By Mike Sokol
Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.
Hey Mike,
Please help me understand this. I’ve been following your recent articles about solar panels and air conditioners, and wonder if there’s a simple chart you can provide that will give us a hint of just how hard/expensive or easy/cheap this is to do. And do we really need a SoftStartRV to run an air conditioner with a 2kW inverter, or is that just sales hype? I really appreciate your straight talk on these complicated topics. —Doug Z.
Dear Doug Z,
Well, since you asked nicely and don’t expect a deep dive into this, I can certainly come up with a basic chart showing the number of batteries and solar panels you need to run an air conditioner for a certain amount of hours.
And yes, if you want to get by with a 2,000-watt inverter you’ll most likely need a SoftStartRVâ„¢ controller to keep the inverter from going into safe mode and tripping off when the compressor starts up with a big surge current. But you can certainly give it a try without any SoftStart technology, and if it works for you, then all is well.
First, if any of you missed my RVelectricity article about this in the Saturday newsletter, read it HERE.
Here are my basic guesses….
I’m going to start with a few assumptions which will then be worked into the graph. First, I’ll calculate the numbers for 100-amp-hr Lithium batteries that can be run down to 0% capacity without any damage. As far as I can see, if you want to run an air conditioner from your RV batteries for a few hours, Lithium batteries are the only practical way to do it.
If you’ll be using some sort of deep-cycle lead-acid or AGM battery that shouldn’t be discharged below 50% of capacity, you’ll need to double the number of batteries. Yes, my numbers are for the least amount of batteries needing the most compact battery technology.
Panel count…
Next, I’ll be showing the number of 100-watt solar panels needed to perform a recharge under normal sun/shade conditions. The number I’ve seen used (and which correlates with my own findings) is around 300 watt-hrs per day for each 100-watt solar panel. Again, your mileage will vary depending on how much sun you get, and what season you’re in. And I’m not considering any type of sun-tracking device. This is for flat panels on the roof of your RV.
Air conditioner compressor time…
Finally, the percentage of time that the air conditioner compressor needs to run hugely influences how long a battery bank can power it. In the blazing sun it might need to run 100% of the time, but in the late evening it might only need to run 30% of the time to keep your RV at a reasonably cool sleeping temperature. I’m picking a 50% duty cycle just for consistency. Again, battery life will be hugely influenced by the insulation in your RV, the color of your RV roof, how cool you need your room, and the ambient outside temperature.
Of SWAGs and WAGs…
My 50% compressor duty-cycle WAG (Wild A** Guess) is just that: a simple guess I can use in this algorithm that will let me create a starting number. Sometimes I just consult my Fuzzy Dice, but I did a little more than just roll the dice for this.
However, it’s more of a WAG than a SWAG (a SWAG is a WAG that is supported by Scientific Observation), since I don’t have a lot of field data to calculate this at 50%. This is more of a gut feel of what I’ve casually observed. However, more testing is needed in the future so I can further refine my basic algorithm.
Chart me up…
There you have it: my best guess on what it will take to run an air conditioner in the evening using battery power alone, and how many solar panels you’ll need to recharge those batteries during the day.
Nope, it ain’t cheap. And nope, it ain’t easy. But as battery and solar panel technologies continue to improve and drop in price, there may come a day when you can run your air conditioner overnight from batteries alone without breaking the bank.
For right now I think a combination of 4 Lithium batteries and perhaps 400 to 600 watts of solar panels, along with a small generator to help recharge the batteries during the daylight hours, might be a sweet spot for some of you. But as I like to say: TANSTAAFL (There Ain’t No Such Thing As A Free Lunch). However, the price is coming down all the time.
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
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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I travel with a cat who rides in the bedroom closet of my 5er. On the road on hot days I set the air at about 83 degrees which keeps her nice and comfy. I have 835w of solar and a 100ah 48v Relion Lifepo4 battery pack. Works very nicely.
On a somewhat unrelated note, I’m dry camping in Quartzsite where the daytime temps have been into the 90s every day. Virtually nobody is running air from a generator. I often see people on RV boards asking what size generator they need to run air, but in my experience very few actually do.
For those of you wanting to go down the rabbit hole with me on this, I’m also considering the power factor of these various types of air conditioners. For example, the PF of a mini-split air conditioner with a PWM drive “inverter” compressor is part of the calculations for determining battery power. And I’m not sure what the power factor of a stock 120-volt compressor would be. For a deep dive into power factor and how it affects the amount of power available, take a long read here. Be warned, this is EE level content that may hurt your brain to think about: https://en.wikipedia.org/wiki/Power_factor
So, Mike, a Sokol WAG is a SWAG?!?
