Today I wanted to share some info about air conditioning on a solar panel system. Charlotte’s heat really came full force this week. I know for many their climate doesn’t get as humid as it does here, for us here, AC is pretty key. Without AC I can’t really sleep, even with a fan and my house being passive cooled. While the humidity is still pretty comfortable, it’s HOT and the humidity is coming. It has been in the high 80’s and low 90’s outside, which made my house in the mid 90’s inside.
I thought I’d do a post today because I’ve been able to run some real world experiments with my tiny house, the AC and solar. I haven’t seen any real world into practice reports on this stuff, so I figured it would be helpful for you all.
I have yet to hook up my mini split system because it has taken me a long time to find a HVAC installer that would install my mini split, the reason being they all want to sell you the equipment if they are going to install it. This was an unknown factor to me when I ordered my unit, but these are the bumps in the road you experience when you live The Tiny Life.
For heating and cooling I opted for the Fujitsu 9RLS2 which is a 9,000 btu unit with a seer rating of 27. To give you an idea, older systems have a SEER of around 8 to 10, modern systems that are labeled highly efficient have a rating of 15 or so, but most today are around 12-13. This is very important because me being on solar, my system simply couldn’t handle the less efficient systems. Read about my tiny house solar panel system by clicking here. The SEER rating is simply a function of BTUs (British Thermal Units) to Watts. The higher the number, the better.
The other big reason I choose this unit versus a window unit was that my air handler is wall mounted, out of the way and above eye level. This does a few things: keeps my limited square footage clear of stuff, it keeps my windows looking nice because I don’t have a window unit blighting a good design, and keeping it above eye level also makes you forget about it because as humans we don’t often look up.
While I’m trying to get an installer lined up I’m using a Portable Air Conditioner which has worked pretty well. The downside to it is it takes up a lot of space and it’s not as efficient; it has a SEER rating of 12, which makes my mini split system 225% more efficient than this.
I decided to “stress test” my system by turning the Portable AC unit on high and setting the thermostat to 60 degrees and see how long it was going to take for my batteries of my solar panel system to bottom out (50% discharge). The charge controller on my system automatically turns off the power to my house if the power in that batteries discharges down to 50%, this allows me to not damage the batteries by discharging too deep.
As you can see by the chart above, keeping discharge at 50% or above gives me a little shy of 2,000 cycles or 5.4 years. I plan to add another set of four batteries to the system pretty soon, which will give me a good capacity and keep my discharge rate much higher than 50% (though I don’t often get that low) In about 5 years we should start seeing some really interesting battery technologies hit market, so I plan to hop on that as soon as my batteries begin to fade.
My stress test turned out pretty good. With the much less efficient portable air conditioner I ran it solid for 3 days starting with a very warm house. At the end of the three days I was very close to hitting 50%, but it didn’t ever dip below. I decided that the test went on long enough to be pretty happy, so I decided to stop. I typically turn off the AC when I’m gone.
The past few days have been a bit trickier because since my system was so low from the stress test, I needed it to build back up, but we have had a series of cloudy days. I’ve had plenty of power to run the AC over night, but it’s lower than I’d like. To give you an idea, on a normal sunny day I make about 1600 watts, on a cloudy day I get between 600 and 800 watts when the clouds are very thick with no gaps.
The really great thing is when it’s hottest, during the day, I can make lots of power. This allows me to run the AC full blast and I can make enough power to run the AC and still be dumping 1000 watts into the batteries. Compare this to heating, you most often need the heat at night the most, which is when the sun isn’t out, so its a major drain on your batteries. To compound the issue of heating, heaters are often more energy intensive than cooling.
The other night I tried an experiment. I got my house very cold and turned off the AC at midnight (when I usually go to bed). Outside it was pretty cool, about 65 degrees and about 45% humidity, so not bad. I left all the windows closed to see how much my body heat would heat up the house and because in the summer, opening the windows doesn’t help even if it is cooler outside because the humidity increase the “feels like” temperature.
As it turns out in just three hours my body heat warmed the loft of my tiny house up to the point that I woke up from being so uncomfortable from the heat. Around 3:30 am I woke up and it was very hot in my loft. I checked the time and was surprised how little time it took. I should note that I’m one that when I fall asleep, I stay asleep all night, even if I get warm, so the fact that I was woken up goes to show how uncomfortable I must have been, because it takes a lot.
I had prepared for this and all I did was crank open my sky light (the highest point in my house) and the loft end window and switched on a fan to draw in cool air. Within 5 minutes the whole place dropped about 5 degrees and I was back asleep.
So that has been some of my real world experiences with the tiny house, AC and solar. I know I had always been frustrated by not enough stories on this stuff, so hopefully I can help others.
Some key resources for those wanting more technical stuff