Solar System 101

We got a solar roof system installed last October and also had a “smart” meter installed by the electricity company to measure our consumption / contribution to the grid. I ran some numbers and thought I would post them here.

We installed 9.4 kW of solar panels with an 8 kW inverter. What does all this mean? Well, solar panel generate direct current (DC) and the inverter is a box that then converts this into alternating current (AC) which is what your house gets from the grid.

So what does that get you? These next 5 charts are based on data from the inverter.

This is the system at peak performance on a summer’s day. The max output is 7.5 kW and its generated about 63 kWh over the course of a day.

This is a good winter’s day. The peak is 4.4 kW and the total output is 28 kWh. The peak is lower (because so is the sun) and the duration is shorter.

The data here shows how the total power output declines over the course of a year from summer to winter. But what does that variation look like day-to-day?

This is a bad winter’s day with lots of cloud and rain. The is peak about 1.3 kW and 5 kWh over the course of a day. To put this in perspective, our fridge is rated to consume 0.8 kWh per day.

This variability can be seen in the output of the system over the course of a month.

What does this look like compared to consumption data? The following chart combines data from the inverter with data from the smart meter.

Here Total Consumption = CONSUMPTION from Grid + Consumption from Solar. As you can see, most of our power in the summer comes from solar but in the winter it abruptly switches over. And this isn’t because we aren’t producing enough power for it to do so (more on this is a subsequent chart). Rather it’s because our pattern of consumption is changing. The biggest energy consumer in our household is ducted A/C - which consumes about 3 kW or 72 kWh if we left it running all day (so about x90 as much as our fridge). In the summer (particularly March in the above chart), this is on during the day and is easily covered by the solar if it’s sunny. In the winter, this is typically on after it’s gotten dark - when there is no solar.

But we don’t just consume the power we make, we sell it back to the grid (although the energy we buy costs x4 - x5 more than we get for the energy we supply).

Even in winter, although we are producing much less energy, we are still producing more than we are buying.

Getting solar has shifted our behaviour. Pretty much all our “discretionary” power consumption (washing machine, dryer, dishwasher) now takes place during daylight hours.

Our next move will likely be to move off gas entirely - which is currently there for water heating and a stove/oven and onto electricity. And then to get a battery - grid power is only get more expensive and the feed-in tariff for our solar is only going to decrease. At this stage, an EV is not on the cards. I have wondered if we had over-specced our system but the combination of seasonal variation, increased future demand for electricity and a battery to demand shift means that I am comfortable with our decision.

Australia has the highest penetration of solar in the world (over 30% of households have it in some form). The performance of solar systems doubles roughly every four years so some of the systems installed a decade ago are already ripe for replacement.

The next step for Australian households is the demand shifting that batteries offer - so solar power can be used at night. Currently batteries are expensive. The payback for a solar system is typically 4-7 years whereas the payback for a battery is about 15. Not great when their warranties are for 10 years. While some energy providers offer to subsidize your battery purchase if you allow them to use it as a “virtual power plant”, the control that they take in return is often opaque. It’s not clear when you will be able to use your battery and when not.

Hopefully we’re going to see:

• Increase innovation and competition driving down the cost per kWh of batteries.

• Options such as using EV batteries to power a house.

• VPP offerings mature.

• Governments offering sensible rebate schemes.

It’s also obvious that increasing retail electricity prices and declining tariffs will play their part as well.