none of that electricity can be used.

This problem applies to net-metered PV systems that do not include battery back-up. Off-grid systems work just fine when the grid is down, but the vast majority of the roughly 300,000 PV systems in the U.S. are net-metered systems without batteries, and most of them lose all functionality when the grid is down.

Given my focus on resilient design (including my founding of the Resilient Design Institute last year), I wanted to install a solar-electric system at Leonard Farm that would have at least some functionality during power outages.

Full islanding capability

I wish we had full “islanding” capability with our PV system. Islanding refers to the ability for a PV system and the loads connected to it to be separated from the utility grid during outages so that no electricity could be fed into the grid and injure utility workers who are trying to repair down lines.

We have three inverters in our system that are housed in a downstairs room in the 1812 barn. The one with Secure Power Supply is the third from the right.

Photo Credit: Alex Wilson

Fully islandable PV systems require specialized inverters along with battery banks that allow them to function off-grid. The battery bank not only provides for functionality at night, but it also establishes the proper waveform during the daytime when the grid is down so that AC power can be delivered to the house.

Some islandable systems, such as the OutBack Radian and Schneider Electric’s Xantrex XW-series inverters, rely on a single inverter that can connect to the grid and a battery bank and switch back-and-forth automatically. Such inverters communicate with and draw electricity from the battery bank during a power outage and also send electricity into the grid during normal operation. These are sometimes referred to as bi-modal inverters.

There are other, battery inverters that can be added to a PV system that already has one or more PV inverters. Inverter manufacturer SMA offers such an option, the Sunny Island inverter that switches between the battery bank and SMA’s Sunny Boy grid-tie inverters with fully integrated controls. SMA’s approach is proprietary, in that the Sunny Island battery inverters only talk to Sunny Boy grid-tie inverters.

The MS-PAE inverters from Magnum Energy offer similar functionality, but can be integrated into systems with inverters from other manufacturers. There are various companies that package this type of inverter with a battery bank and the needed controls to provide islanding, or “AC-coupling” when the grid is down. MidNite Solar is one such packager of retrofit kits.

With any of these options for full islanding capability, there is a significant cost for this type of islanding capability. For a typical, residential-scale 6 kilowatt (kW) system, the cost ranges from about $8,000 to $16,000, according to Mark Cerasuolo of OutBack Power Technologies, who did an analysis of AC-coupling options. This cost includes the specialized inverter, battery bank, and necessary controls.

Detail of our Sunny Boy 5000TL-US inverter. The outlet beneath it provides emergency power during outages (when the sun is shining).Photo Credit: Alex Wilson

A new, low-cost approach

As I said, we didn’t go with full islanding capability, even though I would have liked to do so—and may in the future. The cost of the battery system and other components was just too much for our budget that has been stretched pretty thin with our complex building project—which is finally nearing completion.

What we did do, however, was install a brand-new inverter from SMA that has an outlet that can continue delivering some electricity when the sun is shining during a power outage. SMA calls this feature “Secure Power Supply.” Mounted beneath our 5 kilowatt (kW) Sunny Boy 5000TL-US inverter is an outlet that can deliver 1,500 watts (12.5 amps at 120 volts) during the daytime the power grid is down. Unlike other islanding systems, there is no requirement for battery storage with this option.

This isn’t enough power to operate all the loads in our house that I’d like to power during a power outage, but it’s far better than nothing. The cost is essentially the same as a standard Sunny Boy inverter (though a separate outlet has to be installed). Ours was installed by Integrated Solar Applications in Brattleboro, which installed the  entire 18 kW net-metered system (with 6 kW being owned by a neighbor).

Like other models in the SMA TL line, our 5000TL-US is a transformerless inverter, which is smaller and lighter than standard inverters, and it offers even higher efficiency: roughly 97%.

Emergency power uses

While 1,500 watts is a significant amount of available power, this Secure Power Supply feature is not really intended for loads that have significant surges as they cycle on or that could be harmed by fluctuating current, such as refrigerators. It’s really designed for charging cell phones and laptop computers.

But I’ll be carefully examining power consumption and surge demand when we shop for a new chest freezer—it would be very nice to be able to power that freezer during the daytime during extended power outages.

Our PV array being installed on the structurally reinforced roof with standing-seam metal roofing.Photo Credit: Alex Wilson

There may be a Sundanzer chest freezer, for example (a freezer made especially for solar systems that can work in DC or AC mode), that will work well with the limited output from our inverter. At the very least, we’ll be able to keep our cell phones and laptops charged and power our cable modem and router.

Still in limited supply

I had heard about the new 3000TL-US, 4000TL-US, and 5000TL-US inverters late last year, and heard that they would be shipping in the first half of 2013, but it turns out that we got one of the very first to be installed in the U.S.—or at least in the Northeast. Demand is very high for these systems.

I suspect that within a few years, most grid-tie inverters will include this emergency-power option. I haven’t had to test it out yet, but will be ready for that ice storm this coming winter!

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.