Solar Rule Of Thumb....

One rule of thumb 100 watts solar for every 100 amp hours of battery. It really depends on your usage, you need to calculate the watts of each item you will be using for total watts/amp hours used and what is needed to put back into batteries.

A universal rule of thumb...NO.

Do a simple spreadsheet that takes the wattage of each device that will be in use times an estimate of how long it will run to calculate the total watt-hr.

Say you have a 75w TV runing for 4hours...300watt-hr.
Say you have 5 - 20w light bulbs going for 3hr...300watt-hr

So if that's all you have, you need to replace 600watt-hr each day with solar. Usually you will get around 4hr at the rated output so figure 150watts of solar pannels (600/4 = 150)

There are some catches that go along with that.
- The last 10-20% charging your batteries goes much slower as they can't take a lot of power, so a ton of solar into a small battery bank can result in a lot of the solar being wasted.
- With lead acid, you don't want to drop below 50% charge or you damage the batteries, so you need enough cushion to cover this. (to get battery amp-hr, divide the watt-hr by 12 to get amp-hr (12v).
- Do you want to cover the occasional cloudy day? If you want the ability to go 3 cloudy days without resorting to the generator, you need to triple the watt-hr for storage and increase the solar so it can catch up when the sun comes back out.

Nothing rocket science about the calculations. Make your best estimate and add a little cushion and you will do better than a random number thrown out.

How many watts you need ?,.....if its raining, or shining, or somewhere inbetween ??

First I would look at increasing battery capacity, either 4 GC-2 or another configuration depending on your carrying/storage capacity.

How long do you want to go without charging from another source? A weekend trip or a 2 month stay? If your solar can get you back to 90% then you start the next day at 90% instead of 100% and you likely will end the next day at 80%. You cant blanket say, "this will do it" too many variables. Weather, daily usage, and they compound on each other. If you have a really rainy day, not only do you make less with solar, but you likely will use more from your batteries because you will watch more TV run more lights and other things more.
One of the nice things about solar is that you can add to it.
Start with 200-300 watts and a 40 amp controller, use it a while and see how well it covers your needs. Then, if it is insufficient, add one or two more panels as needed to reach your objective. A 40 amp controller should let you expand to fit your needs without buying a replacement.

You just need to calculate watts in versus watts out.
When buying a panel go as big as possible. Anything less than 100 watts is a joke. The bigger the panel the cheaper it is on a watts/dollar basis. The best deals will be on panels in the 275 to 330 watt range. For these you will need an MPPT charge controller to match the output voltage to your battery bank.
Don't skimp on the panel or the controller. Get a big panel, get a controller large enough to handle a few more panels. A 60 amp controller will handle 2 or maybe 3 of the large panels.

A few rules of thumbs that I've developed, and of course, not scientifically developed:)!

> Put as high as quality, and as many that will fit, solar panels on your roof. (If the roof will fit 12 (Or whatever.) panels, put 12 (Or whatever.) panels on it!
> When you replace your house batteries, put as good of quality and as many (Focus on combined AH's vs numbers of batteries.) AGM's (Or if you're in the mood to do so, Lithium.) that will fit in your battery bay. (Lithium's charge the fastest, and AGM's charge faster then Wet.)
>Don't cheap out on MPPT Controller, and be sure to have an effective Battery Monitor (If Magnum Inverter/Charger, the BMK Meter does well. If not Magnum, check out Bogart's equipment. As far as MPPT Controller, if the Inverter/Charter is Magnum, then no reason not to look at the PT-100. If not, then just be sure you properly size your chosen Controller.)

Whatever gear you end up with, really learn the in's and out's of how to use it, proper battery management can pay dividends of longer living batteries:)!

As said, not scientific. But do it once, and go have some fun:)!
Smitty

KD4UPL wrote:
You just need to calculate watts-hours in versus watts-hours out.
When buying a panel go as big as possible. Anything less than 100 watts is a joke. The bigger the panel the cheaper it is on a watts/dollar basis. The best deals will be on panels in the 275 to 330 watt range. For these you will need an MPPT charge controller to match the output voltage to your battery bank.
Don't skimp on the panel or the controller. Get a big panel, get a controller large enough to handle a few more panels. A 60 amp controller will handle 2 or maybe 3 of the large panels.

Key difference. You may have a peak demand of 200w but if it's for 10hrs, you need a lot more than a 200w panel.

You can tell what size he will need or want until he assesses what he is going to use. If it's just the water pump and a single 15w light bulb for a couple hours, a 100w panel is overkill (of course, that would be a very minimalist lifestyle).

Whether to apply the "don't discharge more than 50%" rule of thumb depends on how much you'll use the battery.

