Solar calculations for batteries

My first (and only) comment is that using resistance heating to heat water with PV power is extraordinarily inefficient.
You would be better off using solar thermal panels for the same job.
If you cannot do that, then use your electricity to power a heat pump type domestic water heater. You will get three or more times the heat for the same amount of electricity, even after accounting for conversion losses on the AC side.

Yes, thanks for your comments. Let's say I'm designing an extremely inefficient solar hot water system so I can report on the results to other people that are interested in trying this.
Cheers,

I think what you are designing has already been done quite a few times. I'd do a literature search before I reinvented the wheel - unless of course you plan on plumbing system inefficiencies to new depths.

Ok, thanks for your advice. I'll scrap that stupid idea.
So I have these 6 x 320 watt panels and I want to get batteries for them to make a 24 volt bank to supply my house which has no grid power.
My power usage for the day when I add up the 24 volt appliances, pump, lights etc is about 4kWH per day. There is no water heating in there as I'll use a thermal panel.
What size battery bank should I use? I'm hoping to buy four 6 volt batteries and have them in series.
Thanks,

Assume some of the load is during the sun shining hours. If you willing to get a generator to charge the batteries on rainy days, you can get 4 of those Rolls or Trojan 450 Amps hours batteries. If you don't want generator you have to get at least the 800 amps hours bank.

Question 1: You say 4kWh per day. Do you have control of when those loads will be used? In particular can you arrange for them to be mostly during the peak solar hours of the day? If you can, the required size of your battery bank may be reduced by a factor of two or more unless you need to allow for several days of poor sun and cannot postpone those loads or run a generator.
BTW, no off grid solution is complete without a generator at least for standby use.

2. For winter you can figure on 2 sun hours solar equivalent per day, so you need at least a 3kW panel array to allow for inefficiencies and energy losses. Even larger if you want to be able to recover from a run of cloudy days. 6 x 320 = 1920 will not do the job regardless of the size of your battery bank.

PS: OK, I take that back. For New Zealand your minimum solar hours per day in winter is probably a lot higher than 2. What is the number, by the way.
But you can still reduce the size of your battery bank if you can schedule loads during the day and have access to a generator so that you do not need five day autonomy.

At 4kWh per day from a 24volt system, that Midnite 150 CC and 1920 watts of panels you would need around a 800Ah rated battery. That gives you about a 5 day supply less losses. It may be hard to find 6 volt 800 Ah batteries so you may need to go to 2 or 4 volt ones.

If you get less than 3 hours of useful sunlight on the worst sunny days you will need to increase your panel wattage.

The Rolls have the 6volt 5000 series 820 amps hours and the Trojan Industrial line have the 6 volt 900 amps hours but they are monsters and heavy over 300 lbs each.

OK that is better. For reference read this thread so you know how and why I came up with my answers.

Lets start with batteries based on you stating you need 4 Kwh/day usage. With that we already now what size batteries is required. We just have to select a operating voltage to find the Amp Hours. So with 24 volts will require a 800 AH battery. So immediately we know there is no way 4 200 AH Golf Cart batteries are not going to work because they are rated at around 200 to 225 AH which is way short of 800 AH. At 800 AH. There are a few manufactures that make 6-volt 800 AH batteries but I do not recommend them for residential purposes because of the weight issue of 320-pounds each. Bit much to handle. Rather use 2 or 4 volt batteries that will weigh in around 100 and 150 pounds respectively. More on batteries in a minute.

Panel wattage depends completely on location, time of year use, shade-orientation-tilt issues. To generate 4 Kwh of usable power each day can take as little as 1000 watts in Tuscon AZ up to 4000 in Seattle WS under ideal conditions. With 1920 watts will require a 80 amp MPPT charge controller and with 1920 watts will pretty much be maxed out. But with that said unless you live in an area with less than 3.1 Sun Hours in winter, 1920 watts is just about right and a perfect match for the battery capacity required. Only way to go up to a possible 5000 watts is to use 48 volt battery. Which brings us back to the battery subject and economics. If you use 48 volt battery you can use a less expensive 40-45 amp MPPT controller. That would limit panel input to 2000 to 2200 watts and a 7th panel is out of the question because 7 is PRIME. At 48 volt battery only 400 AH is needed. Does not change weight or cost, just the size of the cells to 400 AH. At 400 AH there is quite a selection of 6-volt batteries and you would need 8 of them.

Want electric hot water with that using 13 Kwh/day. Well triple what I just went through. What will be really fun is the batteries would cost around $14,000 and need replaced in 5 years or so. Still want electric hot water, I will fix you up.

Oh. I know that they make 6 volt in those higher AH ratings but they may be hard to find and as you pointed out are very heavy. Better to go with a lower voltage at those higher AH batteries.

If he is going for 48 volts system, he can get cheaper batteries even though they not last as long as the heavy duty ones. At least the initial cost would be a lot less. $3200 for eight 400 amps Rolls or Trojan. He can even go to big box stores and buy 16 golf cart batteries wires two bank and replace them every 18 to 24 months for less than $2000.

[QUOTE=Sunking;119695]OK that is better. For reference read this thread so you know how and why I came up with my answers.


Panel wattage depends completely on location, time of year use, shade-orientation-tilt issues. To generate 4 Kwh of usable power each day can take as little as 1000 watts in Tuscon AZ up to 4000 in Seattle WS under ideal conditions. With 1920 watts will require a 80 amp MPPT charge controller and with 1920 watts will pretty much be maxed out.


Thanks for all you replies which I have been reading. The Classic sizing tool shows I can use the Classic 150 with my panels when I connect them in 3 strings of two. I put the battery voltage as 24 volts so hopefully I'm reading that right. I would like to stay at 24 volt as gear is more available. We have alot of sun where I am and I'll just find out a bit more about the chart I attached and what it all means. It's for the correct area we are in and it seems the average winter energy is 2.39 kWH/sq meter.
Cheers,

Only thin gI can suggest at this point is you to consider running 48 volt battery because with 2000 watt input you are at maximum power input for 24 volts. That does not leave you any room to grow. So if you go in now at 24 volts and decide to grow you will have to replace your inverter with 48 volts. You can avoid that by going in with 48 volts now which gives you a lot of room to grow into 5000 watts.

[QUOTE=asdex;119865]The numbers are average annual insolation and panel output for a 1 kW DC installation