Solar Panel Size To Battery Bank Size

I agree, cover the roof and fill the battery box. Run it and see what you have because it ain't getting any better. The upgrade now and planned right is cheaper than down the road. It could just be another panel or it could be new wire and a new controller too. The upgrade from a TS-MPPT-45 to a TS-MPPT-60 is a $100 now, it's $500 down the road.

Hi landyacht318,

I do believe that many folk underestimate their daily consumption and as a result are undersize on both their battery bank and solar. A good energy audit with no fudging should be a prerequisite for bank sizing. I sure wish that LiPo batteries could be charged and low temperatures. It would make life easier.

You have taken action to reduce consumption, and live full time within those constraints.

I have taken the other side of that coin. I want to be able to run everything just as if I were on shore power. Now that I'm full time I do not have enough solar to meet my desires. I do have enough to meet basic needs without any inverter use. I suppose I could urban dock near McDonald's and the like to feed my addiction to the wobbly wide web--but I'd really rather do that in my RV home.

I believe solar is a battery charger, so I focus on:

adequate ability to equalize the bank.

adequate capacity to get back to near or 100% state of charge.

As bank capacity increases the two figures above converge. One reason is that under heavy loads peukert favors greater capacity.

Others believe solar is an energy source.

landyacht318 wrote:
It is far too easy to have too little solar.

My rooftop is maxed out solar wise, So I lowered my battery capacity so that the solar could at least approach the battery manufacturer 'recommended' initial charge rate of 10%, or 10 amps per 100AH capacity, at high noon..

I don't think it is just a matter of replacing the AH used +~10% more, but having a higher wattage to capacity ratio. The happy battery is the one whose properly programmed solar controller decided to float it at 11AM on that gloriously sunny day. Not so gloriously sunny, hopefully float by 3. Cloudy, at least it will negate some daily usage.

Any solar is better than no solar, but a proper amount of solar for ones battery capacity ultimately makes for a happier battery. In my opinion, and experience, full timing.

It is easy to design your second solar system. :)

HTH;
John

It is far too easy to have too little solar.

Too many Newb's do calculations as if a 100 watt panel makes 100 watts sunup to sundown, where instead a flat mounted panel might reach 90 watts for 3.5 hours around high noon near the summer solstice.

I've taken a different track with regard to my capacity/solar ratio's. There is a lot of 'capacity is king' mindset on this forum, and I am not one of them.

My rooftop is maxed out solar wise, So I lowered my battery capacity so that the solar could at least approach the battery manufacturer 'recommended' initial charge rate of 10%, or 10 amps per 100AH capacity, at high noon..

Even though this single 31 battery was cycled deeper than 2 27's, I got a better cycle per dollar ratio from it, and I never needed the extra 100Ah of capacity as a bad weather buffer.

Of course being able to go home and plug in and stop cycling the batteries requires a different amount of solar than a full timer too.

I don't think it is just a matter of replacing the AH used +~10% more, but having a higher wattage to capacity ratio. The happy battery is the one whose properly programmed solar controller decided to float it at 11AM on that gloriously sunny day. Not so gloriously sunny, hopefully float by 3. Cloudy, at least it will negate some daily usage.

Any solar is better than no solar, but a proper amount of solar for ones battery capacity ultimately makes for a happier battery. In my opinion, and experience, full timing.

2oldman and Pianotuna

Absolutely agree with your statements: "Sizing to me is "how many panels can I get on my roof?" and "...For a full time situation I now lean towards maxing out the solar install..."

Have a friend who plans to wind up with 2 kW of panels on his pickup camper and accompanying toy hauler for his BMW motorcycle. I believe that he is fabricating panels that will hang on sides of camper and toy hauler which will be swing up for use.

Reed and Elaine

CT_WANDERER wrote:
When you pro's talk, having solar panels sized to battery amp size, are you referring to total battery capacity or the 50% useable battery capacity? I am working on my Solar system, I have a total house bank of switch-able 410AH on Bank#1(GC2) and 170AH Bank#2 (Group24). I plan on installing 5 X 120watt panels. I figure this should work good. Is my plan good? Thanks Gary

Bank1, 410Ah*14.5Volts*(1/0.77)panel+CC derating*0.05 rate of charge = 386 watts

Bank2, 170Ah*14.5*(1/0.77)*0.05 = 160 watts

Total watts required is 546, so your planned 600 watts is good.

I have to agree with Mex. Better to over estimate consumption than to under estimate it.

I know of few examples where calculated-precisely load to power ratio turned out to be right-on. An overwhelming percentage turned out deficient. This prompted varying degrees of changes of lifestyle to adjust for the shortage. The principal cause was miscalculated hotel loads during daylight.

So I wouldn't worry :) An additional panel or killing the satellite during the day is not the end of the world.

full_mosey - This is all on Gary's dime haha... In my case my solar panels would not produce enough current to charge my battery bank using smart mode charging (90% charge state in three hours)

When I start this mode using my on-board converter/charger being run by my generator I immediately see around 52AMPS DC Current being demanded by the 255AH Battery Bank when they are first hit with 14.4VDC. The battery bank will continue drawing this much current for about 15-20 minutes and then start tapering off. After about an hour this DC CURRENT demand is around 8AMPs DC Current. This is also when my on-board converter/charger will switch to 13.6VDC and the current demand then drops down to around 6AMP DC Current.

This would be the time I would to shut down the generator and let the solar panels continue the charge state. Since I can get around 20 AMPS of DC Current from the solar panels the 8AMPs and less DC Current demand would easy for the solar panels to provide for the next two hours of the battery bank charge.

Being new to the solar world I said "switch to" - actually they will all be connected and protected with in-line diodes so I would just turn off my generator and the current demand would just be the provided by the solar panels.

The point here is I can cut back on using the generator after the high current demand has dropped down from the 52AMPS DC Current after a bit. My solar panels would not be able to meet the high current demand.

Keep in mind this is wanting my batteries to get charged up before the end of the day. Just using the solar panels I would be able to see the 20AMPS of C CURRENT for 5-6 hours on average they tell and several hours of this 20AMP source might get my battery bank back to its 90% charge state. But then again it might not. I know I can get to the 50% charge state but having 60AMPS of DC CURRENT available to use with my on-board converter/charger unit.

If I did just left it on SOLAR PANELS only an it did make the 90% charge state before I lost the high sun then I would not need to use the generator at all. I will learn all of that after I use solar panels for awhile.

I know what happens when I am not able to start my evening with less than 90% charge state batteries. They fall back rather quick and it gets dark on me around 10 PM. My battery performance doesn't work well if I start out with less than 90% charge state. This might be different for other folks??? They may have better batteries than I do... My three 85AH Interstate GP24 12VDC Deep Cycle Batteries were installed in 2008.

Does any of this make any sense haha...
Roy Ken

Hi CT,

Total capacity.

You could go as high as 870 but 600 will work well unless there is lots of inverter usage. If there is, then max out the entire unshaded roof space. For a full time situation I now lean towards maxing out the solar install.

CT_WANDERER wrote:
When you pro's talk, having solar panels sized to battery amp size, are you referring to total battery capacity or the 50% useable battery capacity? I am working on my Solar system, I have a total house bank of switch-able 410AH on Bank#1(GC2) and 170AH Bank#2 (Group24). I plan on installing 5 X 120watt panels. I figure this should work good. Is my plan good? Thanks Gary