Calculating battery capacity

Re: Calculating battery capacity

It comes down to threat assesment, so it's a personal thing. Really!

If it's life support and there is no other backup. One number, likely involving 2 systems with multiple backups of things that are likely to fail. still have outages when a charge controller, inverter or battery dies, but with the flip of a switch your up and running.

If it's what it likely takes to have, sized for general use, with very few outages over a finite number of years, and you can tolerate a 12 hour period without if you had to, then a calculation of the number of days w/o sun (X) the use of your household (X) percent of maximum discharge.

If you have a willingness to do with out and cut back, when you're facing dark days, No laundry, less A/C,... and use what is generated. That's a different level.

I think most people new to solar are in the first group, feeling the electric should be there at the flip of a switch, people who have grown a system and lived with out, or have lived off grid for a while and have maintained battery banks, get farther down on the "needs".

And after almost 10 years off grid, I'm ready (and have prepared for) a transtion from the "use what's generated" to "sized for general use"

Re: Calculating battery capacity

diego96 wrote: »

I'm in the process of trying to figure out how to design it.

You take your load per day:
370w * 24 hr = 8880wh/day

Then you figure out how much battery you need to supply that load for two days:
8880wh x 2 = 17760wh
17760wh / 24v (nominal) = 740ah

Double that to keep the batteries above 50%:
740ah x 2 = 1480ah

So you need a ~1500ah 24v battery bank.


Then, you figure out how much solar you need to recharge the bank while also supplying the load - you need enough PV to carry the running load and have enough left over to charge the battery bank.

So you need 8880wh/day to carry the load (370ah/day) and also need to replenish up to 740ah/day into the battery bank if you want to top the bank off in a single day.

So that's 1110ah/day you need to generate. Mulitply by charge voltage of 28v to get the watts:
1110ah x 28v = 31080w

Divide that by hours per day of "good" sun (I'll use worst case and say 4 hours):
31080w / 4hr = 7770 watts per hour.

So you'd need a 7.8kw PV array.

BUT...the PV only puts out around 80% of rating on average, so add 20%:
7800w x 1.2 = 9360w

AND...lead-acid batteries are only around 80% efficient, so add another 20%:
9360w x 1.2 = 11232w

So you'd really need a 11.2kw PV array.


That's pretty much how it's done.



There are some fudge factors...

For instance you could plan on recharging the battery bank over the course of two days instead of doing a full recharge in one day - so you could possibly cut the PV array in half.

You might get more sun on average than I used in my example. If you average 6 hours a day of good sun instead of 4 then you might be able to cut the PV array by 1/3 - as long as it didn't create a significant shortfall in winter.

You could plan on one day's backup, and cut the battery bank in half.


There are various ways to play with the numbers, but the reality is that a 370w continuous load is a bloody big load to be running from PV and batteries.


At the very least, you'll need to supply 9kwh/day just to run the loads, so with say 6 hours/day of good sun, you'd need a bare minimum of about 2kw of PV + whatever you need to recharge the batteries when they need it.

Re: Calculating battery capacity

mike90045 wrote: »

That is a bad idea for battery life. When the battery is below 75-80% charged, the sulfate deposits on the plates begin to harden. After 24 hours, they become permanent. This then reduces your battery capacity a little bit each time it happens

wow, I'm shocked and saddened that it happens so fast. I've probably shortened the life of my trainer set a bit just to recharging over 2 days on occasion. bummer...