Battery calculations

Well depending on the chemistry of the batteries I always use a multiplier around 25% or less to help determine the amp hour size. So if you plan on 2880wh a day then I would make the calculation of 2880wh/12v/25% = 960amp hour system. This way I would not use more then 25% of the total amp hour the battery system is rated. If you have a lot of days of no sun shine then you can divide the amp-hour by a smaller % which increases the amp hour rating of the battery system.

Now a Li chemistry battery has more useful amp hours available, you then have to determine if going to Li is worth the cost. I always determine the amount of watthours I can use safely each day from the battery system then times the number of cycles the battery is rated. That gives me an estimated total watthours. I then divide that number into the cost of the battery system which will get me a $/kWh value. Then I can determine if a FLA or Li type battery is worth the cost.

Thanks, i must have made some mistake in my calculations. This is for our bus our daughter is living in. We have 400 ah 24volts in our offgrid house so she wouldnt need that much. Ill need to do the usage calculations again. In the bus is 460w of panels and 430ah of batteries at 12 volt. The batteries are failing so im goint to put new lead carbon batteries in. I was hoping for around 300ah otherwise it will be a bit expensive.
Cheers,

How do you get that 1,174 watt-hours per day, which last time I checked was equal to 1.174 kWh/day, to be equal to 2.88 kWh/day ?

You sure you understand what the on-line calculator is telling you ? Or did I misunderstand something you wrote or am I ignorant of something else ?

Or are you being told to not discharge the batteries more than ~ 40 % per day ? Or something else ?

As to your question, one of the design parameters used when designing an off-grid PV system with battery storage is how many days of sunless (or sun reduced) weather to design into the system so that power is available for a sunless period of that duration as well as some consideration for such period's temporal frequency and pattern.

From an online calculator. It was northern arizona wind and sun i got 1174watt hours per day.
Then 1.2kwh x 2 x 1.2 =2.88kwh.
Then 2.88 divided by 12v x 1000 =240ah battery bank.
Ill attach the email from them of my power usage.

Here are a couple of stickies from a forum favorite:



and:

So, 1.174 kWh/day is the daily load and 2.88 kWh (not kWh per day) is the battery storage size ? If so, maybe that's where I got confused.

I think that is so. The x2 is 50% battery soc and the 1.2 is battery efficiency. Also i have installed a mppt controller recently replacing tbe pwm morningstar and kt shows daily charge 1.35kWhrs so its near the 1.174 figure. I expect the 1.35kwh is solar input.
So how will these usage figures wodk out to battery bank ah with maybe two days no sun? We have the lead carbon batteries and they charge up alot quicker than our previous flooded lead acid Crown batteries.

I am also confused by this.
Usually, the formula for calculating the daily power consumption of a device is: E(Wh)=I(A) x U(V) x T(h)

hi, you can't simply consider the solar panel, but first, solve the battery capacity problem. If you have 3 days of continuous rain, and you can't charge for 3 days, then the battery's capacity should be considered 3 times the original. If you don't consider this problem, the calculation method of the solar panel is probably:
total power consumption / 6h (possible light time in a day) / 0.7 (the actual power of the solar panel)，
240*12/6/0.7=685.72，
7 pieces of 100w solar panels need to be installed in parallel.
12v/60a solar controller

Sure learned a lot from him. Too bad he doesn't post here any more.