Still Don't Get It (Solar)

Thanks all, I'm considering portable. I use most power during winter when running furnace all night. I roughly figure I use about 40-45ah a 24 hr cycle based on my multi-meter readings in my 208ah bank. I read 12.4 first morning (running furnace overnight and watching 2 hr movie that evening off inverter), and after charging with WFCO onboard for two hours on the Honda 2000, ran heater all night and watched a movie again, and read about 12.3 the second morning. My Rockwood Minilite likes the heat it seems. In summer, without the furnace, I figure about half that consumption rate.

I'm thinking a 12 solar panel, maybe 80 watts, or two 60's, and using one of the Morningstar junctioin box mounted controllers.....

Gman

I have a 230w panel and MPPT now instead of my preferred two 12s and PWM because of price.

Here at the usual prices it is $1/w for 24s and $2/w for 12s. The Solar30 PWM controller is $35 and the Eco-Worthy MPPT is $102.

A. 230 + 102 = 332
B. 460 + 35 = 495

Yep, IMO its still a bit cheaper, no matter how you want to misrepresent it !!

pianotuna wrote:
Hi,

Panels requiring MPPT controllers are as low as 30 cents a watt and are fairly often on sale at $0.76 per watt. Nominal 12 volt panels are almost always well over the $1.00 per mark. I don't call that "a bit cheaper".

The controller is the key.

RJsfishin wrote:
Cost difference ?????? Its a wash,....panels are a bit cheaper per watt. controllers are NOT.

Charging the batteries is the same for your on-board converter or the solar controller.

If you want to charge a single 12VDC deep cycle battery it will need 14.4VDC@17-20AMP of DC CURRENT available to re-charge the deep cycle battery up to its 90% charge state if it was discharged to its 50% charge state. This will take three hours of charge time.

If you have two 12VDC batteries then you will need 14.4VDC @ 34 to 40AMPS DC CURRENT capacity to charge both of them to a 90% charge state in a three hour period.

You have probably already noticed that your panels will not produce that much current. The rule of thumb is a 120WATT Panel will only produce 5-6AMPS in the high sun for 6-7 hours. That equates to around 30-42AHs of DC CURRENT.

SO now you are down to having a large panel in the high sun all day long (normally 6-7 hours is all your get a day) and hope you can produce enough DC CURRENT to re-charge your batteries back up to at least their 90% charge state before the sun goes down. If you don't start your night shift with at least a 90% charge state you won't be able to make it much past 10PM or so.

This is where experimenting with things will come into play...

When you ask folks how they do it you will get alot of different answers. You have to know up front what you want to run off your batteries and how many DC AMPS will re-charge them. These guys you are asking may only be running some LED lights and maybe just watching some TV over night.

In my case I will be drawing 20AMPS of DC CURRENT solid for at least 5 hours before we go to bed and then the parasitic drain will always be there there drawing 1-2 AMPS or so for the whole time period. Doing this will drain my 255AHs of battery capacity down to around its 50% charge by 8AM the next morning.

This will most definitely get me behind the charge curve of my batteries and I will have to replace back all of this current I have been draining from the batteries. The only time I can do this is when the panels are in high SUN which is around 6-7 hours per day if it isn't cloudy or raining.

So the big question remains will the 10-12AMPS you will be producing from your mounted panels will be enough to replace all of the DC CURRENT you have discharged from your battery bank and get your batteries charged back up to their 90% charge state so you can drain them all over that night after the sun goes down.

If you don't at least get back to a 90% charge state your batteries will not perform at their stated specs and will discharge much quicker than planned

I am NO EXPERT in this field but have been running off my 255AH capacity battery bank for around five years now and pretty much know what I need. I am just now considering mounting solar panels on my off-road POPUP trailer. I already know I won't be able to get enough DC Current from my planned solar panels and will have to run my trailer on-board converter/charger from my generator for an hour or so first to get my high current battery charge demand down to 8 amps or so then my solar panels will finish off the charge period for me with the full day of high sun...

Roy Ken

Budget should only be considered if both types will fit on the roof and meet your needs.

Hi,

Panels requiring MPPT controllers are as low as 30 cents a watt and are fairly often on sale at $0.76 per watt. Nominal 12 volt panels are almost always well over the $1.00 per mark. I don't call that "a bit cheaper".

The controller is the key.

RJsfishin wrote:
Cost difference ?????? Its a wash,....panels are a bit cheaper per watt. controllers are NOT.

Hi,

Are you 1000% certain 270 watts is enough? If not, choose a controller that allows for expansion.

The watts are the same. The difference will be the choice of controller. Many folks advocate low end controllers. My feeling is that unless there is lots of wattage, this is an error.

I would go with a controller that allowed for expansion of the system. It was my one mistake when I designed mine.

The ideal controller will have:

temperature compensation
adjustable set points
room for expansion of the system.

If it is MPPT, that is a little bonus--but going MPPT will almost always cost more. Again it depends on the wattage. At 270 watts, a quality PWM controller would be more cost effective.

If I limit my inverter use, I run out of drinking water before I run out of power.

When I was not full time, my system provided all my battery charging needs for many years and I did not have a generator. That worked because I had a larger that usual battery bank, which would recover to fully charged when the RV was in storage. I had a 2500 watt inverter for my 120 volt needs.

Having fewer panels means fewer holes in the roof.

While being able to aim and/or tilt the panels does give more charging ability, I'm inherently lazy and know I would never do so. Making powered tilting mechanisms is possible but extremely expensive.

They are 12v batteries,.....6 or 12 matters none.
Like asking what charges more, portable or fixed,....in my case, fixed.
Keep in mind, 240-270 watt panels were not designed for RVs. If you can make them work, and it makes you feel like a "big strap", go for it.:)
Cost difference ?????? Its a wash,....panels are a bit cheaper per watt. controllers are NOT.

One system might charge marginally better than the other but practical considerations like space and cost should rule in the end.

For a small 2 battery system, both should be able to keep up unless you are in marginal light conditions or you are draining the batteries way down at night and still drawing significant amounts during the day, but in that case, you probably want to upgrade the battery bank also or add another charging source.

The first step should really be an energy audit where you estimate how many amp-hrs you will use when off grid. Then based on that, you can determine how big of a battery bank you need and how much charging capability you need.

Grodyman wrote:
What will charge 2 6v golf cart batteries better, a single 270 watt high voltage panel with mppt or two 135 watt 12v panels (parallel) with PWM and why???? Thanks.

Gman

I think as a practical matter the difference will be meaningless. Or as BFL showed, it will be a moving target. The most important decision in that scenario is probably the charging controls and specs of the controller. If you have sun, either 270W system will charge 2 6v batteries with power to spare.