Battery storage for residential grid-connected solar power owners is about to experience a growth phase that will probably outstrip what we are seeing in solar PV itself. (Feb 2015).
The question on everybody's mind is "what battery technology is the best?"
In reality that question should be: "Which battery technology will come out on top, and when?"
As of 2015 we would be foolish or arrogant to make that call, so here's what we are doing to prepare customers for the storage onslaught.
Storage is an open ended question and requires a lot more than just a blanket approach but to give you an example as of Feb 2015 you are looking at around $1500/kWh of storage capacity that will last for around 5 - 10 years. The inverter upgrade is extra on top of this and depends on a few factors.
The SMA Sunny Island configuration is one of the most established architectures in this space at the moment but the system is expensive (around NZ$10k for the Sunny Backup Unit c/w the AS Box to make it AS4777 compliant and sundrys) and will only properly work if the customer already has an SMA Sunny Boy inverter on their existing grid tie system.
We have bench tested and installed many variations of Hybrid system including mixtures of Enphase, Outback, SMA, and other inverters and have developed "the sam" (same hardware as the Feed-in Fighter but with a battery management module) to be able to integrate almost any battery inverter with any existing grid-tie system with all the functionality of the Feed-in Fighter as well.
So back to Battery types: at the moment reliable Lithium Iron Phosphate is still too expensive and the cheaper ones are too risky and we have bench-tested several of these to destruction - I give this technology 1 - 2 years to mature before we see a cost-effective reliable technology, this will probably mostly be thanks to Tesla's drive but others will be hot on their heels.
Lead Acid: AGM... 5 year lifespan in perfect conditions at 40-50% DOD - must be well managed. Cost is around 19c/kWh of used storage over the lifetime of the battery - similar to current feed-in "cost".
Sodium Ion (salt water batteries): We have just got pricing for these and they are very exciting! Cost per kWh used lifetime storage seems to be around 12c/kWh BUT.... Only suitable if you are willing to spend $20k for the bank or more AND you install our load management system. These batteries don't perform well under certain load conditions and therefore MUST only be installed by experts and into the right environment otherwise both installer and customer will end up in a very costly agruement over promised performance. http://en.wikipedia.org/wiki/Sodium-ion_battery
The list goes on but get's progressively worse for grid-tie storage applications....
We are currently installing high end AGM units starting at around $5k plus a 5kW inverter upgrade at around $6k plus labour and sundrys and customers are treating this as an interim arrangement for 4 - 6 years at which stage there will be a good reliable long-term Lithium or Sodium solution at a great price.