# Calculation test with GC2s

JoshK
Solar Expert Posts:

**232**✭✭
Ok, I am rusty on this so I need some help. Please double check this math carefully.

The Sam's Club $84 GC2 is 6v. And I think 208AH.

So 6 of these batteries could provide a grand total 7.488 KWH over 20 hours.

And assuming 50% Depth of Discharge, I can use 3.7 KWH over 20 hours.

So 187 watts are available for 20 hours. (3740 Wh total)

And assuming an inverter with 77% efficiency I could run a 144 watt load continuous for 20 hours and only drain the batteries 50%.

So 6 GC2 are perfect for a 144 watt continuous load for 20 hours. At that time they are at 50% DOD and need a recharge.

The Sam's Club $84 GC2 is 6v. And I think 208AH.

So 6 of these batteries could provide a grand total 7.488 KWH over 20 hours.

And assuming 50% Depth of Discharge, I can use 3.7 KWH over 20 hours.

So 187 watts are available for 20 hours. (3740 Wh total)

And assuming an inverter with 77% efficiency I could run a 144 watt load continuous for 20 hours and only drain the batteries 50%.

So 6 GC2 are perfect for a 144 watt continuous load for 20 hours. At that time they are at 50% DOD and need a recharge.

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## Comments

5,657✭✭✭✭✭Well sorta...

More of a system sort of thing, and typically you wouldn't design a system to discharge to 50% with out recharging. Particularly on a less than reliable charging source, such as most renewable energy sources...

You parameters are screwed up if your talking solar, charging is typically done at 10-13% rate for bulk and less to top it off, so 4 hours even under optimum conditions would leave the batteries under charged...

It would be better if you told us what you wanted to do?

- Assorted other systems, pieces and to many panels in the closet to not do more projects.

232✭✭So what I would remember would be:

"For every 140 watts continuous, I need a minimum of $500 batteries."

and if I'm lucky enough to remember more details:

"That's six GC2 batteries and 50% Depth of Discharge"

5,657✭✭✭✭✭...of course I just told you it won't work for solar....

If you give us a real situation, we can give you real answers...

- Assorted other systems, pieces and to many panels in the closet to not do more projects.

232✭✭795✭✭✭✭3✭JoshK,

I understand your question as when it comes to battery and loads there is a lot of variables.

I don't agree with everything this link I am suggesting below recommends as it is their theory. But I do believe their knowledge shared when it comes to their real life experiences. As see how adding a larger battery bank can actually be a great benefit to the life of the batteries and how to wire them all together properly. Good luck

http://www.smartgauge.co.uk/technical1.html

2008 Class C Four Winds Chateau Sport 28A

3,739✭✭✭✭Also, your battery will have less capacity when it is cold or old... when designing a system you should take that into consideration.

No, 187 watts are available for 20 hours.

Watts is a rate of energy use. It can be expressed as energy per time (energy ÷ time). When you multiply energy per time by time, you get energy. Thus 187 watts X 20 hours = 3700 watthours which is an amount of energy.

When you say 'watts per hour' you are saying energy per time per time. It's not what you mean, but 'watts per hour' does have a meaning....

Suppose you are drawing 75 watts at noon, and then drawing 150 watts at 1 PM, and then drawing 225 watts at 2 PM, and then drawing 300 watts at 3 PM (etc). You rate of energy use is increasing at 75 watts per hour.

I also assume this is three parallel strings in a 12 volt configuration. Having 3 parallel strings will work for awhile, but it is not an optimal configuration. Ideally the OP would have a single string of batteries.

--vtMaps

232✭✭And everyone is right that this is not enough to design a system based on. For those already running a battery based system, they have a 'feel' for correct answers. I don't have that yet, so I question my math skills. But this clears up the basics. Thanks.

232✭✭795✭✭✭✭I recommend going to Batteryuniversity.com where you will find a lot of information on battery technology. How to connect and maintain them, including all of the various chemistries with the advantages and disadvantages of each type.

Good luck

232✭✭