12, 24, or 48 Volt system

Re: 12 volt or 24 volt battery bank?

Are you using 615 Watts average per hour or are you using 615 Watt hours? There is a big difference between the two. The terms used and where used can not only change the calculations they can confuse the issue.

For example let's say you use only 615 Watt hours per day. On a 12 Volt system that's (615/12) 51.25 Amp hours from the battery. This could be handled with a pair of GC2's (220 Amp hours @ 12 Volts) with about 25% DOD and be recharged from about 400 Watts on a PWM type controller. Small, simple, inexpensive.

Now if you average 615 Watts over 8 hours that's 4,920 Watt hours and a completely different system needed to supply it. There you're looking at using a 48 Volt system which would be roughly 102.5 Amp hours used needing a 400+ Amp hour battery bank. You're looking at two parallel strings of eight GC2's and 2750 Watts of array on an MPPT controller.

Hence why it's important to understand the terminology and use it correctly: systems are based on the Watt hours you need to supply. When you need more power it is better to go up in Voltage rather than up in current if at all possible.

Right now you have eighteen 6 Volt batteries meaning nine parallel strings of two and that's not good. What's more that would total around 1980 Amp hours and there is no way 250 Watts of panel would be of any use on that. Nor would 760. What's more that would be 5.9 kW hours of stored power which is even more than the 48 Volt system example above.

I doubt you need all that battery capacity and I'm certain they are being destroyed through lack of proper charging. It would be nearly impossible to keep the current even through nine parallel strings even if there was enough charge power (3500 Watt array, three controllers) to do the job.

The wind turbine will most likely prove a disappointment even in Newfoundland.

So lets work out what the real power requirement is in Watt hours per day and go from there.

Re: 12 volt or 24 volt battery bank?

fiddlerkelly wrote: »

The 615 watts is / hour and that is only if I have every light plus TV ,receiver and laptop on. But as I said on average I am consuming 3 amps /hr. The invertor suggested to me by the dealer was a magnum invertor /charger /transfer switch with 4400 watts capability either 120 volt or 240 volt.

Yes, well. Good inverter but not necessarily what you need.

You don't use Watts per hour because Watts is a rate and it would be a rate of a rate. Watt hours is a quantity: 1 Watt draw over 1 hour = 1 Watt hour.

The other thing is there is a difference between what is going on with the AC side and what is going one with the DC side. If you are using 3 Amps @ 120 VAC that's 360 Watts. If you use that over 8 hours that's (360 * 8 ) 2,880 Watt hours AC.

First thing that happens: inverter conversion efficiency. Let's say you have a good one and it can convert at 90% efficiency. Now the 2,880 Watt hours AC is 3,200 Watt hours DC. Then you add the inverter consumption. Based on running only 8 hours and being "truly off" the rest of the time it would be a number like (20 Watts * 8 hours) 160 Watt hours DC to be added. Total: 3,360 Watt hours DC per day. If you round that up you can make the math easy: 300 Amp hours @ 12 VDC (ridiculously high), 150 Amp hours @ 24 VDC (much better but still large), or 75 Amp hours @ 48 VDC (easily manageable). Or expecting 25% DOD 1200 Amp hour battery @ 12 Volts, 600 Amp hour battery @ 24 Volts, 300 Amp hour battery @ 48 Volts.

I think we'll split this off into its own thread for further discussion.

Re: 12 volt or 24 volt battery bank?

fiddlerkelly wrote: »

Yeah like I said in reality I only have between 250 and 300 watts /hour that I am burning with more hours of use in winter than summer, 8 hours use in winter and 5 hours in summer. so based on that usage how many 6 volt batteries do I need and what size panel array would suffice taking into consideration our weather / hours of sun.

If you take the higher power demand number of 300 Watts and multiply it by the longer time demand of 8 hours you get 2.4 kW hours. Not an unreasonable number to achieve.

The next question is: if you do not achieve it and yet need it what will happen? You run the Honda. I'm not far removed from this because it's almost identical to my own system.

So if we target 2.4 kW hours and factor in the inverter conversion efficiency and consumption we get 2.8 kW hours per day. Not an onerous amount of power to supply. This can be done reasonably on a 24 Volt system, as it is about 117 Amp hours @ 24 Volts used. That would be a 468 Amp hour battery bank.

If you've got eight of those GC2's that are still good and can count on some power direct from sun to improve efficiency and possibly curtail usage if needed you could probably get away with two parallel strings of four batteries for 440 Amp hours @ 24 Volts. If not or you think consumption may be higher you have to make the decision of whether to go with 24 Volts or take the leap to 48. It's right on the threshold.

If you go with 440 Amp hours @ 24 Volts the solar should look like this:
44 Amps * 24 Volts / 0.77 = 1371 Watt array, round up to the nearest available size. That would be minimum. You could go up to 2X that if the weather is not all that co-operative.

If you go up to 440 Amp hours @ 48 Volts the minimum array size doubles.

Now about that inverter: do you need 240 VAC for anything? Will the total loads demand 4kW? If the answers here are "no" then there's other choices that will do the job rather than the Magnum.

Re: 12 volt or 24 volt battery bank?

fiddlerkelly wrote: »

Well I had considered if ever I should want to run an electric oven at the cottage but I would need a lot more panels if that were the case more than the 1370 watt array as you suggested. Again in reality I can be a bit stinger on the lighting in the night requiring only 200 watts . I only get there with water pumps running for 5-10 minutes pumping water to cabin or running shower or if kitchen light are on which are long tube flourscent lights.

You don't want an electric oven. Trust me on this. You'd be looking at needing to supply up to 12kW for a few hours at most. Massive system size for an occasional use item.

If your lights are already CFL you're probably doing good. If they're incandescent shifting to LED will be a large power saving.

If you can select to use the water pump only midday when the batteries are up and the panels still able to produce power that otherwise wouldn't be realized that will be a big improvement. I do this, storing up power in large pressure tank. Takes about 6 minutes to fill from empty to provide about 50 gallons. If you've got room you can put in more tanks and run the pump longer to store up even more.