What size batteries for Panel Output of 48v and 30 amps?

[BilljustBill]

Let's say I'll have a 48 volt system with strings of four panels of the following:

135 watts
120 watts
100 watts
90 watts
75 watts

The total is about 30 amps @ 48 volts. What amp-hour size of eight 6-volt batteries would I begin with to be sure I have enough panel power to prevent the cells from going bad due to low charging? What battery brand would you buy?

I live in Texas and have a site that provides sunlight from 8:00 a.m. to 6:00 p.m.


All the ratings I listed have 4 of equal wattages on each strings, i.e. Four 75 watt Siemens panels. Three of the five strings have matching wattages and brand names.

Each string equals 48 volts and has as equal Amperages as the Spec. sticker/Amp Meter reflects. Unless I get a very good offer on some of the used panels, and sell them to buy new and matching panels, here is how I see using what I will have:
The 75 watt string and 90 watt string on one controller, the 100 watt and 120 watt strings on another controller, and the new Kyocera 135-SX panels are wired to another controller. The distance between the two arrays and the batteries, and amount/low cost of the #6 copper wiring isn't a problem.

So, do you believe that there will still be a higher than normal loss? Would you have to put a blocking diode on the lower wattage strings to prevent the higher wattage from back flowing?

Bill

[Sunking]

Well to answer your thread title question, it cannot be answered accurately as it depends on what your daily watt-hour usage is. Two things have to be known to match all the components to work with each other.

The very first thing you have to know is how many watt hours you will use in a 24 hour period. You cannot even start a design without having that info. So for example let's say you will use 1000 wh or 1 Kwh in a 24 hour period. From that we can immediately determine the battery Amp Hour capacity needed by using a simple formula which is AH = [1.5 x 5 x wh] / Battery Voltage. So let say it is a 48 volt battery system. [1.5 x 5 x 1000] /48 = 156.25 Amp Hours.

Ok the solar panel wattage and charge controller completely depends on your location, and daily watt hour usage. So let's use two examples to demonstrate the extremes. I will run one for Tuscon AZ, and a second for Seattle WS. OK for a battery system we design for worse case, which in this example is winter.

OK for Seattle the winter Solar Insolation is 1.2 Sun Hours.So to determine the solar panel wattage we run another formula which is Solar Panel Wattage = [1.5 x Daily wh] / Sun Hours. So 1.5 x 1000 wh / 1.2 hours = 1250 watts.

For the charge controller amps is a straight forward formula. Amps = solar panel voltage / battery voltage. 1250 watts / 48 volts = 26 amps.

OK for Tuscon they receive 5.6 Sun Hours in winter. Rather than drag out redundant info the answers are:
Solar Panel Wattage = 267 watts
Charge Controller Amps = 5.5 amps.

So as you can see there is a heck of a lot of difference between Seattle and Tuscon. Only thing in common is the battery capacity and daily watt hour usage. The system in Seattle will cost roughly 5 times more than one in Tuscon.

[BilljustBill]

I live in Texas.

This solar project is but one of the things I'm working on. For now, I'm not expecting to "live" with the arrays' power since it will be powering a two story, 1000 sq.foot, cabin/storage/hobby area. If this system works well and oil/economy causes my monthly power costs to skyrocket, then later it would be taken off the cabin if it justifies the cost of an "Outback VFX-3648" to power parts of my home.

Since I have to buy EIGHT of the 6v batteries to get the system operating, because it is a 48v system operating an "Aims 1500 watt PSW Inverter", in view of the large cost of the batteries, what is the minimum size battery you would start with?

Thanks,
Bill

[Sunking]

If you are looking for a rule of thumb, 10% of the battery 20 hour Amp Hour rating.. So if your battery AH capacity is 300 Amp Hours @ 20 hours, you need 30 Amps of charge current. So at 48 volts you are talking about a minimum 1500 watt solar panel array using a MPPT type charge controller. If you are using an inexpensive shunt type controller like PWM much higher wattage like 2200 watts.

But be advised any battery solar system will cost you a minimum of 10 times more than commercial power in 5 year up front lump sum cash., more like 20 to 30 times more.

[BilljustBill]

If you are looking for a rule of thumb, 10% of the battery 20 hour Amp Hour rating.. So if your battery AH capacity is 300 Amp Hours @ 20 hours, you need 30 Amps of charge current. So at 48 volts you are talking about a minimum 1500 watt solar panel array using a MPPT type charge controller. If you are using an inexpensive shunt type controller like PWM much higher wattage like 2200 watts.

But be advised any battery solar system will cost you a minimum of 10 times more than commercial power in 5 year up front lump sum cash., more like 20 to 30 times more.

Thanks for your help, Sun-King.

I understand that Off Grid systems with batteries are more expensive, but there are reasons I'm doing it this way. I have a Co-op electric company that I plainly do not trust after they installed updated high-line poles and wire and damaging everything I have that operated on Standby or a remote control that didn't have surge protectors.... I'm not finished with them as I have a complaint letter going to the state utility comission outlining what was done and how they twisted the facts and wouldn't pay for nearly $800 in damages... No way I would put a grid-tie system on their grid as they have proved that they will not take responsibility when they are in the wrong...

Also, "IF" things in our country should "Go to Hell in a Handbasket", buying enough batteries with the quality that will last through tough economic and inflationary times ahead, I can't afford to shorten battery life because of low panel output.

Yes, it is a 'different' way of building a solar panel project. As the national and local economic times have been somewhat "nail biting" in the last year or so, and having some panels to support a fridge or freezer in case the bottom fell out was a good feeling.

I've been collecting the parts to this Off Grid solar project as bargains are found. I'd found two used 120 watt American Signal/Siemens panels for $22.50 each, four new 100 watt 'China Made' panels from an Internet auction for $500, and four used Siemens 75 watt panels for $160.

If I had a large cash budget, I'd buy all of them with the same wattage and of the same brand. But, alas, I collect the panels and support items as time, money, and bargains are found. I began this solar project after testing the 120 watt panels last August of 2009. Funding this project so it's paid in full as I go, takes time, work and flexibility. For instance, I used $265.00 in clad silver I've found while metal detecting toward two new 90 watt 'China Made' panels making their cost less than half price. Because I'm actually a closet "American Picker", I have had a large garage sale and two different months of selling at the local flea market, and raised a little more than $3,000 in cash.

With some of those profits, I've purchased the TriStar 60 amp PWM controller for under $195, an "Aims" 1500 watt, 48 volt PSW inverter for $180, a 500' reel of new #6 copper wire for $85, metal for array frameworks $50, four new black-frame Kyocera 135-SX panels (including shipping to Texas) for $385 each, and two sets of six carbon fiber wind generator blades for under $60 each.

Since seeing the need for adding a 48 volt perm. mag. homemade wind generator into this project, and having an average to good wind site, I'll probably use the TriStar controller for it. Then, build the framework for two ground based tracking panel arrays. Putting strings of the 75 watt and 90 watt panels into one array controller, the strings of 100 watt and 120 watt panels on another controller, and the newest 135 watt panels on a third controller is my plan... subject to my learning curve and help from this Forum. This way, I can more closely match each array with sets of strings and two smaller MPPT controllers to get the most out of this 'hodge-podge' of bargain panels.... ;>)

What do you think?

Bill