ASK THE EXPERTS: Solar Energy Wasted

Grid-tied inverter systems are less expensive
Watt for watt, grid-tied inverter systems are less expensive and take up less space than battery-based inverter systems. However, they won’t supply power if the grid goes down.
Battery-based inverter systems are more complex and expensive
With more components, battery-based inverter systems are more complex and expensive than comparable grid-tied systems.
Grid-tied inverter systems are less expensive
Battery-based inverter systems are more complex and expensive

A recent New York Times article, “Solar Companies Seek Ways to Build an Oasis of Electricity,” started by saying, “When Hurricane Sandy wiped out the power in areas like coastal Long Island and the Jersey Shore, what should have been beacons of hope—hundreds of solar panels glinting from residential rooftops—became symbols of frustration.”

I read in the article a sense of missed opportunity. I’ve had a grid-tied system for a few years now. I understand that with my current configuration, the essential strategy was to ensure my system wouldn’t energize what would be expected to be powerless grid lines to protect utility repair personnel from electrical shock. However, in situations where restoration of power from the grid may take days or weeks, wouldn’t it be useful for the homeowner, as well as the solar-electricity industry, to have an isolation cut-out device between the grid and the service entrance that would prevent electricity from the PV array from being sent to the grid, but provide electricity to the house and neighboring houses?

Wouldn’t it have been great to read that homes with PV systems were supplying enough electricity to provide a refuge for neighbors who would have been shivering in the dark? Instead, I read about solar energy going to waste.

With all of the gadgets and systems mushrooming in the industry, can’t someone come up with an option that would allow for emergency electricity while protecting the power line workers?

Julius Hayden • Covington, Georgia

There actually is such an option—a grid-tied system with battery backup—but it is significantly more costly, requires more and slightly different equipment, and requires batteries. In HP110, you can find an article on such a system that provides electricity for my office. (Additional articles about grid-tied systems with battery backup can be found in HP139.)

However, the problem is not so much the danger of sending energy back onto the grid when it is down, since there are standard and inexpensive ways of dealing with that. The problem is that a solar-electric array puts out a fixed amount of energy and cannot make up for surges and other quickly changing energy requirements of a home. Plus, sunshine is variable, only available during the day and when there are few or no clouds. That is where either the grid or batteries enter to buffer surges and changes in the solar resource and in usage. 

A good example of the problem is a fridge—something that folks really would like to have energy for during a blackout. That fridge might only take a couple of amps to run, but when its compressor motor tries to start, it might draw 10 to 15 amps for a brief period. And when a cloud skitters over the PV array, without batteries, the fridge—and everything else—would go out.

A battery backup system is more costly—1.5 to two times the cost of a batteryless system—and about 10% less efficient. We usually recommend that people consider batteries only if their homes experience frequent and/or prolonged outages. Of course, with storms occurring with greater frequency due to climate change, more people may be willing to spend the extra money for battery backup.

Michael WelchHome Power senior editor

Comments (8)

bob tarzwell's picture

the trick with multiple parallel batteries is to use isolator switches and do a equalize with only one bank, then with the other banks individually, each month . yes lots of big wires and big 1/4 by 2 inch buss bars and wire each group separate to a big buss bar , I have shunts on each set of batteries and can monitor with a volt meter how each bank is doing, in this application my draw is not that big most of the time during the day 10 to 30 amps , as we have up to 10 to 11 hr of good sun in the Bahamas its at night I need a lot of batteries and I have some of the 100 210 w evergreen panels, south east , south and south west to grab as long a solar day as I can on both systems . one problem we have is shipping , if I go with bigger ah battery's to reduce strings the shipping via boat is no problem its the dock/ local shipping here has a hard time with heavy items ie island problems. The s560's I just had delivered were by a very overloaded pick up truck with 5000 lb of batteries don't know why he did not bust the suspension and we hand unloaded each one . The second system is a 50 panel 210 kangaroo 120 volt dc battery system so only two parallel stings . with a china inverter ( don't buy china inverters JUNK) , don't like the `120 volt dc as to dangerous for my local maintenance man . im electrical eng so I know how not to get dead , but will buy 2 more radiants some day $$$$ and switch it to 48 volts . We have the 5000 sq ft house and four one bed room apartments and the laundry room on one system 10kw 120 volt dc and the pool dock and a few apartments on the other system 22.5 kw 48 volt dc . Problem is air conditioners just kill the battery's at night, but with $5,000 power bills a month we are now down to $1000-$1500 just added 10 flat plate and evacuated solar heaters , 6 pool panels and two hot tub solar panels ,/ our power charge is between a high of 48 cents per kw and 43 cents per kw , and power on the island is very flaky. Last years we had so many power outages it was a major problem for our guests . Been putting solar up now for three years as it was illegal on our island had to battle the power company monopoly rules. we have won and solar is now legal to 75 percent of your load, no grid tie so its off grid back up batteries and I now have 40 s560's and 88 6 volt 190 ah golf cart batteries. As the golf cart batteries die I will replace with better and bigger cells . hey its living with want you got on a brahmas island cant be that bad.

