Solar Education Needed!!

[Uncle Sam]

I have tried to study and understand sizing solar panels that are designed for RV use. I have a load that I want in my RV that uses 23 watts on a continuing 24/7 basis. It is winter time and the solar panels can only charge the battery and run the device about 8 hours per day. The device runs on 120 VAC and needs an inverter which is OK. The solar panel is primarily used to charge the 12 VDC battery and the battery runs the device through the inverter.

The information I read about these solar panels is that they produce about 17 VDC and maybe 4-6 amps of current. It takes only 14 VDC to charge a 12 VDC battery, so can I get 6-8 amps (or more)of charging current from these panels to keep the battery recharged from the night time drain?

I have never used solar before, so I have no practical experience for understanding. My calculations indicate I need about 100 watts of solar panel capacity to run the device and recharge the battery with only 8 hours or so of daylight or sun. Is this even feasible or are my calculations all wrong? Don't hesitate to correct my numbers if they are wrong; I just want to learn.

Uncle Sam

 

[seattleguy]

Sam - Check out this blog

I wish I had found it before I added solar to my RV.

 

[Happycarz]

Jim,

Seattle is correct about Handy Bob. I have had solar on my motorhomes for close to 20 years and just now trying to optimize my current setup.

It is a math thing; you have 2 amps for 24 hrs coming out of the batteries so you need to replenish 48 amp hours to recharge them. If you have panel(s) capable of charging 10 amps, you would only need 5 hours of sunlight per day to get the job done. Tilting the panels 50* and facing the south, greatly improves charging, as does clouds diminish charging.

If you have the time, read Bob’s blog. I did, and now Bob and I have been emailing back to tweak mine to higher efficiency.

You can get 7 amps out of a 130 watt panel, optimally. Realistically, you will get less. You drop the panel 17.7 voltage to 14.8 through a controller to charge deep cycle batteries, not marine/rv batteries. The controller, wire gauge, distance to batteries, and battery type are all critical factors to get satisfactory results, but easily accomplished.

Give me a call.

 

[seattleguy]

Just curious - what are you running at 23 watts 24/7?

BTW the vending machine I bought from you has been running 100% problem free. Absolutley nothing wrong with it! I wish my RV ran as trouble free.

 

[Greg Pack]

Hey Jim,


There is a loss of energy every time you store to the battery & convert back to AC. There are some rough rules of thumb that you only harvest about 50% of what you actually generate with a panel.

What a panel is rated for and what it actually generates are two different things. A typical number used is calcs is a derating of .77. Look at insolation tables for your area. You generate the peak power of 4.5-5.0 hours a day at best. And that is assuming that it is not cloudy, or very hot, which lowers the efficiency of panels.

Also, most inverters are about 85% efficient at best, and current of the inverter at idle can be large. If you can take the inverter out of the equation for such a small load you may solve your problem. If you absolutely have to have one a unit such as the morningstar suresine will offer the best efficiency.

23Wx24 hours a day is 552 watt hours. That doesn't sound like a lot but in the world of solar it actually is. There is a way to do the math on this but using a rule of thumb you're going to have at least 200 watts of panels to keep up with that load reliably.

 

[Uncle Sam]

Thanks for all the responses----

I think I am beginning to get a handle on the most efficient way to use the latest in solar tech. I read the blog that Seattle mentioned in his post; Handy Bob is another engineer who does not mince words. He did not have many good things to say about the solar industry in general and the RV solar people in particular. He is full-timing it with his wife in his RV with solar only, so that is a pretty good recommendation for me.

After some more study I think the whole solar battery charger system is just 2 things; the proper dc voltage to charge the battery and use of the least amount of amps for your load. Batteries are rated in ampere-hours, so reliability depends on low amperage use over time. I think I can get by with 1-125 watt panel putting out 18 vdc and about 7 amps dc, 2 -12 volt golf cart batteries, and 24 vdc power to my load. I found out yesterday that I can run my load on 24 vdc which eliminates the inverter, transformer, etc in my system.

Seattle, I am not at liberty to divulge my use for the solar thing right now, but it will come soon. Thanks to all of you for your input and ideas on solar.

Uncle Sam

 

[seattleguy]

Jim - You might want to look into using 4 6 volt golf cart batteries instead of the 2 - 12 volt batteries. Longer life while deep cycling.

-Ian

 

[Greg Pack]

Yes, golf cart batteries are best bang for the buck in deep cycle. There are better batteries but they cost a good bit more. You will need more than as single 18V output to charge a 24V system. You can use two panels in series with an MPPT controller though....

