More Amps = Faster charging?

[Zillard]

Ok i have done some calculations for the LED lighting system alone. The fridge running for the whole day (6 hours guestimating it actually running at full power) with the LEDs running for 6 hours will require a panel wattage of about 730W, wich is already too large in physical size to accomodate.

The LED lights run on 12V, so im stuck with that on battery voltage. I have 14 light fittings, so i can use 14 LED lights running at 4W each. Now i wont be using all of them at once of course, but at least 8 of them at once, but im designing for maximum usage, so using 14. And even then i wont be using them for more than 6 hours a day.

So my daily Whours are = 4W * 14 units/lights * 6 hours = 336Wh.

Now i found the formula for battery Ahours calculations on the forum, which im hoping is correct: Ah Capacity = (daily wh * 7.5)/V
Using that my Ah = (336 * 7.5) / 12 = 210 Ah. Now i dont know why they have the 7.5 in there, so can anyone advise what its for?

Now, assuming i dont want to discharge my battery bank below 80% of total capacity, i would assume that if 20% equates to 210Ah, my battery bank needs to be at least 1050Ah? Would i be correct in my assumption?

Ok, since i need to be able to replace the energy used each day, my panel size i calculate as follow, assuming 5 sun hours, and a 50% efficiency using a PWM charge controller: P Watts = Total Wh / Charge loss / Sun hours
P Watts = 336 Wh / 0.5 / 5 = 84W. So a 85W panel should do the trick. Doing a quick search on locally available panels, i find one 85W panel that might fit. It has a maximum voltage of 18.2V, a maximum current of 4.7A, an open circuit voltage of 22.2V and a short circuit current of 5.3A.

BUT, if i want to have a C/8 maximum charge rate or 13%, i would need to be limit the charge at max 27.3A. And a minimum rate of 5% or C/20 giving me 10.5A. So i would need to be in the range of 10.5 to 27.3 A to charge the batteries. So would i need to then have 3 of the 85W panels above in parralel just to get above the minimum limit, giving me 3 x 4.7 = 14.1A?

If im correct with my above statement, using the Charge controller = Solar panel W / Battery Voltage formula, my charge controller should be = (3 x 85) / 12 = 21.25 Amps, so a 30 A controller should be sufficient.

Ok, this is my calculations and assumptions. Please check if im going about it the the correct way, or if im cocking up big time.

Thanks in advance.

[Sunking]

So my daily Whours are = 4W * 14 units/lights * 6 hours = 336Wh.

Now i found the formula for battery Ahours calculations on the forum, which im hoping is correct: Ah Capacity = (daily wh * 7.5)/V
Using that my Ah = (336 * 7.5) / 12 = 210 Ah. Now i dont know why they have the 7.5 in there, so can anyone advise what its for?

It is 5 not 7.5. To maximize battery life and to give you a couple of cloudy day autonomy you want to limit your daily discharge to no more than 20% per day. So in essence you are designing your battery for a 5 day capacity and only using 20% per day. In practice you never want to discharge your battery more than 50%, so in reality it give you 2.5 cloudy days.

Now with that said I really have to ask what are you thinking? You have commercial power right? Why on earth would you want to take anything off grid and on to batteries?

Going off grid, especially on such a small scale is going to cost you at least 20 times more for electricity. National average is 10.8-cents per Kwh, down from 12.1 cents 3 years ago. The system you are thinking about will generate about 2 cents of electricity per day. That works out to 60-cents/month, $7.20/year, $36/5 years.

That 12 volt 200 amp hour battery alone is going to cost you about $300 and will need replaced in a few short years. Assuming just the battery cost and it can acutally last 5 years, it will not, you will be paying about $0.50/Kwh. 5 years from now the price of that battery will be $400. That is just the battery cost and nothing else.

Have you stopped and thought about that?

[Zillard]

Yes i have looked at what it would work out on, and it would work out more expensive for the next 3 years or so.

Here in South Africa our electricity supplier Eskom, barely has enough capacity to supply the whole country, and during peak times we tend to have rolling blackouts, or as its called here, load shedding. Peak hours is when everyone gets home after work, starts cooking, watching TV, bathing, etc. Now to have no power for an hour or 2 during that time is not so nice. I think if i at least have some lights running off battery it would soothe the annoyence.

Now to compensate for the low capacity, they are building 2 new power plants, but those will be up and running in 1 to 2 years time. And as all African systems seem te be operated, there isnt enough money around to finance completion. So now we will be getting rates increases ranging from 21 to 28 % every few months from Eskom, to try and get funds available. As it stands, it appears South Africa already has the most expensive electricity in the world, and that will just keep getting worse the way these increases happen. In the last year our electricity bill went up by about R160 or $21 per month. And ours is just a small household. Some larger homes are almost paying R1000 or more each month.

So i understand what you're saying, but i think we will get to a point where the grid supplied power will costthe same, if not more than the solar option.

The other concern is that with these load shedding appearances, the power surges quite a bit when its diverted back. And during the last 3 years we have heard lots of stories of fridges, TVs, etc blowing up due to the power spikes. This is the 2nd concern to get the essential appliances on to a independant of the grid system.

So that being said, i need to confirm that the factor of 5 you said is then to allow for a larger capacity bank. Which means that i only need a battery bank of 210Ah, not the 1050 i calculated.

Thanks for your feedback guys, it is really helpful.

[Mike90250]

The other concern is that with these load shedding appearances, the power surges quite a bit when its diverted back. And during the last 3 years we have heard lots of stories of fridges, TVs, etc blowing up due to the power spikes..

You need to install "house size" surge protection, and maybe a 90 second time delay for your re-connect. I really like the SPD that Midnight Solar sells, seems to be more useful than the 7KV Deltas.

So your backup battery, needs to only last you for 3 hours or so, and then it can be recharged from the grid. (I expect your grid power is still less expensive than solar)

[Zillard]

Thanks Mike.