How many solar panels are needed to run an air conditioner off-grid?

So, how many solar panels does it take to run a solar an air conditioner on solar? Well, let's look at it in terms of running a ton of air conditioning, or 12,000 BTUs of air conditioning.  Do you want the punchline first?  If it's a solar air conditioner heat pump with variable-speed DC motors, like Airspool's MS12,  with plug-and-play solar panels and backup grid power, then the answer is 3.   Assumption:  The unit needs to run all year long even in harsh desert conditions like Las Vegas, where extra watts are needed to drive the compressor in the summer, and the panels are 450 watts (now becoming standard).  

So what's the problem running a 'normal' air conditioner directly off of solar?  Well, normal air conditioners (or heat pumps, meaning that they have a  reversing valve and can provide heat in the winter or when it's cool) run off of alternaing current (AC).  Since solar power from solar panels is inherently direct current (DC), the panel power first needs to be converted, or inverted, to alterneting current.  There's normally a 3-to-5% loss in this conversion. Not a big deal.  

 A bigger deal is that such units have only 2 speeds, on and off.  As such, when they turn on, there's a huge surge, or inrush, current that's needed to get the unit running of roughly 4 to 6 times the normal running current.  You may have heard, or even felt, an air conditioner come on, since this huge current draw is a quaking jolt.  To accommodate this first few seconds of the fans and compressor coming on a normal unit, yes, you need either 4 to 6 times more solar panels or you need to store this energy in a battery bank to accommodate this every time the unit cycles on.   And, these units cycle on a lot.   They shut off when they exceed the setpoint temperature, and then come back on when the temperature has drifted back above the setpoint temperature.  The process repeats itself again and again. And, as a result, these on/off units have an efficiency that's usually significantly (30+%) less efficient than units with variable-speed motors.

 Finally, what about nighttime?  If a solar-powered air conditioner can garner 100% of its energy from, say 8am to 4pm (on average, season dependant), what about the other 16 hours where little or no energy comes from solar?  There's a divergent philosophy here on how to manage these times.  If you really need to be off-grid 100% of the time, you're going to need storage.  And, what does that storage look like?  Well, let's assume the stander-efficiency unit you're working to power requires 1,800 watts per 12,000 BTUs to run in the heat of a hot summer night (when you need it most).  Sure, spring and fall are less burdensome in terms of energy consumption, but let's plan for a worst-case scenario.  1,800 watts x 16 hours = 28,800 watt-hours, or 28.8 kWh.  If you're running lead-acid batteries with a 50% duty cycle, that around 36 batteries.  Oh, and if you want to try the Tesla Powerwall instead, one $7,500 Powerwall battery is 13.5 kWh.  So, yes, that's why people say that, without lots and lots of money (or without net metering--another emerging problem, by the way),  solar-powered air conditioning just doesn't make sense.

So, how does it make sense?   At Airspool, we believe in embracing the grid.  Exploiting the grid.  However, you want to put it.  Run your Airspool unit hard and free off of solar during the daytime.  At nighttime, or when the clouds are thick, it needs the grid, if you need to run at these times, and to access the grid, plug in Airspool to any 120v outlet.  And, during such times, it runs at a super-efficient 22 SEER, so that's very likely 50+% more efficient than your existing home unit.  Oh, and, did we advise that many of our clients actually use Airspool where they really just need air conditioning during the day?   People who have a garage or work shed they want to air condition during the day, for instance.  Or commercial users.  If you're not there, you don't need to condition the space.

Or, maybe you do?  If you have an RV that's 120 degrees in desert heat or 90 degrees in 78% humidity, let Airspool cool and dry it every day during the day when it's hottest for free.  Something to think about.

To discuss the possibilities, email comfort@airspool.com or give us a call at (725)699-1200.

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