24v Panels - 12v Batteries

harry

Hello all;

Have been setting up my little solar project around Waco Tx. using your help. For a few years I have been using some old Marine batteries I have around, charging at home. I'm using 300watt/day or less per a Kill-A-Watt meter.

What I have now is;
Morningstar SureSine 300watt
Morningstar MPPT 45amp (future growth)
and plan on 2-6v 220amp batteries

Borrowing from Bill's examples for 500watt/day;

50 watts * 10 hours * 1/0.85 inverter eff * 1/12 volt battery bank * 2 days of no sun * 1/0.50 max batt discharge = 196 AH @ 12 volt battery bank

220 AH * 14.5 volts charging * 1/0.77 panel+controller losses * 0.10 rate of charge = 414 watt nominal array

Okay, What I'm a little confused is using a 24v panel. Would this be 1/2 the watt array above? I'm looking at something in the 31v 250w 8a - panel.

Thanks;

Cariboocoot

Re: 24v Panels - 12v Batteries

Watts are Watts: Volts * Amps. A 24 Volt panel produces the same Watts with less Amps than a 12 Volt panel would. If you could find equivalent panels.

Where are you getting ten hours of sun from? Texas is good for daylight, but it's not that good! Just because you have long days doesn't mean it all pours directly on to the panels. Try it this way:

500 Watt hours per day on a 12 Volt system:
500 / 12 = 42 Amp hours * 4 (25 % DOD - never mind "2 days of no sun") = 168 Amp hours minimum. Round up to nearest available battery size like a Crown 185 Amp hour 12 Volt.
Calculate the array size based on that:
18.5 Amps * 12 Volts minimum = 222 / 0.77 = 288 Watt array. Round that up to nearest available size like two 190 Watt panels for 380 Watts.

Now check the array for AC production:
380 Watts * 5 hours equivalent good sun * 0.52 over-all efficiency = 988 Watt hours AC per day.

This system is very small. If there are no plans for expansion you could scale the array back to a couple of 140 Watt 12 Volt panels in parallel on a PWM controller. It would be cheaper and still deliver more than 500 Watt hours AC per day (about 700+).

If you are planning future growth think about what that would be. Chances are you'd be better off going to a 24 Volt system to begin with, rather than try to push a 12 Volt one to, say, 2 kW hours per day.

BB.

Re: 24v Panels - 12v Batteries

You have everything correct.. Just to clarify, you are using 300 Watt*Hours per day? (seems so, you used 500 Watt*Hours per day for your battery sizing).

And assuming you get a fair amount of sun:

500 Watt*Hours * 1/0.52 system losses * 1/(414 watt array) = 2.3 hour of "noon time equivalent sun" per day

So--That will carry you deep into winter weather/poor sun.

From a "simple" point of view, Watts is Watts, no matter the voltage. So, up this now, the 414 watts is at any voltage (a 12 volt panel with Vmp~18 volts, Vmp~30-40 volts, etc.).

The voltage selection/installation issue comes into the discussion when you select your solar panel(s) and charge controller.

In general, larger solar panels (>>100 watts) are not available in Vmp~18 volts, but Vmp in the range of 30-40 volts (60 to 80 cells in series) panels. But the >>100 watt panels are usually cheaper $$$/Watt and easier to install and wire (1 large panel vs 3-6 smaller ones).

So, that then forces you to look at PWM vs MPPT charge controllers. For smaller systems a PWM charge controller with a Vmp~18 volt panel is fine (in hot climates, may lose some charging capacity because solar panel Vmp falls when cells are hot).

To make use of panels with Vmp>>18 volts (efficiently), you need a MPPT type charge controller to efficiently take the high voltage/low current of the panel/array and "down converter" to low voltage/high current needed by the battery bank (sort of like the DC equivalent of an AC variable transformer).

If you where to use a "24 volt panel" (which in "battery world" is a panel/array with Vmp~35-40 volts) on a PWM controller and 12 volt battery bank, you will lose approximately 1/2 of the panel's output wattage (a 250 watt Vmp~40 volt panel would only output ~125 watts to a 12 volt battery with a PWM controller).

So, many times, you are left with the choice of multiple (somewhat more expensive) 140 watt "12 volt" panels with a PWM controller, or a couple less costly >200 watt panels and a more expensive MPPT charge controller.

For smaller systems (say less than 400 watts), PWM controllers are generally fine. For larger systems (over ~800 watts), you end up with MPPT controllers being the better choice (because you can run Vmp-array >> battery voltage and use smaller gauge wire for longer runs from array to battery bank plus charge controller--and some other reasons too).

If you are looking at 2x250 watt panels--You would need a minimum of 30 amp MPPT type charge controller. The Rogue is a good/solid 30 amp model (3rd generation just came out--Have not seen anyone here use one yet, but the previous two models were very nice controllers).

Or you can look at larger MPPT controllers (45-60-80+ amps). MorningStar makes 45 amp/60 amp models--Midnite and Outback make larger ones.

-Bill

BB.

Re: 24v Panels - 12v Batteries

To add to what Marc said... I think the MorningStar 300 watt TSW 12 Volt AC inverter is a good reason to stay at 12 volts if you do not need more than 300-600 watts of power.

There is no other small AC inverter with the "search mode" and "remote on/off" controls that are great for smaller off grid systems to keep power usage to a minimum.

If you need >>300 watts, then you need a larger AC inverter, larger battery bank, larger charge controllers, more battery bank.... And before you know it, you will be spending a lot more money for that larger system.

A "small" system may be 1,000 Watt*Hours per day... Many folks want to plug in a refrigerator (off grid cabin or home solar backup)... And then you are at 1,200-1,500 watt minimum AC inverter and a ~3,300 Watt*Hour (3.3 kWH) per day system. And in pretty near the realm of a 24 volt battery bank.

-Bill

harry

Re: 24v Panels - 12v Batteries

Thanks for the education from both of you. I'm definitely lacking in this stuff.

To clarify;
- Now I'm using 300 total watts per day per Kill-A-Watt meter. Probably shouldn't have put this in here as it confused the topic.

- I just used 50 watts for 10 hours a day = 500 watt hours (for some growth). Not sun hours. I do get about 5 hours per day

- I have now a MS 300w inverter and MS MPPT 45amp charger. I'm very impressed with the MS 300w inverter over my old cheap-o thing.

- I bring a battery from home for now.

- This is a small solar project for my get away. No plans for anything much larger. If so, I'll get another inverter.

- Some days when I turn into a sissy, I run my 8000-BTU A/C off a Honda eu2000.

So, it seems something about ~400 watts of solar panels should work.

Have I got the 8 ball in the corner pocket?

Thanks;

BB.

Re: 24v Panels - 12v Batteries

harry wrote: »

Now I'm using 300 total watt*Hours per day per Kill-A-Watt meter. Probably shouldn't have put this in here as it confused the topic.

The one "minor quibble"... Watt*Hours is the amount of energy you use per day... Watts is the rate:

50 watts * 6 hours = 300 Watt*Hours
50 gallons per hour * 6 hours = 300 gallons pumped

Watch the units--If they don't match, then the equations are probably wrong. We are so used to seeing miles per hour, gallons per minute, liter per hours, etc... That we (even me after all these years) still want to type Watts/Hours sometimes (Watts is already Joules per Second--So it is a rate).

Sounds like a very good setup... The generator to run a small A/C when needed is usually the best option for seasonal cabins. A lot cheaper and less maintenance vs building a "larger" off grid system for rare, higher power, usage.

-Bill

harry

Re: 24v Panels - 12v Batteries

Minor quibble is a major learning step for me. Thanks for your help. I'll be back.