It’s now over a year since we installed our solar panels at Red Cottage and it’s a good time to analyse how successful we have been in reducing our power purchases from Mercury Energy.

First to recap, we installed a PV array on our roof in mid-February 2013. It was supplied by What Power Crisis  for a total cost of $11,667.85.

Our 9 x 250W panels have a theoretical output of 2.25kW, but because the orientation of our roof is not perfect (the panels face NE), there is a consequent loss in efficiency. As can be seen in the daily production chart (below), the day-to-day variability can be up to 4kWh on either side of the 14 day moving average (red Line). On our worst day in April 2103 we generated only 0.05kWh, but our best day in January 2014 saw 15.57kWh pass through our meter box.

Daily power production from Red Cottage

As there is no power storage on site, any power that we generate over that being consumed is exported to the grid. The purchasing agreement we have with Mercury Energy has them buying power from us at the same rate it is supplied ($0.155/kWh) up to the amount we purchase from them in a given month; any production over and above that quantity is purchased at a discounted rate ($0.035/kWh). Therefore it is more effective for us to use as much of our own power as we can during the day. Energy intensive activities should therefore be undertaken in the middle of the day – a transition that we have not completely adjusted to.

We pay normal consumption levies to Vector Limited and the Electricity Authority on power consumed and daily fixed charges to Vector and Mercury as usual. These extra charges mean that we have never had a monthly power bill that was in credit, but having had the Vector dividend from AECT ($330), paid to our power account we have not had to pay a power bill since September and still are in credit.

Over the year we bought 2,657 kWh from Mercury and sold them 1,882 kWh.

Unsurprisingly, since the panels were installed our power purchases have at least halved in every month – which equates to a saving of $1,167 compared to last year’s power bill. Assuming static power prices, it will take 10 years to pay the system off, but will still have at least another 10 years of life left.

Obviously this is a power strategy that requires high up-front expenditure and a long pay-back period that will not suite most consumers. There is more feel-good factor than budgeting sense in this strategy, but it is a good example of the potential to improve energy efficiency and sustainability on a wider scale. Our location in the Hauraki Gulf means that we get plenty of sunshine and this helps to compensate for the inefficient roof orientation of our system, but for new homes where design can be tailor-made for peak efficiency, this is an option that has much merit.

We would certainly advocate for increasing pressure/incentives by local and state government to encourage uptake of energy efficiency measures – especially for new housing where measures can be optimised.