Wednesday, October 5, 2022

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RVelectricity – Just Ask Mike (J.A.M.): How much solar power do I need?

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. Today I discuss solar power.


 


A question from the Hershey RV Show this week…

Hey Mike,
One of the RVs I’m looking at has 150 watts of solar panels and a 100 amp-hr lithium battery. The salesman says that a 150-watt panel is plenty to keep my battery charged for a few days of dry camping. It also has a new 12-volt DC refrigerator, and I’ll need to use a CPAP machine at night. Is this guy correct, or is he blowing sales smoke up my skirt? —Alfonso

Dear Alfonso,
Ah, yes… This is a case of over-promising and under-delivering. Either he has no idea how solar works, or he’s just lying to make a sale. Let’s look at the numbers to see how this actually works.

How much battery storage does it have?

As I’ve noted many times here, a lithium battery can be discharged down to 0% State of Charge (SoC) thousands of times without damage. To calculate available energy we just need to multiply 12 volts times 100 amp-hrs to see that it equals 1,200 watt-hrs of energy.

Of course, if you had a 100 amp-hr flooded cell or AGM battery, they should only be discharged down to 50% State of Charge for best life, which would be 600 watt-hrs of energy.

How much energy will a 150-watt solar panel provide?

The general rule of thumb is that each 100 watts of solar will be able to provide between 300 and 400 watt-hrs of charging energy per day, depending on cloud cover, angle of the sun, and length of day. So if we multiply 1.5 times 300, that equals a minimum of 450 watt-hrs of charging per day, and possibly as much as 1.5 times 400 which equals 600 watt-hrs per day in a sunny climate. Let’s use 400 watt-hrs per day to account for real-world losses.

How fast will that charge the battery?

With everything turned off in your RV, that 150-watt panel will take at least 3 to 4 days of sun to completely recharge a 100 amp-hr lithium battery. That’s because 1,200 watt-hrs of battery divided by 400 watt-hrs per day of charging equals 3 days of time.

How much energy will that refrigerator use?

Of course, this depends on a lot of variables. But my experiments with 8- and 10-cubic-foot 12-volt refrigerators have shown that you’ll need around 900 watt-hrs per day to keep it running. So with a 1,200 watt-hr battery, it will drain the available power in a less than 2 days.


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Can 150 watts of solar panels keep up?

No, because it can only replace 400 to 500 watt-hrs per day. So, that might extend the refrigerator another half day of running.

So how much battery and solar do I really need?

If you want to dry camp (boondock) with a 12-volt DC refrigerator and CPAP machine for several days (let’s assume the CPAP machine will need 200 watt-hrs of energy per night), you’ll need a second lithium battery, which is 200 amp-hrs or 2,400 watt-hrs of storage. You’ll also want at least 600 watts of solar, which could provide 2,400 watt-hours of charge per day. This would allow the sun to completely recharge your batteries in a day. Even if the sun doesn’t shine, you’ll still have at least 2 to 3 days of stored battery power.

Caveat emptor.

Just like any other purchase, you’ll want to research the claims made by the salesperson. I’ll be doing more in-depth studies of solar panel and battery selection for boondocking, so stand by for more data. But in the meantime, I can tell you that 150 watts of solar panel and 100 amp-hrs of lithium battery storage will not be enough for your camping expectations.

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….

Send your questions to me at my RVelectricity forum here.

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.

You don’t want to miss Mike’s webcasts on his YouTube channel.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign

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Larry Lee
16 days ago

Another way to approach this analysis is to ask how long could he boondock with 100AH battery & 150 watts of solar. Fridge+CPAP+misc=1200 watt-hrs/day. If he arrives at 2 PM then by 2 PM on day 2 he will have about 400 WH left in the battery (having used 1200 WH the battery started with and replaced 400 with solar). Now to run CPAP through the night he must turn the fridge off when he goes to bed and the lithium battery will have near zero charge in the morning. If he is a weekend camper it is now Sunday and time to pack for home. Once his vehicle is attached and engine running he can restart the fridge. Obviously a very marginal weekend only arrangement. Next just add a 2nd 100 AHr Lithium battery and he can leave the fridge on all weekend even with only 150 Watts of solar. Adding 1 more 150 watt panel would mean about 800 WHrs each day so he would fall behind by 400 WHrs/day and the 2 battery set would be done after 6 days.
One more panel & he goes full-time.

Bob Palin
16 days ago
Reply to  Larry Lee

I like your thinking, but the numbers assume that it’s summer and there are no clouds or trees.

Joseph Phebus
16 days ago

Well, I hope the potential buyer goes back to the dealership and says to the salesperson with the sales manager present

“At best you don’t know your product and at worst you lied to me. Therefore you have lost this sale and my business forever. Moreover, I will be telling my family, friends and neighbors of the experience that what this dealership using deceptive sales tactics and suggest they go elsewhere. If you didn’t know the answer, you should have said so, done your job, and gone to someone who does. This also reflects on the sales manager and management who obviously have little interest in hiring a well trained professional staff.”

Brad Teubner
16 days ago

My experience is that roof mounted solar panels provide less than 50% of rated power (averaged 4 hours) at winter solstice at Quartzsite.

Bob Palin
16 days ago
Reply to  Brad Teubner

I agree, I spent 3 months on the road last winter and that was my experience. I had 400W of solar and 300AH of AGM batteries, propane fridge, no furnace. I had to run the generator every other day for a few hours.
I’ve upped my game to 800W and 600AH of lithium for this winter, but I’ve added Starlink and a small (700W) Instant Pot…

Last edited 16 days ago by Bob Palin
Jesse Crouse
16 days ago

When someone says-“Trust me, it will work” a bell goes off in my head. Statements like this should be covered under the “Truth in Lending” when you sign the sales and loan docs. I’m not holding my breath on that.

Michael Roach
16 days ago
Reply to  Jesse Crouse

So True Jesse, whenever anyone says trust me , I’ve lost all trust in that person.

Admin
RV Staff(@rvstaff)
15 days ago
Reply to  Michael Roach

Ha!. That reminds me of when my now-46-year-old son would say, “Trust me,” I knew something was seriously wrong. 😆 Have a great day, Michael. 😀 –Diane

Bob p
16 days ago
Reply to  Jesse Crouse

The salesperson (notice I wasn’t sexist) is there for one reason only… sales! The person probably knows less about their product than you do. That’s been my experience anyway as I do my research before I do my looking.

Tom
16 days ago
Reply to  Jesse Crouse

I’m from Washington, and I’m here to help. My favorite lie.

Admin
RV Staff(@rvstaff)
15 days ago
Reply to  Tom

Hi, Tom. That depends on which Washington you’re referring to. My long-ago boss had a client in Washington, D.C., who called our office. When he identified himself as, “I’m so-and-so, from Washington,” I don’t think he appreciated it when I said, “Oh, you’re from ‘the other‘ Washington.” (To me, Washington state comes first. 😆 ) Have a great day. 😀 –Diane