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Stuart, this is a very good idea , but as you already assumed, other people already had the same idea and there are already systems on the market which follow more or less your thoughts.

In the most simple form, the panels are directly (or via a discharge protection diode) connected to the battery and the inverter is then directly fed from the battery. A control system has to regulate output power of the inverter, so that:

- during daytime, the pv panels charge the battery to an acceptable fullcharge level

- during times with no power from the panels, the battery is then discharged down to the minimum discharge voltage

This simple system has a lot of disadvantages. To name one: the panels cannot be operated at their MPP and so the overall efficiency is not very good. But for a DIY system, this could be a good starting point.

But there are more sophisticated systems already avalibale (in Germany) which work quite good, with very good control and excellent efficiency.

If the battery and panels were switched in/out of operation then the MPP tracker would work fine.

If you ignored any low power points during the day, and just switched the battery bank in after dark, I could see this working quite well.

If the battery and panels were switched in/out of operation then the MPP tracker would work fine.

Yes, sure, but you also want to charge the battery at some point. This usually takes hours and during this time you loose MPP operation. And a lead acid battery should be filled up to more than 70-80% every now and then to keep it healthy (and then you also want it to be filled up during daylight time to be able to discharge during the night). A clever control can do a lot here, but is not as easy as it seems at first sight (I know quite well because I developed such a unit last year for professional use . The system is in production and sold in small numbers but is nowhere near perfect).

Regards, Jörg.

PS: the question with systems like this is if it really pays back after a reasonable time and this depends on a lot of auxilliary conditions.  

You are looking at £2250 (300V/12= 25 batteries)just for the batteries and they will have to be deep cycle types, better mayby to have just one or two batteries and a small inverter to drive just the lighting in the house low energy bulbs of course.

And think of the hydrogen you'll be producing.

I'm thinking of just removing the background load on the house, probably 200-300W.

Of course, the battery will need re-charge, but if I'm disconnecting them from the PV, then that would have to be at 240V mains, a bit wasteful there as I'm converting dc=ac = dc !

.......have to be at 240V mains, a bit wasteful there .....

This is actually a much better approach as the whole system becomes completely independant of MPP voltage (and PV configuration at all). A setup like this is also useful as an add-on for existing PV systems to increase 'own consumption'  for which you get higher payment (here in Germany!).

Modern solar inverters have an efficiency of ~>96%, the same goes for rectifiers where efficiencies are now also in the 95-96% range (or even higher). You loose much more energy if the PV generator does not work at the MPP.

 EDIT: the small (and cheap) 300W grid tie inverters from china are all well below 90% though. But this could still be a good starting point.

Just thinking about pushing the battery power into my SMA 3000HF inverter !

Hello i have a your idea!! but i think to control inverter grid tie to reduce power (from 500w) to house consumtion  ex..200w

and energy excess (panel 3 x 240W) to charge with mppt regulator  battery automotive used !

and energy storated to recycle in night mode with relay to inverter_grid pin (10 battery x 12v 120v to inverter 2-4 amp) and  control inverter grid for house consumtion !

to control inverter i think to modify dc current monitor to overcurrent mode for reducer power out on arduino control power 

feedback house power consumtion !! 
 

any ideas ??