diy solar

diy solar

Adding Battery To existing System + Additional Solar


New Member
Sep 5, 2022
Hi, I am in the UK and looking for any advice/suggestions/recommendations on what would be the best way to proceed.

Currently I have two solar systems, one system is a 3.99kw Grid tied FIT system with a Solax X1 Dual Mppt (fitted 2016 on grid grid tie inverter).

The second system is 2kw Solar off grid array on two strings feeding into an EP500pro.

I have a manual transfer switch that allows me to switch over all (mostly all, have a socket in the kitchen and the attic that stay on the mains so can spread the loads) of the plug sockets within the house from grid tie 3.99kw mains to off grid 2kw EP500pro.

My goal is to add more battery storage, such as a Fogstar 15kw or Seplos Mason 15kw. In addition I would like to if feasible tie in the 2kw off grid solar so that there is nearly 6kw potential solar for charging the battery.

As it stands I know that I could add an off grid battery (in place of the EP500Pro) and a hybrid inverter (I was looking at Solis or Solax Dual MPPT Hybrid inverters) and utilise the off grid circuit already in place (without needing to inform the electric company or the DNO) but I feel I would be loosing out on the power from the grid tied system. Is there a way I could use the excess power from the grid tie system to charge the off grid battery?.

Alternatively I was thinking that I could DIY replace the grid tie Solax X1 with a hybrid inverter (of similar spec) and add a battery direct to this. If my understanding is correct, to do this I would need to replace the current FIT meter with a by directional one and inform the FIT company of the change?. This way would mean that the 2kw of off grid solar would not be being used to charge the battery. Is there a way I could use the excess power from the off grid system to charge the grid tie battery.

Another option, if my understanding is correct is to add the off grid 2kw solar in an AC coupled fashion and apply to the DNO to allow this additional solar as it will then be above the 3.6kw 16a export (I understand I wont get paid for it but that's not an issue). From the research I have done so far this would cost me between £300/£600 just to get a confirmation. However and again if my understanding is correct this would mean in the future I could potentially have a battery on both the current grid tie inverter (when changed to a hybrid inverter) and also a battery on the AC coupled side giving me the combined power draw from both batteries when under larger loads like the cooker/shower.

Any suggestions/thoughts would be a great help, not sure if I am completely off the mark in what I am trying to do or my understanding of what I am allowed to DIY when referring to the grid side is as simple as I think it is when replacing the current grid tie inverter or adding an AC coupled inverter/battery.
I added a Sofar ME3000SP and battery to take advantage of my FIT PV system. Worked a treat! I've now changed the Sofar for a Sunsynk 5.5kw ECCO Hybrid as I wanted a higher charge rate AND wanted to add additional PV, BUT the additional PV will only feed the batteries NOT the grid so as not to impact on my FIT