If you're less than a full-time boondocker, the difference may simply not matter. Only 4% of campers do over 30 days per year. The majority camp less than 15 nights. And even fewer if only considering boondocking.

You can find pessimistic estimates for generic deep cycle lead acid of 200 cycles @ 100% discharge. Then look at a premium battery - Trojan Battery's literature for their SPRE 06 255 solar battery (same size as a T-105) shows a life expectancy of about 1900 cycles @ 50% discharge, and about 1200 @ 80%.

The inexpensive Interstate GC-2's sold by Costco are supposedly 650@80.

So, take the extreme high end of 30 full battery cycles per year, and use the pessimistic expectation of only 200 cycles of lifetime, and that's still almost 7 years. Not many RV batteries last longer than that regardless of how well they're treated (now someone will describe their 20 year old battery bank).

But let's say, age and cycles combined, the battery lasts 6 years instead of 7. We did that by limiting how much battery capacity we used - 50% instead of 80%. So, over those 6 years, we avoided using 30% of the available capacity to save (7/6=1.17) 17% in battery cost. It's still a false economy.

Use more realistic figures, and things only get better. Use the Interstate 650@80 numbers, and that's over 21 years. Cycles simply aren't a significant factor at that point. Even a battery on constant float charge isn't going to last that long.

Good point made above concerning number of cycles per year.

One important factor in going below 50% is if you have high inverter loads which pull down battery voltage when running those loads. As your SOC goes down so does the remaining AH capacity and you are more likely to trigger the inverter's low voltage alarm and eventual shut down. The batteries are still ok for lighter loads such as running the furnace, but you can't use the inverter anymore.

One way to solve that is to give the inverter its own battery bank and use another bank for the normal things. Now you can run those below 50% and everything still works.

mike-s wrote:
Whether to apply the "don't discharge more than 50%" rule of thumb depends on how much you'll use the battery.

If you're less than a full-time boondocker, the difference may simply not matter. Only 4% of campers do over 30 days per year. The majority camp less than 15 nights. And even fewer if only considering boondocking.

You can find pessimistic estimates for generic deep cycle lead acid of 200 cycles @ 100% discharge. Then look at a premium battery - Trojan Battery's literature for their SPRE 06 255 solar battery (same size as a T-105) shows a life expectancy of about 1900 cycles @ 50% discharge, and about 1200 @ 80%.

The inexpensive Interstate GC-2's sold by Costco are supposedly 650@80.

So, take the extreme high end of 30 full battery cycles per year, and use the pessimistic expectation of only 200 cycles of lifetime, and that's still almost 7 years. Not many RV batteries last longer than that regardless of how well they're treated (now someone will describe their 20 year old battery bank).

But let's say, age and cycles combined, the battery lasts 6 years instead of 7. We did that by limiting how much battery capacity we used - 50% instead of 80%. So, over those 6 years, we avoided using 30% of the available capacity to save (7/6=1.17) 17% in battery cost. It's still a false economy.

Use more realistic figures, and things only get better. Use the Interstate 650@80 numbers, and that's over 21 years. Cycles simply aren't a significant factor at that point. Even a battery on constant float charge isn't going to last that long.

I had a pair of T-125's that I consistently discharged to 25% SOC around 40 times/year, then when home charged them fully. kept them watered. After 10years the SG was still near new. Then they went to daughters in-laws for another 4 years of abuse. Last year they finally were getting pretty worn out, SG on a few cells was dropping, but they still were lasting a weekend. But total number of cycles to 75% DOD was around 500, close to what Trojan claims for life.

And I asked trojan what determines EOL. They said when capacity is 75% of new. So even the cycle life they claim really isn't taking them down to dead and gone, just 75% of initial AH.

So, yes for most campers, you CAN deeply discharge GC batteries and have reasonable cycle life. What that means is that when comparing AH, if you use 50%DOD for 12V, vs 75% DOD for GC, GC has a significant advantage in useable AH. And even trojan 12V deep cycle won't give near the cycles at the same DOD as the GC does.

Now, there are tradeoffs, as BFL mentions, and as I have experienced, GC are really weak when it comes to heavy discharge currents, the long life design sacrifices internal resistance. coupled with not sharing current between parallel batteries, and GC are at a signficant disadvantage when used with high inverter loads like a microwave. A pair of GC has a hard time with a 750W inverter load below about 80% SOC, where a pair of 12V won't grunt till 50% or less.

we boondock a lot, and like to occasionally use the microwave on the inverter. and not have to be anal about battery capacity, so to solve both problems I have 4GC. with that I CAN run the microwave with the batteries down near 50%. with a pair I was pressing my luck at anything below 80%SOC.

My 4GC with an 550AH won't give the CCA that a single 12V 75AH starting battery will give.