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How big should a battery bank be for off-grid that only consumes in the day?


solarman
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I understand (think I do) the calculations for a home off-grid. I want to find out if you put a off-grid system on an remote office where eskom is not an option that runs from 8am to 5pm every day (laptops and printer etc.). How big the battery bank should be. The average daily consumption is 10kwh.

Home vs Office example. (bulk of consumption during daytime vs evening)

For example, If a home consumes 10kwh a day (24 hour period) which is mostly in the evening (10% day and 90% evening). A battery bank with 200Ah Lead-Acid batteries 12V and a 50% DoD works out to 8,33 batteries needed. So adjusting for inefficiencies, building in a buffer and taking 48V in consideration. 12 Batteries will be used? (This is just taking in consideration 1 day of autonomy for simplicity) 

How will this change for a battery bank where 90% of the power is consumed during the day?

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Let me take a different tact on this, rather than the usual capacity calculations. Ideally the bank must be large enough to accept the full charge current from the solar panels when the load is zero (which might typically happen on weekends when no-one is around). Yes, there are software solutions and all, but to keep it safe and simple. To make 10kwh a day you need at least 2kwp of panels. Might need a bit more in winter, but lets go with that for now. 2kwp at 50V (again, to make the math easy) means 40 amps at the peak. 40 amps needs to be about 15% of the battery Ah capacity, definitely no more than 20% (assuming lead acid), which gets you to between 200Ah and 260Ah. So the smallest bank you can get away with is one string of 200Ah batteries.

One string of 200Ah batteries will give you a total of 9.6kwh storage, let's say 4kwh at just under 50% DoD.

The rest of the sums depend on early morning and late afternoon. Assuming the average load is not going to be more than 1kw, then in the middle of the day (10AM to 3PM) we can probably assume everything will be taken care of by the sun. So now you just need to determine if you can get through the night (which takes 1kwh if we assume 90% of the 10kwh is used during office hours), and through the 4 hours before 10AM and after 3PM with that battery capacity. It seems a bit tight, but it depends on your load.

So conservative answer would be two strings. You don't really want 3 strings anyway. Multiple strings causes all sorts of trouble. Optimistic answer would be that you could probably get away with just one string, if you keep the before-10AM and after-3PM loads down as much as possible.

Edit: But then... what is there is no sun on the day? Will you run a generator? Perfectly acceptable answer, just something to think about. If you want a day's backup, you need the larger battery anyway.

Edited by plonkster
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