Tuesday, August 24, 2010

Solar electric system update and statistics

It’s been over six months since we threw the switch on our off-grid solar system. To refresh, we have 1,250 watts of solar panels, 12 6-volt batteries wired together for 24 volts, a Xantrex XW MPPT solar charge controller, and a 1,100 watt true sine wave inverter providing all of our power needs for our home.

We have had no real issues with our system. It has worked as it is supposed to. We are able to power lights, computer, fans, washing machine, refrigerator, vacuum cleaner, sewing machine, and even occasionally an iron. As you may recall, our refrigerator is a chest freezer with an external thermostat. It has provided the only real challenges. First, there is no freezer for ice cream! Second, it accumulates condensation in the bottom which must be sponged out periodically. And, third, it sometimes surges beyond the capability of our inverter when starting.

This third challenge is not a big deal. When the appliance can’t get started within a couple of seconds, it stops trying. It tries again in about 30 seconds. If it can’t start on that try, it stops and waits another 30 seconds before it tries again. It may take it a few tries, but it always gets started. This is not an all the time occurrence, and the problem would be eliminated if our inverter could handle a surge over 2200 watts. I didn’t realize how much the freezer can surge when starting when I put our system together. When running, it only consumes about 200 watts or less.

Our charge controller keeps stats on the power that is harvested. During the last six months, we’ve harvested 469 Kilowatt hours of electricity. That equals out to an average of 2.43 Kilowatt hours per day. Some of this power goes into charging the batteries and some is used during the day.

Our battery monitor keeps track how many amp hours are charged into the batteries or discharged from them. According to it, we have used a total of 244.8 Kilowatt hours from the batteries during the last 193 days that our system has been online. That is an average of 1.27 Kilowatt hours taken from the batteries every day.

The difference between the 2.43 Kilowatt hours per day average harvested and the 1.27 Kilowatt hours per day average used from the batteries is the extra we have used during the day when the sun is shining. We are able to run ceiling fans and floor fans to keep cool enough to survive the many 90+ degree days we’ve had this summer.

We are able to charge our batteries with our generator if the battery level drops low enough (it’s not good to cycle lead acid batteries too low). We have only hooked up the battery charger three times, and all of them have been during the summer months. The first time it wasn’t really needed, but I wanted to see how well it worked. The other two times followed several days of cloudy but hot weather. Our usage exceeded what we could harvest.

solar harvest graph

When looking at our usage by month, we have harvested a lot more power during the summer months than we did during later winter early spring. This is no doubt impacted by the amount of sun available during the summer, but another factor is our usage. When we do not use as much, the batteries are charged more quickly and the charge controller only harvests enough power to maintain the batteries.


So far, we have not felt deprived. We are able to live our lives normally with our limited consumption (one of the keys to making an off-grid solar power system work). It’s been a great blessing to be able to run fans and to have the convenience of the washing machine and refrigerator. So, I would say that so far, our system has done quite well.

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