Effect of discharge rate and DOD on battery life
I think that, for quite a lot of folks on here, battery life is a major consideration. I have posted this graph about LiFePO4 battery life elsewhere in the forum, so apologies if you find this repetitive. The graph is from a manufacturer other than Sunsynk, but it is likely to be broadly representative.
You can read the graph for yourself but, broadly speaking, what it is saying is that (for the right-hand half of the graph where most of us will be operating) discharging at 1/2C instead of 1C could reasonably be expected to increase your battery life by something in the region of 40%.
Reducing DOD from 100% to 90% could increase the life by about one sixth. Going from 100% DOD to 80% DOD increases it by about one half. Going from 100% DOD to 70% DOD roughly doubles it.
I would guess that other factors, such as temperature, would have significant effects on battery life.
If the predominant reason for installing a battery system was for UPS's then I would agree, but in the UK people are mostly installing these systems to save money so I think under specified systems are a problem. Legislation can be by installation type keeping the range of options fully open while giving the consumer greater re-course against being miss-sold a woefully underrated system, in my view something the industry commonly guilty of at the current time. I agree MCS is probably not the best body to set these requirements, but I think it probably a good path for giving installers a defined procedure to determine required capacity and for enforcement on non-compliance e.g. removal of MCS certification for repeated miss-selling of systems should be the minimum penalty.
For reference, lifetime costing should include environmental impact of battery production and disposal, not just end consumer cost/saving benefit.