To battery or not to battery – that is the question!

Published December 7, 2012

We are frequently asked about batteries, what happens when the grid is down, etc. So let’s review three types of solar systems; grid tied, grid-interactive (or battery/grid hybrid), and off grid .

This is by far the most common, requires the least maintenance, and is the least expensive. However grid tied systems are required to disconnect from the grid in the event of a power outage. This means you won’t have any power either. Whether this is an issue for you depends on the individual. If you only lose power 2 or 3 times a year for an hour or 2 this shouldn’t be a problem.
You can go out to dinner, see a movie, or just light a candle and have a glass of wine.

On the other hand, if you require electricity for medical purposes or live in an area with more frequent and sustained power outages, or live at the end of the line (these are the last places to be restored), you may want to consider the other options.

This type system is grid tied but has batteries which provide emergency power. This is more expensive than grid-tied obviously because of the need to have not only the batteries but equipment to charge the batteries, regulate them, provide over current protection and disconnecting means, an enclosure, more wiring, an emergency panel and possibly an additional transfer switch. So if this significant expense is worth it to you, keeping in mind the batteries will have to be replaced in 5 to 10 years, then this may be a viable option for you. These systems will be alittle less efficient than grid tied systems because the chemical reaction that charges the batteries takes energy. Additionally energy is lost in the chemical conversion from the battery to the inverter when the batteries are drawn upon to provide electricity.

These systems are of 2 basic types, AC Coupled and DC Coupled. This refers to the coupling between the PV and the batteries.

In a DC coupled system the PV can charge the batteries directly with DC current via a charge controller. In this type system you may have one inverter with both grid tied and battery charging capabilities. The disadvantage to DC coupled is your strings are usually limited to 150 volts, so you may only have 3 panel spear string for example. This means more wiring and if there is too much distance between the panels and the batteries you will need larger wire and conduit.

AC coupled systems on the other hand go from the PV to an inverter. Here the electricity is inverted to AC which will run loads that are present and at the same time send AC to a battery inverter that has a battery charger built in. Besides providing charge control to the batteries this inverter inverts the batteries to AC which feeds an emergency panel in the event.