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I made a system like this but based on SMA sunny boy inverters, Eltek battery chargers, a Trace /  Xantrex inverter charger, Rolls batteries and an adapted version of Robins sketch. It also heats the DHW and  house with any additional surplus solar power. At the moment it's got my grid usage down to 48KWh in the past month. Before this it would have been about 25KWh a day.

I shall soon be changing to a SMA backup system combined with the Eltek Chargers and 3 different versions of the diverter sketch running on 2 mega 2560 Arduinos and one uno.

Can you share some additional info ? Thanks !

As set up at the moment,

Basically Robins diverter is used to turn the chargers on in sequence to charge the batteries. As the batteries become charged they require less and so then the immersions and then the convector heaters turn on.

Once the outside light level gets lower than a set point.ie little pv generation, the inverter/charger switches to inverter mode and powers the house from the batteries. The inverter has the charger side connected via the diverter as well to charge the batteries when the PV allows.

The new backup system;

The SMA backups will charge the batteries automatically as and when the PV is available. They will also power the house when the pv can't supply enough.

One Diverter system will work monitoring the Grid and turn on lmmersions/Heat pumps as surplus PV becomes available. (The convectors are just temporary at the moment and will be replaced with a 2000l buffer tank for the central heating). This sketch will also have a facility to control the loads by using measured frequency when off grid. 

The  Arduino uno will monitor the AC supply to/from the battery backups so that the diverter (above) will be disabled when the power coming from the batteries is higher than the power going into the batteries. (3 backups so the sum of the 3 phases). The diverted loads are connected on the output side of the backups rather than before the backups. This way the backups can be used to power the diverted loads for a short time from the batteries eg to stop the heat pumps cycling.

The second diverter system is going on our second grid supply to charge the batteries and then divert to more immersions / heat pumps when the pv on this is able to. This supply has virtually no base load on it. This won't need a disable facility or frequency part as this grid supply won't have battery backup.

This all sounds similar to want to do.

I was going to start with diverting spare PV into an immersion and build up into a PV store when I can afford the equipment. I only have a rent-a-roof 2.4kw system so have to be careful on where I site my cables so as not to disturb the currently installed system.

Can I ask how big your PV is, number of batteries and capacity?

Regards

Nick.W

Its grown into a big system. I have about 25kWp of solar split as 20kWp Ac and the other 5kWp as DC straight to the 48v 1300Ah  Rolls batteries via an MPPT controller.

Depending on your hot water usage and where you live you may find that there is not much able to be diverted to batteries etc.  

With two kids under ten, we heat a tank of water each night for baths using Gas.  Must work out how much that is costing me. Any links to formula to calcuate that?

PV store would be a great pipe dream but I really do need to get the PV router into my system.

If you know the volume of water, the temperature difference between the water out of the tap and the cold inlet water, it's simple basic physics (See here). Then factor in pipe losses and boiler efficiency - both probably unknowns. I suspect the better way is a gas meter reading before and after, compared to a similar period when you're not heating water for the baths (or do it when you're not using gas for any other purpose).