Batteries are Complicated

When I started I thought it would be easy to figure out how many batteries I would need for the system. I wanted 20 KWh to cover the possibility of 4 cloudy days in a row. As it turns out, batteries are not rated in KWh, so you have to do some math. Batteries are rated in AH or Amp Hours, and the number of AH available depends on how quickly the energy is used and the temperature. If energy is used quickly, less electricity is available. When the temperature surrounding the batteries goes above 77 degrees, the capacity of the battery goes down. At 95 degrees, the batteries are only half as effective.

In our case, the installers suggested using 8 Trojan L16H-AC 6 volt batteries. These are big deep cycle batteries—11 5/8” long by 7” wide by 16 3/4” high, and they are heavy—125 pounds each. Deep cycle batteries are designed for putting out low amounts of electricity for a long time, while car batteries are made for putting out a lot of energy in a short time. Each are rated to produce 25 amps for 935 minutes, or 75 amps for 245 minutes, or 357 AH (amp hour) if drained over 5 hours, or 435 AH if used over a 20 hour period of time.

Since we are hoping the batteries would last for four cloudy days, the 20 hour rating is the most likely to apply to our situation. In theory, using the power over 96 hours would give us a little more electricity than the rated amount, assuming the garage doesn’t get too hot. So, if we multiply 435 AH by 8 (the number of batteries), we should have 3,480 AH. If you multiply amp hours by the number of volts in the battery (6 in our case), you get watt hours. 3,480 AH multiplied by 6 volts is 20,880 watt hours or 20.88 KWh. In theory then, we should have four days of reserve power.

The cost for each battery with shipping and sales tax is about $500. They are sealed, so there is no maintenance, but they only last for 5 to 8 years. We should probably ignore the fact that if we do save $182.50 per year, we won’t have saved enough money to replace the batteries. At least the technology is really cool. The monitor has a lot of lights and numbers and switches.

In the picture at the top of this post, you can see the closet where the batteries are stored. If you look at the bottom of the picture, you can see a glimpse of the batteries. The batteries are mostly covered so the grandkids can’t touch them.

I’ll let you know as soon as we go live.