Backing up your grid-tied system with batteries

As we have discussed in the introductory pieces on solar, there are two principle kinds of solar systems: grid-tied and stand alone. Grid-tied systems are directly connected to the power grid and any energy that is collected is converted into AC by an inverter and supplied to your house or sold to the grid. Alternatively, stand alone systems supply power to batteries, which are then directly plugged into the house. A third kind of solar system is a combination of the two: grid-tied with a converter that can also feed into batteries. This alternative setup allows the solar owner to have more control over how and when their power is used. Let’s talk more about how this would work and why it may or may not make sense for a typical home owner.

One of the common questions asked by people new to solar systems is: do solar panels power the house when the power supplied by power lines goes off? If you are among the majority of solar panel owners with a grid-tied system, the answer is no. When the power grid goes down, so does the grid-tied solar system. The inverter, which is responsible for converting the DC power of the solar array to the AC power used by the house, goes down with the grid, thereby blocking the power from being used in any way. It could be the sunniest day of the year and it wouldn’t matter; all the energy collected by the panels wouldn’t be used without the inverter to convert the DC to AC.

One way to get around power outages is to use bidirectional inverters, which are capable of converting both to AC and DC. Essentially, they are capable of converting energy gathered by the sun to usable energy for the house (i.e., DC to AC) and energy from the grid to charge batteries (i.e., AC to DC).

Another benefit of bidirectional inverters and batteries is they allow you to decide when you buy energy from the grid versus when you sell energy back to the grid, which gives solar owners the potential to save money on tiered pricing. It basically comes down to using less power from the grid and selling more power back to the grid when you would be paying more per kWh for that energy. In some states, such as Hawaii, too many people are attempting to sell power back to the grid, which has prompted utility companies to disallow selling energy back to the grid. Even in these states, however, having a bidirectional inverter and a battery system allows you to decide when to buy the least amount of power. Once again, paying attention to the tiered system would allow you to save the most amount of money possible if you used your batteries most when the price of energy was the highest.

Although bidirectional converter and batteries are an attractive option, there are some important factors to consider before getting out your checkbook. The biggest deterrent for buying bidirectional inverters with battery storage is price. A typical inverter is thousands of dollars and the batteries are hundreds. A complete set of batteries with a standard bidirectional inverter would run for about $8,000. The battery life is mostly determined by the ambient temperature in which the battery resides. In a typical basement, the battery should last about a decade.

As battery technology improves, we may see the prevalence of battery systems increase. However, for now, the high cost will continue to deter many potential customers. Closely considering all the factors that go into the cost of energy where you live and your energy usage is important before pulling the trigger on a battery kit. If you would like to discuss your personal situation with a representative and figure out if buying such a system is in your best interest, give us a call. We’re happy to help.

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