PV Gap

Susan Said:

How many of these batteries do I need for a off grid system?

We Answered:

There is more info needed.
1. What are you using to charge the batteries. wind, solar? and how often do you have days when the batteries do not get "fed"; so you would need enough reserve to use off the batteries for more than a day.
2. How large is your wind or solar charger. If you are putting in 5kw from your "generator" and only have a couple of batteries they will charge too quickly and possibly be damaged.
3. If it is a solar system that charges every day for lets say 6 hours you do not need battery capacity for those hours because the solar will be directly feeding for that time. If it is a wind system and you live in an area that has relatively constant winds you may only need enough battery for 12 hours or if the winds blow hard 1 day and nothing the next 2 you could need a huge battery bank.

I will go under the assumption that by 9000 watts per day you are 9kw hours. Also assuming that the 8G8D is the battery you are looking at. The spec sheet stated 265ah and on the bottom it says 225ah so I will use the lower number so doing the math 12v * 225ah = 2800watt hours. Then 9000 / 2800 = approx 3.2 So you would need 4 to get over 9000 watt hours.

These large gel batteries are quite expensive and this size battery is common in large industrial machinery so you may be able to find a better price at a farm implement dealer or industrial supply house.