Calculating PV requirements given known amp-hour load and known watt-hour capacity

[steve&renee]

This is about TucsonJim's Victron BMV-712 Smart Battery Monitor - Like a Fuel Gauge for your Batteries, post #2 (relevant info copied below from another forum) where he estimates PV capacity required.

I'm interested in your feedback on my title. I think I understand things better if Mikec557's post (see below) is correct when he says: "So the same can be said: volts x amp-hours = watt-hours."

I don't understand how Jim calculated panel requirements given the information he presented.

What confuses me the most, I think, is that Jim was basing estimates on amp-hours at noon. That's just way too optimistic. Right?

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Originally Posted by TucsonJim


Lessons Learned

However, this experiment indicated that I do not have enough solar charging capacity to top off the batteries each day. I had to run a generator for an hour a day to get them topped back off. My solar array produces about 16 amps at noon. Of course it's less efficient in the morning or evening. If I'm using an average of 132 amp hours per day, and want the system to charge in five hours, I'll need to produce 26.4 amp hours on average to complete the task. So, I'm going to double my solar array in order to meet or exceed my requirements. My current array is three 100 watt panels in series. I plan on adding another three panels in series and then running both arrays in parallel to the controller.

Jim

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So Jim, Could you walk me thru the way you calculated your solar panel requirements. I've got a recent-version Victron charge controller that reports instantaneous amps going into the battery and cumulative watt-hours for the day, but not amp-hours.

How did you determine you were short 3 panels based on knowing that your PV array produces 16 amps (instantaneous?) at noon?

-Steve


Another forum member, Mikec557 replied with this:

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I'm sure Jim will get back to you. But in the mean time...

Remember volts x amps = watts. So the same can be said: volts x amp hours = watt hours.

If you divide your Victron reported daily watt hours by 12 volts, this equals your daily amp hours. I have a Victron 100/50 and the instant read is interesting, but you can easily see the effect of a cloud going by. So to me the instant read is not that useful.

He wants to generate 132ah during 5 hours of sunshine. He gets 19ah during the noon hour. Presently he generates 19ah*5hr=95ah. And that's optimistic because 2.5 hrs before noon is not as good as noon, the ah(s) are tapering up to the noon reading. Ditto the second 2.5 hrs, the ah(s) are tapering down from the noon reading. So at 95 he's short 37ah (132-95). Doubling his 3 solar panels will give him (95ah*2=) 190ah. A good deal more than the 132ah he needs. So even on cloudy days, and considering the tapering up and down, he'll probably still get the 132ah per day he wants.

Hope the math helps.

Mike

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[TucsonJim]

It's very complicated to try and estimate exactly how many panels you'll need to charge a battery bank in a given time. Angle of solar incidence, shading, cloud cover, time of the year, etc. all come into play. Will the panels be tilted continuously towards the sun? Will they be flat? What is the efficiency of the panel? What wire size and length will be used, and how much loss will there be?

This is where a rule of thumb comes into play. A typical 100 watt panel can produce approximately 30 amp hours of power per day. Of course, on a cloudy day, it will be much less. On a summer day in Southern Arizona or Texas, it can be much more. But if you run with the 30 amp hour rule of thumb, you'll be close for most applications. My meter shows that I use approximately 140 amp hours per day. 140/30 = 4-2/3 panels. I was running with only three 100 watt panels, so on a sunny day, I'd almost get fully charged, but not quite.

BTW, I'm currently upgrading my charging system to 800 watts. I'm hoping to be able to charge my battery bank in a few hours of good sunlight. And when we dry camp in Quartzsite in January, I'll probably get quite a bit less than the 30 amp hours per day.

Jim