Presumably you don’t enjoy FiT as your diy installation will not be MCS, hence all you will save is the import cost of energy at the Tarrif applicable to the time of use.
I like the idea of (free) pv charged battery packs, but the economics just don’t stack up. In your case, with a 5kWh battery - assuming that you get one cycle per day and use the stored capacity at a 15p/kW Tarrif time period, you have saved the (drum roll)… sum of 75p! Extrapolate that to a years use and you may have saved circa £270.
Most lithium battery packs are warranted for circa 6-8 years (with incrementally diminishing capacity), so let’s say 7-years life x £270 = £1,890 potential savings. Compared to the purchase price of the Growatt charger+battery @ £2,990. Add the cost of AC and DC isolators etc and the total battery set up could cost easily twice as much as you could realistically save.
I appreciate that you bought your set up second hand so almost certainly paid substantially less than the retail price, but given the reduced capacity from a used battery (I.e. you won’t get 5kWh) I’d guess that you will still struggle to get much of a financial benefit over the life of the battery system.
I originally looked at purchasing a Tesla PW2, touted as being 13.4kWh capacity, but although Tesla warrant the PW2 for 10-yrs, it is only for 80% capacity. So, averaging the 20% loss over the warranty period means an adjusted capacity of 13.1kWh, then factor in the software limited 80% DoD (depth of discharge) and you end up with circa 10kWh real world capacity.
So, using the same rationale as per the Growatt, this gives us £1.50/day saved or £390/yr x 10yrs verses an installed cost >£6k … the sums just don’t add up over the life of the battery pack.
The final nail in the coffin is that it will be a rare day indeed where a FiT eligible, <4kW pv installation in the UK is going to produce enough surplus energy to recharge a fully discharged PW2 battery, hence owners will inevitably resort to topping up with off peak grid supplies, further eroding any financial justification for having a battery pack.