Balance between PV Array and Batteries

tr0y

I was wondering if there was a "best practice" in figuring out the balance between a PV array and a battery bank, this of course after you determine your electrical power needs.

Are there any guidelines, general rules of thumb, formulas or an online calculator of some sort ?

Thanks !

Cariboocoot

Re: Balance between PV Array and Batteries

Yes: simple mathematical formulae.

There are two factors to consider: one is the peak charge current, the other the Watt hours harvested.

For the first the "rule of thumb" is between 5% and 13% of the battery bank's Amp hour capacity. Some battery manufacturers recommend more, but generally 10% is a good "target". As in:

200 Amp hour battery charged at 20 Amps. This is multiplied by the system charging Voltage to get the need Wattage;
20 * 14.2 = 284 Watts, 20 * 28.4 = 568 watts, et cetera.
You then have to figure in a certain amount of panel "derating", as a panel's actually averaged output over the "equivalent good sun" hours will be less than nameplate.

That's where it gets tricky. Your actual site location will determine the derating. We use 77% as "typical" just to get in the ballpark. But if you're at higher altitude the panels will do better. Lower altitude or other factors decreasing the insolation will adversely effect panel output.

The second equation is for Watt hours harvested. You can use the PV Watts program to get a very reliable figure for your area. The rough calculation looks like this:

Panel watts * derating factor * equivalent good sun hours = expected harvest.

Usually if you get the charge current right and don't draw the batteries down to 50% you'll be okay. But solar installs are highly site specific, and things like high ambient temperature or high humidity or long wire runs all have to be taken into account.

Dave Angelini

Re: Balance between PV Array and Batteries

I am with Marc but if this is for offgrid, I would say you can add solar as you go, as long as you are reasonable about lowering your useage in the early years. Use a generator to make sure you charge at least once a week. More is a good thing and charging every single day is the goal if your location supports it. You can never have too much solar!

tr0y

Re: Balance between PV Array and Batteries

Thanks for the answers guys, I actually have been off grid for just over a year

(The nearest power pole is 13 miles away and 19 million dollars for the utility to run the lines in. )

and actually had great beginners luck with usage and my system working very well, so well in fact that in my small community I have sort have become the "solar guy" by default. I think that happened because I have a background as an electrician and when I did my solar install, I just sort of followed code and did nice looking work, and I really credit lurking in this forum for along time before building the house.

The reason for posing the question was I have people asking me for advice and I would rather help them learn, than let them get served poorly by a guy that lives in our community and sells unlicensed electrical work and overpriced second hand equipment as a "Solar Consultant" what the hell is that ?

I don't know nearly enough about this to sell services but if I can help with solid information I feel as I am doing my community a service.

Thanks for your input, I enjoy the learning !

Troy