By Mike Sokol
One of the most-asked questions I’ve received this year is about the possibility of powering a rooftop air conditioner from solar panels and batteries without shore power or a generator. At first I thought it was a silly idea without any chance of working at all. However, as new technologies become more readily available such as Lithium Batteries, Hybrid Inverters and Soft Start controllers, there’s the possibility of running an air conditioner for at least a few hours in the evening in order to cool down your bedroom enough to sleep. Getting the batteries recharged from solar panels alone is still a difficult proposition, but I’m working on it….
Now, I don’t promise this will be super cheap or that you would be able run multiple air conditioners 12 hours a day, but I’ve just begun experiments on how to integrate these various technologies to create a storage system that will run a single rooftop air conditioner 2 to 4 hours on battery power alone.
In this article I’m going to lay out the science of what such a system needs to do and detail the various components I’m going to integrate into this test. And over the next month I’ll gather empirical data to let you know how it’s all working. Finally, I’ll detail how well I think it’s all working and how you can put your own system together.
“Air conditioners are energy hogs and batteries are poor storage devices.” —Mike Sokol
That’s right. These two technologies do not play well together. A typical RV air conditioner needs around 14 amperes of current at 120 volts AC when its compressor is running. And while that’s pretty easy to get from shore power, it equates to around 140 amperes of current at 12 volts DC from your storage batteries. Yikes! Let’s do a little math and get this into energy numbers.
I’m going to assume a 50% duty cycle of the compressor for these rough calculations. That suggests you’re not sitting in 110 degree heat with the air conditioner running at full blast and not able to keep up. So your mileage will vary depending on a lot of other variables such placing your RV in the sun or the shade, amount of insulation in the walls of your RV, color of your RV roof, air temperature and humidity, etc. But I need a duty cycle number for these initial predictions, so 50% it is.
Let’s do the numbers…
We need to get everything into watts of power and watt-hrs of energy, so here’s our first calculation. If the air conditioner uses 14 amps at 120 volts, then 14 x 120 = 1,680 watts. (Amps times volts equals watts or I x E = P in a textbook.) Now, 1,680 watts is a measure of power, but we need to find energy consumption. So if it was at 100% duty cycle and run for an hour it would use 1,680 watt-hrs of energy. However, we already estimated it would only run 50% of the time, so 0.50 x 1680 = 840 watt-hrs of energy per hour of operation.
Now let’s go see how much energy a single 100 amp-hr battery can supply. If it’s a Lithium battery most of them can safety discharge it down to 0% of total capacity without any damage, but a lead-acid battery needs to be limited to a 50% discharge or you’ll quickly kill the battery life.
First let’s determine how this would work with a single 100-amp-hr lithium battery. To calculate total energy available in watt-hrs we just have to multiply 12 volts times 100 amp-hrs and see that there’s 1,200 watt-hrs of usable energy stored in this Lithium battery. In a lead-acid battery of the same amp-hr capacity there’s 50% less available power without damage, so you can only use 600 watt-hrs of energy.
In our initial example of an air conditioner at 50% duty cycle requiring 840 watt-hrs of energy per hour of running we can quickly calculate that a single 100 amp-hr Lithium battery might be able to run one rooftop air conditioner for 1.42 hrs or around 85 minutes. A standard lead-acid battery with the same amp-hr capacity would only have half that running time, or maybe 42 minutes. Again, this is assuming that the air conditioner is operating only 50% of the time. In a high-heat situation with the compressor at a 100% duty-cycle those run time numbers would drop to 42 minutes for a Lithium battery and 21 minutes for a lead-acid battery. Hardly enough time to cool the bedroom.
We’re gonna need a bigger battery bank
To make this work we’re going to need to increase the battery capacity while doing everything we can to lower the air conditioner energy requirements. So to get more energy storage capacity we’re going to need more batteries. At the very least that suggests two Lithium batteries which can supply 1,200 + 1,200 watt-hrs of energy for a total of 2,400 watt-hrs. Or better yet, four Lithium 100 amp-hr batteries which can supply 4 x 1,200 watt-hrs of energy for a total of 4,800 watt-hrs.
All things being equal, with a 50% duty-cycle of air conditioner run time this suggests that 2 Lithium batteries might operate it for 2 x 85 minutes or 170 minutes (just shy of 3 hours) and 4 Lithium batteries might operate for 4 x 85 minutes or 340 minutes (maybe 5 1/2 hours). Of course, these run times would drop to 1.5 hours and 2.75 hours if the air conditioner was running 100% of the time. Hey, I’m not Reddy Kilowatt giving out free energy here.
But there are a few other tricks that can be done to help the situation. For example, the SoftStartRV controller I’ve been experimenting with for the last few months would reduce the peak amperage draw on your inverter considerably, allowing you to start and run your air conditioner from a 2,000-watt inverter. This at least gives your battery a fighting chance of getting your air conditioner compressor started. You can read more and get a discount price on a SoftStartRV unit by clicking HERE.
Second, I’ve just received a new hybrid inverter from CarGenerator which allows you to combine amperage from your battery bank with limited 120-volt AC power. Instead of a standard inverter which is an all or nothing kind of thing, a hybrid inverter allows you to use two power sources at the same time. So you can utilize your battery bank to add battery amperage to your available 120-volt AC amperage. This technology allows you to start and extend the run time of an air conditioner using something like a 1,000 watt CarGenerator or 1,000 watt generator. Find out more about CarGenerator HERE.
I’ll also integrate a pair of 100-watt Xantrex solar panels in the demonstration to gather info on recharge times (I think I already know those numbers, but need to see for myself).
For a deeper dive into this please read my RVelectricity newsletter tomorrow (Sunday), where I’ll discuss how I plan to set this up. Now please don’t yell at me if this costs more money than you want to spend or doesn’t allow you to run three air conditioners at the same time. I’m just a scientist/engineer not a magician who can say Abracadabra and “poof” there it is. Or am I?
Yup, you can buy a Winnebago Travato with their Pure3 Energy Management System that uses the equivalent of 8 Lithium Batteries and a second alternator on the engine for charging them, along with solar panels on top. It can power the rooftop air conditioner for up to 8 hrs. on a single battery charge, and running the vehicle engine for a few hours will recharge it completely, or so they say. I’m asking Winnebago for a demo unit to try it for myself, which I’ll report on later. So there’s at least one real-world example that proves this can be done.
See you tomorrow on my RVelectricity newsletter.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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