new member pv off grid question

[martin pearce]

hello.

I have 4x 235w pv panels intended for grid tie. I want to use these for off grid to a battery bank via 2x 30amp charge controllers and then on to a immersion element in my hot water tank. i have a small diagram below and if any members have any thoughts on this they would be much appreciated.

 

[Worcester]

Your 4 x 235W panels are not going to produce 940W very often, it will always be significantly lower, a well designed system should produce about 750kWh in a year. The panels are unllikey to put out more than about 7 amps each, so in parallell you'l get up to 14Amps through each charge controller, depending what the load is at the time and what the batteries can take.

The one thing you're going to have to watch the is depth and number of cycles that the batteries can take, car 12V batteries don't take kindly to regular deep cycling.

Your system about should work in principal, you are going to have put some kind of control system in for the immersion heater though, otherwise you'll just kill the batteries. You'll also need to size the HW tank properly, otherwise you are in danger of always having cold / tepid water.

You also don't want one bank of batteries just charging another, so you need to add some kind of control mechanism (diodes..)

 

[martin pearce]

Your 4 x 235W panels are not going to produce 940W very often, it will always be significantly lower, a well designed system should produce about 750kWh in a year. The panels are unllikey to put out more than about 7 amps each, so in parallell you'l get up to 14Amps through each charge controller, depending what the load is at the time and what the batteries can take.

The one thing you're going to have to watch the is depth and number of cycles that the batteries can take, car 12V batteries don't take kindly to regular deep cycling.

Your system about should work in principal, you are going to have put some kind of control system in for the immersion heater though, otherwise you'll just kill the batteries. You'll also need to size the HW tank properly, otherwise you are in danger of always having cold / tepid water.

Thank you very much for your thoughts. I am beginning to think that the load is just too great for the batteries and I agree that I would have to limit the battery usage to an hour or so a day which is not great! As I have the panels, controllers and batteries already I wonder what you think might be the best use of the energy? The house is ongrid - I am wondering if a heat pump might be an efficient use, certainly putting the energy via a 240v inverter also seems a bit of an inefficient use. Again than you for your thoughts, much appreciated.

 

[solarfred]

I think it will not be a good investment. If you are trying to save money that is. Too many losses and insufficient PV. Why do you need the batteries? Why do you need a 13 amp fuse? Just plug in a 12v or 24v DC immersion from the controller? I have not done the math but that would eliminate all the expensive kit. Of course having a PV system just heating water is not very green. Maybe run some house loads and any spare into a small immersion.

 

[martin pearce]

Thanks again. I happened to have the batteries and the pv panels. The fuse I thought to protect the charge controllers. Maybe I need to up the pv to make this system any sense. the house being on grid is all 240v but maybe I could run a separate 24v/12v main that I could use for house loads at that voltage. Seems a shame to have the panels and not to use them at their best efficiency, even if they are only a small array.

 

[solarfred]

If you want to charge batteries then string them all together in parallel at 39 volts with a decent MPPT charge controller and that will charge your batteries in one bank. Then run the DC loads in 24 v but it means rethinking your plans specifically for a hybrid system. It will not be cheap to do this properly but could be worthwhile to give you some ideas about the future if the power bills start to rocket. Having an inefficient and badly planned battery system will be expensive and disappointing.

 

[martin pearce]

Thank you, your thoughts have been really useful. I will have a think and if i come up with some ideas perhaps i can follow up on this thread. again thank you very much.

 

[Worcester]

Seeing as the panels were intended for grid tie, it would just make best sense to get them put on the roof, registered for the FIT and powering the home... as Nike would say "Just Do it!"

 

[martin pearce]

Inverter to Consumer unit connection

Seeing as the panels were intended for grid tie, it would just make best sense to get them put on the roof, registered for the FIT and powering the home... as Nike would say "Just Do it!"

Thanks for the advice on this, much appreciated. I have taken your advice upped the array to 8 x 235w panels and a 2kw grid tie inverter. One question is connecting this to the consumer unit. DC isolator between panels and inverter, ac isolator between inverter and my own hour meter, ac isolator meter to inverter - not sure why the meter needs isolator either side but hey! Going into the panel do I connect via a spare RCD and if so what rating? and as the flow is from the inverter to the consumer will the RCD work 'backwards'.. - sorry if there is an answer to this elsewhere on the forum, maybe you could direct me to that. Again many thanks for your help!

 

[Worcester]

@martin pearce, the system will need to be installed by an MCS registered installer to qualify for the Feed-in-Tariff, you can't 'Do it Yourself'.

The RCD rating depends upon the make of inverter... - it is G83 certified ?
You don't need two AC isolators if you are right next to the CU, the AC isolator needs to break both poles and be lockable in the off position only.

 

[martin pearce]

thank you..yes I know final connection will need to be mcs. but I will do as much as I can..its a g83 fronius unit ig20. thanks for your help.again much appreciated