 

[seattleguy]

Exactly - I had a cheap controller that I fried and now I have a Blue Sky Energy controller that seems to work well.

 

[Greg Pack]

Also check out the morningstar MPPT controller. It has a built in load feature with low voltage disconnect to keep your batteries from being totally tapped out. It has a remote meter option that helps you keep track of voltage. With this, 24V worth of batteries,wires, and enough panels that should be all you need.

http://www.solar-electric.com/mosumpsochco.html


I see you think you need 150 watts of panels. Did you account for cloudy days? For long term health of the batteries the system needs enough surplus generation capacity to "catch up" the battery charge after a couple of days of cloudy weather. If you are not careful you will have chronically undercharged batteries, fighting for their lives.

 

[Uncle Sam]

Solar update

Thanks for all the input on this use of solar to operate my load. There are many conflicting ideas about solar charging of batteries which is still somewhat confusing to me. I am limited in the size of the panel I can use, so I have to get the highest cell output I can. Right now it looks like 6" X 6" grade A cells (4 watts, 0.5 vdc = 8 amp dc @ 17% efficiency) is the way for me to go. 36--6" X 6" cells hooked in series gives me 18vdc; the same hooked in parallel will give me 8 amps charging capacity at the charge controller if the wires coming from the panel are big enough to eliminate most of the voltage drop.

Uncle Sam

 

[pitzerwm]

What is the payback time, 36 units?? or 72, 36 in series & 36 in parallel?

 

[Greg Pack]

Jim, would this panel work?


http://www.solarblvd.com/Solar-Panels-Systems-Individual-Solar-Panels-175-Watt-+-%2811%29/c1_25_203/p2272/Suntech-Power-175-Watt-24-Volt-Solar-Panel/-Black-Label/product_info.html

 

[Uncle Sam]

What is the payback time, 36 units?? or 72, 36 in series & 36 in parallel?

Bill,

I think you confuse a solar panel with a solar cell. The cells are 6-inches square with 2 tabs (one positive and one negative) to tie cells in series. The panel I am using contains 36 of the 6-inch cells connected in series to add up to 18vdc. Each cell is rated at approx 4 watts @ 0.5 vdc; that would produce approx 8 amps DC in good sun. I am trying to find a panel that would produce these numbers for $300 or less.

Uncle Sam

 

[Greg Pack]

Did you decide to go with a 24V DC to power your device, or 110V AC through an inverter? If the former, you will have to step up the voltage through a transformer if you generate 18V(dealing with conversion loss again), or you will have to generate higher voltages and store in a 24V battery system.

Most manufactured panels will likely look better and outlast a DIY job. Most reputable panels have 20 yr output warranties. I do not have experience with the panel I linked above, Sharp and Kyocera panels both have great reputations.

You can get regular PWM controller in 24V such as this one:

http://www.solar-electric.com/ss-10l.html

But the more expensive MPPT controller will maximize panel output. If panel size is real issue you may want to look into one of these. They are more expensive but it will harvest 20% more compared to PWM controllers.

Your load uses 46AH at 12 Volts, 23 AH at 24V Rule of thumb to maximize life of a lead acid batteries is to not exceed 50% state of discharge so you need a 12V battery at roughly 100AH or 50AH to run this unit for 24 hours with no solar input. If you need to run it for three days then you will need to triple the storage capacity. To keep Lead Acid batteries healthy and charging properly over the longer term your solar panel need to be capable of generating 5% of the battery systems capacity.

I do not know your budget but panel, MPPT controller, batteries and wiring you're probably talking $800.00-1000.00 to do it with manufactured components that have a reputation for long life and reliability. You could go with a slightly higher output panel and PWM controller and might save a little-maybe $100.

If you really want to see what's going on in your system, take a look at buying a Doc Watson meter. You can use it on the load side for a few days to get an average usage, then on the generation side to figure out how much solar you're truly generating. It will save you from the guessing game.

http://www.rc-electronics-usa.com/ammeters/rv-battery-monitor.html

I'm not knowing if I'm going in the right direction, so I'll shut up now. Good luck and have fun.

 

[Uncle Sam]

cfcw,

I am still learning, but your numbers and my numbers are pretty close. Whether I go 24 vdc or 12 vdc has not been determined yet, but ampere-hours surely will come into play. I'll let all of you know how it comes out.

Thanks to all of you for your input and knowledge.

Uncle Sam