Help check me on solar needs calculations

[MN_Traveler]

I had thought that this topic has been exhaustively discussed - but can not really find anything on search. Apologies if I missed critical threads. Also, I am posting this to this forum because the question is specifically around the needs of large, all electric (or nearly so) MH's.

I am (finally) getting serious about adding a solar system to my coach. This posting is specifically about how much to add, not whether to add. please do not create that debate in this thread. My assumptions: no AC usage, minimal (or none) genny run time to power the microwave or coffee maker (have a grill and propane burners to supply that need).

I have it from one installer (who has worked in shops in the Quartzsite area) that a rule of thumb is one 150W panel per 12Volt house battery (or per 2 6V batteries in series to make a 12V cell). My coach has 6, 6V house batteries (wired in series/parallel) - which would imply a recommendation for my coach of 3 150W panels (or as near an equivalent as can be bought).

Now ... my coach has 6 Lifeline GPL-4CT batteries, each with 220 A-hr capacity. wired in series/parallel, this equates to 3 12V (220 A-hr each) cells wired in parallel, giving a total capacity of 660 A-hr. The manuals for the batteries indicate that capacity consumption at 12V is 55% ... indicating a total energy draw from fully charged to 12V is 0.55 * 660 A-hr = 363 A-hr. Assuming that I want to be able to draw my batteries down to that 12V level overnight (I DO have an electric residential refrigerator), that seems to imply that I need to be able to re-charge during the day with 363 A-hr energy from the solar cells. (I know there are many "but what about's" in that statement ... but for now lets assume conditions, etc. are ideal).

Given the rule of thumb (coming from many directions, and seems reasonable to me) that each 100W of solar panel will supply 30 A-hr/day, this would seem to imply that I would need 363 A-hr/day *100W/30A-hr = 1210 W of solar panels. Lets pick on the Renogy 160W panel ==> this would imply a need for 1210W / 160 W/panel = 7.5 panels (no Im not going to saw a panel in half.

So ... here is the question: why the huge mis-match between the rule of thumb recommendation (3 panels) versus the calculation result (~7 panels)???? What am I missing? Is it an overly high expectation of me to think about recharging from a 12V depth of discharge? Are my calculations wrong? Do most people accept that the solar is something to augment the genny (extending times between needing to run it) and not a replacement, etc??? (also ... please do not get into a discussion about my actual energy usage - I am picking on the 12V floor because the of risk to the batteries, not because I am implying any specific energy usage.).

Please let me know your thoughts and experiences.....

[Dutch Star Don]

I think your thoughts about just augmenting your generator is accurate. There are soooooo many variables from one RVer to another. I think you can get your math pretty close to YOUR actual need, but then you run into roof real estate and cost issues.

There are diehards who will winter in Quartzsite with enough solar power to run everything in their coach without a minute of generator use (they usually are not running residential refers). Then you have people like me.....I installed 450 watts of solar to augment the generator. On mine, 600 watts would probably be a better setup for my use, but we rarely boondock. I did build my system, with a combiner box on the roof, so that I could simply add another panel.

[Domo]

Residential fridge or dual fuel?

As an EE, I calculate my daily expected load to figure how many panels are needed. After that, I'd size the number of batteries for storage.

Here's a pretty good article from the Wynns;

[garbonz]

Originally Posted by MN_Traveler

I had thought that this topic has been exhaustively discussed - but can not really find anything on search. Apologies if I missed critical threads. Also, I am posting this to this forum because the question is specifically around the needs of large, all electric (or nearly so) MH's.



I am (finally) getting serious about adding a solar system to my coach. This posting is specifically about how much to add, not whether to add. please do not create that debate in this thread. My assumptions: no AC usage, minimal (or none) genny run time to power the microwave or coffee maker (have a grill and propane burners to supply that need).



I have it from one installer (who has worked in shops in the Quartzsite area) that a rule of thumb is one 150W panel per 12Volt house battery (or per 2 6V batteries in series to make a 12V cell). My coach has 6, 6V house batteries (wired in series/parallel) - which would imply a recommendation for my coach of 3 150W panels (or as near an equivalent as can be bought).



Now ... my coach has 6 Lifeline GPL-4CT batteries, each with 220 A-hr capacity. wired in series/parallel, this equates to 3 12V (220 A-hr each) cells wired in parallel, giving a total capacity of 660 A-hr. The manuals for the batteries indicate that capacity consumption at 12V is 55% ... indicating a total energy draw from fully charged to 12V is 0.55 * 660 A-hr = 363 A-hr. Assuming that I want to be able to draw my batteries down to that 12V level overnight (I DO have an electric residential refrigerator), that seems to imply that I need to be able to re-charge during the day with 363 A-hr energy from the solar cells. (I know there are many "but what about's" in that statement ... but for now lets assume conditions, etc. are ideal).



Given the rule of thumb (coming from many directions, and seems reasonable to me) that each 100W of solar panel will supply 30 A-hr/day, this would seem to imply that I would need 363 A-hr/day *100W/30A-hr = 1210 W of solar panels. Lets pick on the Renogy 160W panel ==> this would imply a need for 1210W / 160 W/panel = 7.5 panels (no Im not going to saw a panel in half.



So ... here is the question: why the huge mis-match between the rule of thumb recommendation (3 panels) versus the calculation result (~7 panels)???? What am I missing? Is it an overly high expectation of me to think about recharging from a 12V depth of discharge? Are my calculations wrong? Do most people accept that the solar is something to augment the genny (extending times between needing to run it) and not a replacement, etc??? (also ... please do not get into a discussion about my actual energy usage - I am picking on the 12V floor because the of risk to the batteries, not because I am implying any specific energy usage.).



Please let me know your thoughts and experiences.....

The rule of thumb is 1 to 1.5 watts per Amphour of battery, so you have 600 amphours of batteries, so 600-900 watts of panels total. Or simply 100 watts of solar per battery. You are thinking of the 6 volt deal incorrectly. More is needed in northern and winter than summer.

[MN_Traveler]

Originally Posted by garbonz

The rule of thumb is 1 to 1.5 watts per Amphour of battery, so you have 600 amphours of batteries, so 600-900 watts of panels total. Or simply 100 watts of solar per battery. You are thinking of the 6 volt deal incorrectly. More is needed in northern and winter than summer.

Actually we are saying the same thing: 100w/panel / 30 amp hours/day is about 3W (solar needed)/Amp hour charging per day. Given 50% depth of discharge that brings you to your 1.5 Watt/A-hr of battery (which is basically the same as my 363 A-hr at 55% depth of discharge at 12 V (per the battery curves). So I think we end up at basically the same place (with the noise being in 50% versus 55% depth of discharge.) So I think the difference is expectation of no-run on the genny, versus extending periods between running the genny??

[HaRVey 74.5]

Your calculations are approximately correct but there are panels that will get you what you need with less realestate. I have four 170W mono panels (680 total) with MPPT controller which produce ~36 amps at peak or approximately 200 amphour/day, which I sized around a four 100ah LI batteries (360ah @ 80%use), but use my gen for occasional big draw items or top off.
I think the Wynns recommend ~1000W+ and 600 amphours of battery for nominal and I do wish I had just opted for the 6 panel kit instead so I could use the gen less.