The basis on which the majority of people would have engaged with the VPP would have been to reduce their electricity bills, with backup protection a secondary consideration, especially as the latter was only offered as an option for which they would have to pay extra.
This was clearly stated in AGL’s “Virtual power plant in South Australia, Stage 1 milestone report 31 July 2017”.
“A review of historical solar pricing and installation rates data coupled with qualitative market research showed
that to achieve any sort of energy storage system uptake beyond the niche ‘early adopter’ a simple payback
approaching seven to eight years was necessary.”
Given that the early entry price for the AGL VPP was about $4,000, AGL is clearly saying that customers will save an extra estimated $500 per year by joining the VPP.
In reality, the Sunverge battery system turned out to have cost us money, rather than saving us money, and AGL, to their credit, clearly recognised the need to offer alternatives. The evidence now clearly shows that the Tesla Powerwall 2 system is superior, and may deliver the savings that AGL considered necessary in order to get people to participate in the VPP.
518 days of Sunverge data show that the battery did not contribute to paying itself off, but was a drain on the profits we were getting before Sunverge, when we just had solar panels.
181 days of Tesla data suggests, from Table 10 above, that the current subsidised VPP Powerwall 2 VPP battery cost (with backup) of $8,389 could be paid off in 14.6 years. For those not in the VPP, the latest retail pricing for a fully installed Tesla Powerwall 2 is estimated to be $12,500 - $15,000, with a time of 21.7 years to pay it off at the lower price. These Tesla numbers could become worse with time, given that the 181 days of Tesla data are from the more profitable time of year when solar output is highest.
There are five other aspects of solar batteries worth considering; “free” solar energy, warranties, system repair costs, battery replacement costs, and system monitoring.
There is a strange paradox in solar energy, in that it is not free. In South Australia, every kWh that we use directly during the day costs us $0.16300 in lost revenue, and every kWh we draw from the battery at night costs us $0.1900. The latter cost is $0.16300 increased by the round turn losses of 14.2%, because the energy used at night has to be replaced the next day, and that energy goes through the charge/discharge/invert cycle with its attendant losses. In other words, every kWh used at night uses up 1.166 kWh of solar energy the next day to charge the battery, instead of being exported.
The REC solar panels we have used have a 10 year warranty and a 25 year linearity warranty. This means they are warranted not to degrade by more than 0.7% per annum, which means that after 25 years they will still perform at 84% of their original specification. Most solar batteries seem to have a 5 to 10 year warranty, but often with no linearity warranty. The Tesla Powerwall 2 is, at least, warranted to have 70% of its initial 13.5 kWh usable after 10 years. If the batteries have to be replaced after 10 years or so, this could greatly affect the economic viability of solar battery storage.
Repair costs for solar battery systems will clearly be greater than for a simple solar panel array without a battery. We had two installation faults and at least three run-time failures with our Sunverge battery, so if we were responsible for the repair and maintenance costs, that could have been a significant additional cost. We have had no downtime or problems with the Tesla system so far.
Given the payoff time for a solar battery system currently appears to be in the order of more than 10 years, and the estimated life of the battery appears to be less than 10 years, battery replacement costs may weigh heavily on the decision to install a solar battery.
The only monitoring provided in relation to the Sunverge system was via AGL’s app and the Solar Command website, and neither made it easy to monitor in detail, and in real-time, the battery charge status, household consumption, solar production, and grid exports. The Tesla app, and the Windows Powerwall Companion app, do provide easy real-time access to all these parameters. Unfortunately neither AGL nor Tesla provide any downloadable historical data via a website. I have asked about the possibility of such facilities being provided in the future, and both AGL and Tesla have indicated it might happen sometime, but it is not a priority. I would hope they would both see the need to differentiate themselves in a competitive market place, and offer customers access to their own data.