You got that right, Larry! 😆 —Diane at RVtravel.com
Yes, I guess so.
Mike you sure are a SWAG Scientific wild-a– guess! Can’t get more to the point than that!
Snoopy
For those of us in the abnormally dry (and getting dryer) SW U.S., there is another, much more efficient use of solar panels and batteries to cool us in summer–evaporative AC. Turbokool has a 16#, 4.6A, 12v unit that can fit over any existing standard RV 14″ roof vent.
So you can keep your OEM AC unit for traveling to the cloudy, humid East and use the Turbokool in the sunny West.
The “swamp cooler” on our house in Colorado has been cooling us for nearly 50 years at a fraction of the cost to power refrigerated AC. And, in all those years, our indoor temperature has rarely been above 75 degrees even with outside temps measured at over 100 degrees (I have a met station that records outdoor and indoor temps). As a side benefit, the wet pads have even acted as filters to wash the smoke particles out of this fall’s red skies.
Great idea for this Nevada boy, Steve! Thanks!
But a swamp cooler in a wet southern state like Louisiana in the middle of summer just won’t work due to the high humidity. Still it’s an interesting concept that bears more study. I’ll look up the Turbokool unit for a future article.
I agree that 300 wH per 100W per day is a good approximation for an average day. However, from my experience on a few days you’ll get almost twice that and on hot overcast days almost nothing!
Full sun on a cold windy day in late spring or early fall is best!
• The sun is still fairly high in the sky.
• Solar panels are more efficient when cool. Cold air flowing top and bottom really helps. A hot roof really hurts.
The weather factors impacting low watt-hours can easily occur for several days up to a week. You always need either a generator or the ability to move to a 120v outlet. (Or do without power.)
You can get by with a small generator that can recharge the batteries over several hours but that couldn’t meet peak demand on its own. (Or the inverter that hangs off your car battery while you run its engine.)
Yes, there’s a lot of variables in play when powering from solar panels, so you always have to have a backup plan in case the sun don’t shine. Hence my WAG (Wild A** Guess) on the amount of power available per panel. But at least it’s a start as to what really required to run air conditioners while boondocking.
Another consideration is the time of year and latitude. My roof solar panels barely did 50% rating during the winter solstice in Arizona. According to the manufacturers, LiFePo4 battery life varies greatly with depth of discharge, so that also needs to be taken into account. I think Mike is incorrect is suggesting taking the batteries to 0%.
I’m not speaking for Mike, but IMO and what I’m reading is that is what you would HAVE to do, but not the IDEAL thing to do…also, I’d never even try to run my A/C system off my batteries. I run two 6V AGM with a total 440Ah. They charge faster than Lithium batteries from what I understand anyway. Never been close to a 50% discharge!
A number of Lithium Iron Phosphate batteries include an energy managment system that allows you to discharge them to 0%, but they keep a reserve charge so as not to damage themselves. Again, this entire article is just a WAG based on a lot of smaller guesses. But I’m pretty sure I’m not too far off.
Battle Born batteries advertise 100 usable amp hours per 100 ah battery. The built in battery management system disconnects the battery when it drops below the state of charge that would harm the battery. While depth of discharge does seem to affect all types of battery life, lifepo4 batteries still have about 10 times the life cycles of lead acid when used in similar circumstances. Marketing hype? Perhaps. Perhaps not. Time will tell. As to charge time, my lithium batteries charge multiple times (estimate 2 to 4 times) faster than my 6 volt golf cart batteries.
Bill, that’s a great question that I really can’t answer. Many RV’ers seem to demand all the comforts of a bricks and sticks house in a portable package. I have seen 5th wheels with 2 full bathrooms and full-sized residential refrigerators. I’ve been asked about how to install a garbage disposal in an RV kitchen sink. And some 60% of RV owners I surveyed had 2 or more air conditioners. That’s also why campgrounds are overloaded electrically with some larger RVs struggling to run from a 50-amp hookup. So they want to know how to tap into the 30-amp pedestal outlet as well. By the time I was 16 years old I had already camped in 40 states and half of Canada in a Cox pop-up camper where the only thing electrified was my portable AM radio. But even though I don’t understand some of these lifestyle choices, I do my best to answer how to make them work electrically. I hope it helps…
Hi Mike. I see lots of folks asking about/wanting to run A/C units off of a battery bank, I am assuming, while boondocking. My question is why would you want to. People have been boondocking for years without A/C. RV’s are designed to be moved so why not move to a shadier or cooler location?
Because all the cooler and shadier locations are already filled up with locals who reserve 4yrs in advance and then leave their campers parked there empty until the weekend. 😉