bob tarzwell's picture

I live in the Bahamas which is a hurricane magnet, we are not allowed grid tie yet but its coming, so I installed a 10kw system on the house with outback radiant inverters and 40 rolls s530 batteries. when we get to grid tie I have a disconnect switch so I can de power off the grid and power the house like I have now in a off grid design, a quick program change on the radiant inverter to go back to off grid. ON the 10 apartments I have 25 kw with 3 8kw outback's and 80 rolls s530 batteries , expensive yes but we pay 45 cents a kw in Bahamas for power . problem is as stated you run a extension cord to a close house and they don't know how to live with solar and off batteries , told the person now this is solar battery so don't draw any more then you need , checked the batteries a few hours later and almost dead , walked over and he had three air conditioners running thinking it was free unlimited power, had to have a long discussion with my friend and I don't lend power any more with out a small fuse in line.
one trick I did was build a box that sounds a alarm when the gird goes out and another alarm when it comes back on so I know when my apartments are with out power. Im off grid 100 percent so I never know.

Kemgadi Nwachukwu's picture

Very interesting story of how you tackled the challenges. For the power sharing fuse, wouldn't it be easier to use a circuit breaker with a max rated power of what you are willing to share? I have a power share agreement I'm looking to implement in an extended family vacation site where we built a 25KW off grid system, and I was thinking of possible options, including a smart meter if possible that can disconnect based on some max KWH setting.

Jesse Roberge's picture

40 s530? 5 Strings (Outback Radian is always 48V)? That is a lot of parallel battery strings. You check your charging current balance lately? 5 strings is hard to keep the current balanced between the battery strings. Hopefully you are using bus bars to parallel your battery string and using religiously equal lengths of wire. Battery temperature difference can also disrupt charging balance. If your strings are not charing equally, you will have shortened battery life - some batteries are over charging while others are undercharging and sulfating. When it is time to replace your battery bank, better to get 2 strings of 2V S-1200s (24 batteries per string, 48 batteries total) instead of 5 strings of 6V S-530 (8 batteries per string, 40 batteries total).

Michael Welch's picture

Good story, Bob. I especially like the idea of using a small fuse to limit the energy you lend -- though it could become a hassle to always be changing the fuse. I suppose after awhile, the neighbor would figure it out.

You should not need a disconnect switch when you finally get grid tie capability, the Outback inverters should perform the switchover from grid to battery automatically.

bob tarzwell's picture

Unfortunately in the Bahamas were a BIT behind and they will require a main disconnect when they decide to allow grid feed, it took 4 years of work but the are now finally allowing solar off grid limited back up, as it was illegal before , many like my self still put up solar and said to bad sue me if you don't like it . we have only four solar PV systems so far on the island ( I put up three of them ) and may be only 40 solar hot water systems and we get 9 hr a day of sun shine 305 days a year . a big waste of free energy. but I got a great deal on panels bought 150 210 w evergreen for 100.00 each , some one brought to the island and then found out they were illegal . Still trying to work a way to even out the loads , right now using only a bit of what I can generate so upping the batteries and trying to work a load shedder to divert unused power to the pool might as well have a warm pool , it has 6 pool panels but not quite enough.

Tom Lampros's picture

Hi Michael,
SMA now has a feature on their Sunny Boy - US series of inverters called a Secure Power Supply that allows the owner of a batteryless grid-tied system to disconnect from the grid and power a 15A/110V outlet while the PV system is producing power. Not a lot of power, and only during the day, but its better than nothing. They also have a feature on their Sunny Island inverter/charger that allow them to speak with and control other SB inverters and power them from a battery bank (similar to other off-grid inverters), forming a micro-grid.

Michael Welch's picture

Thanks, Tom. Yes, we've been eying that new SMA inverter for a bit, and are including an announcement of its availability in our Gear section in an upcoming issue of Home Power.

We look forward to seeing how it works for folks who have installed it, but anyone that does will need to be careful of the size of the load they plug into it.

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