The rooftop PV system we installed seven years ago has just passed another good round number – 16 000 KWh – having produced 4 000 KWh since my last update, in September 2016.
The daily average in that period is therefore 5.9 KWh/day, a little lower than the average of the first five years. The drop in output is so small that it’s not really worth worrying about but three explanations come to mind, and all may have contributed to it:
- We don’t bother cleaning the panels, so full-sun output may have dropped;
- Our trees have kept on growing, so the panels may be shaded for longer, especially in winter;
- The period we are considering includes two full Wet seasons but not quite two full Dry seasons (the month-by-month variation is shown here).
How much money are we saving now?
- Looking at our last four quarterly accounts, I found that we had been credited with $390 (886 KWh @ 44 c/KWh) under the “Solar Bonus Scheme.”
- A few minutes with pencil and paper showed that we had generated 2150 KWh during those twelve months so we must have used ~ 1270 KWh here, saving us ~ $320 worth of grid power at 25.9 c/KWh.
- Our total annual benefit was therefore ~ $715, pretty well in line with what I found last time I did this sort of calculation.
- It appears that our installation and occasional use of two big air-cons late in 2016 didn’t make much difference. That’s not so surprising when we remember that this well-shaded high-set house has always been cool enough so that we managed without any air-con upstairs for twenty years.
Would we do it again if we moved?
Definitely – but differently, mainly because of pricing changes. Systems are now readily available for under $1 per watt (I’ve seen ads for a 6KW system for $5500) and the big feed-in tariff (the Solar Bonus Scheme we’re still enjoying here) has done its job and been closed to new customers.
These days the typical system is 4.5 KW, three times the size of our present 1.5 KW installation.
If we had generated three times as much electricity in the last year and only used the same amount, we would have fed ~ 5200 KWh into the grid instead of ~ 900. At the (new, current) rate of 9.4 c/KWh as per Ergon’s feed-in tariff page that would have paid us ~ $480, to which we should add the $320 savings from using our own power. Assuming no increase in tariffs (for what we buy or what we sell) that would give us a payback time of about five and a half years, which is fine.
If either the feed-in tariff or the consumption tariff went up, payback time would drop, as it would if we began to use more power during the day by (e.g.) spending more time at home with air-cons running, or charging our not-yet-bought electric car. Furthermore, in the next six years we may find that retro-fitting a battery, or even going off-grid entirely, may become economic. On the other hand, I can’t see any likely scenarios which would make the payback time significantly longer.
What about batteries?
They are still not worthwhile for most urban customers because the payback time is too long and other benefits are small. Gizmodo’s Australian home battery storage buying guide (May 2017), for instance, concludes:
…none of the available options, including the Powerwall, make economic sense for the majority of households in Australia . That will change … but they won’t magically save you bucket loads of cash from day one. …
There are many solar and battery storage companies, as well as media articles, that promise these systems will pay for themselves in under 10 years, or even shorter time periods. The vast majority of these claims are false, and only work if you don’t include key factors about battery storage, such as efficiency losses.
Furthermore, many of the claimed returns are bolstered by a large solar array, which would actually save more money if used without the battery storage.
Solar Choices (2018) agrees, saying:
Previously we’ve looked at the topic of when battery storage will become a ‘no-brainer’ investment. In that article, we said that the ultimate goal post is the point at which the addition of batteries actually reduces the payback period of a solar PV system – rather than lengthening it.
If we assume that the batteries are charged only with the sun, by our estimates battery storage pricing needs to come down to about $200-$300 per kilowatt-hour (kWh) of storage capacity (for a lithium battery with a 10-year warranty) for it to make sense purely from an investment standpoint. … Currently, the lowest prices we’re seeing are about $750-$800/kWh of storage capacity.
Once again, the only certainty is change. Battery prices are falling fast, and the wider energy market is also changing fast as the cost of renewables continues to drop. Power storage (big batteries, pumped hydro and more) and network integration look like the two next big things.