Panel voltage for 24 volt off-grid system

[Kebast]

I'm looking at some panels that provide 30.7 volts. Is this voltage high enough to properly charge a 24 volt battery bank? Would it depend on the charge controller used? Even without losses, it is pretty close to the absorb voltage.

[Sunking]

Nope you need a minimum of 34 to 36 volts Vmp.

PWM voltage is extremely critical and any less than 34 volts will not be able to deliver full charge, and anything more than 36 will waste a lot of power. For MPPT you can go as low as 35 volts, but much more efficient at higher voltages up to 150 volts Voc

[Kebast]

Thanks!

Nope you need a minimum of 34 to 36 volts Vmp.

PWM voltage is extremely critical and any less than 34 volts will not be able to deliver full charge, and anything more than 36 will waste a lot of power. For MPPT you can go as low as 35 volts, but much more efficient at higher voltages up to 150 volts Voc

Thanks for the quick response! I suspected as much, but wanted the confirmation.
My watt-hour requirements aren't quite high enough to justify two of these in series plus the cost of a MPPT controller.

[Sunking]

Thanks for the quick response! I suspected as much, but wanted the confirmation.
My watt-hour requirements aren't quite high enough to justify two of these in series plus the cost of a MPPT controller.

Are you sure? Once you get to about 200 to 300 watt panels is the threshold.

[Kebast]

Maybe

I went through your Solar Off-Grid Battery Design and came up with needing 235 watts of solar panel. An MPPT controller might be cost effective depending on what panel(s) I end up using. Buying a second of the 250 watt 30.7 volt panels would be overkill in my case though.

[Naptown]

Look for 72 cell panels the ones you are looking at are 60 cell.
This will give you the voltage you need.

[Sunking]

GTI panels are less expensive than battery panels.

[Naptown]

GTI panels are less expensive than battery panels.

there are a number of GTI panels that are 72 cell

[Sunking]

there are a number of GTI panels that are 72 cell

Yes I know. Combined with MPPT controllers once you get above 200 to 300 watt panel wattage MPPT controllers is the most economical answer for a given minimum daily power requirement.

[Kebast]

I'll keep looking. I've got time on my side currently, so no need to rush into anything. I'm still not entirely sure about controller types. As you said, grid tie panels appear to be the cheapest route to take. It looks like I can get a 250 watt panel for near the $300 range. When I said it might not be worth it for me to go with a MPPT controller, what I meant was, with that panel, either controller would give me all the power I need currently. So in that sense, even though MPPT is more efficient, it would just be added cost for power I don't really need. Now there are other benefits, like easier expansion later and such, so I might go that route anyway.
Right now I'm just waiting on a panel deal I can't pass up, I'll figure out the controller later .

[Sunking]

it would just be added cost for power I don't really need. Now there are other benefits.

You are not seeing the big picture. A PWM system at very best is 50% efficient, and MPPT is 65 to 75% efficient.

Here is how that applies. Let's say you need a minimum 1 Kwh/day and live in Kansas City where winter insolation is roughly 3 Sun Hours. If you use a PWM controller will require a 700 watt battery panel and a 40 amp PWM charge controller. To generate the same amount of power with MPPT controller will require a 500 watt panel and a 40 amp controller.

Now here is where it gets interesting. If you go with a PWM setup you are forced to use more expensive battery panels. Go MPPT you can use much less expensive Grid Tied panels. So when you factor in the lower wattage needed, and less $/W cost of GT panels, the MPPT system is less expensive than a PWM system.

[nirooafza]

but if you use MPPT CC you can get more power with high voltage panels ... and during cloudy days you can have enough charging current to charge at least 50% of Your battery . and of course it works much more better if your battery charge level is very low ....

[Kebast]

Cost factor

Sunking, after researching panel prices I see your point now. I can essentially buy one panel (~250 watt) at around $1.2/watt with the MPPT controller for the same price as the PWM and more expensive panels.

nirooafza, thanks for the info. I just finished reading a study on MPPT controller efficiency on cloudy days that backs up what you stated (comparison-mppt-pwm.pdf). Not the most scientific, but the important part is at the bottom.