Solar update - 3 years in

How are our power bills?

Negative bills for Jan, Feb, Mar 2025

We changed from Contact to Z Electricity in June 2024. Similar pricing - still 3 hours free, from 3am to 6am, as well as half price electricity 9pm to 7am. But the main change is we've moved from a quite low but safely predictable 8c/kWh solar export price - to the rollercoaster of the wholesale spot price. That means we earn whatever the spot price is at the time and it changes every 30 minutes.  Spring solar export rates approached 0c/kWh but in summer it's been up around 20, 30 even 40c/kWh. We were often getting more for exporting solar power than we paid for importing grid power. February's bill was negative $95!

How is our Payback? How have things changed in the last 3 years?

4½ years more and the panels are paid off

There's been ups and downs, but the payoff has stayed fairly consistent at 8 years. 

Is this modelling is breaking down because of inflation? Power prices are going up, a larger power price will make it look like the panels are saving us less when they're actually saving us more, right? But turns out that's not happening after all. I looked at how our prices have changed over the years and because we've managed to juggle plans to our advantage we're actually paying roughly the same for power as we were in 2022.

In terms of how our panels are doing, we aren't seeing any degradation in the solar panels either which is great. Certainly nothing visible that we can discern from variations in weather. We'd expect this given that they have a 25 year warranty so any noticeable degradation after 3 years would be quite a worry.

Annual variation

We've had the panels long enough now to see how they perform in winter vs summer. 

2024 seems like a 'typical' year - 773kWh in January compared to just 200kWh in June. 75% less! That 25% of midsummer generation is still enough to keep our daytime power free-ish, when the heat pumps aren't warming up the place. We get a bit of power trickling in even on cloudy and rainy days. This is a good reason to over-spec your system though - how much power do you want to bring in, even in mid winter?

We bought an EV!

This will change things! In March we bought an MG ZS EV. It's got 320km of range and recharges from empty to full in about 24h on a trickle charger, and from 20% to 80% in 30 min on a fast charger. 

For instance, when Angela drove it back from Christchurch to Wellington, it needed one fast charge in Kaikoura. There are now plenty of fast chargers in NZ and it's charged up before you know it - 30 min is not quite enough time for a coffee and a muffin. 

24h on a trickle charger sounds like a long time, but it really isn't - because we don't commute in our car, it stays parked and plugged in in the garage and that basically means it's fully charged whenever you get into it. In fact because apparently it's slightly better for the car to not leave it sitting with a 100% charged battery, the issue is more managing it to not charge up fully and also to use the free power or the cheap power to charge it. None of which is actually necessary but it keeps me entertained. I'm turning into my Dad, who insisted on pushing our car around on the driveway because it was better for it than cold starting it.

Did we buy a battery yet?

Well, no. But my neighbour just told me about how prices are still trending down down down:

The lowest automotive prices are down to US $53 per kWh for a cell, and about US$75 in a vehicle battery pack. Might be an oversupply as EV sales are down in some markets

They are the later LFP chemistry which is far less fire prone

So A$1000 per kWh for Tesla Powerwalls is extreme, and other storage batteries for solar also look way overpriced.

I'm still hopeful to see 10kWh batteries available for much cheaper soon - which is all we need to capture a day's solar to use in the evening, and cheap/free overnight electricity to use in the morning. $1000 NZD, not $10,000 NZD would be a good price, and I'd be more than happy with second life batteries out of a parted-out EV. There is a bewildering array of batteries available now, this market is finally heating up like it should do.  But 50kWh of home battery storage is still a lot pricier than our MG with a 50kWh battery - and it doesn't come with a free car. That seems ludicrous.

I did some reading and seems like it's mainly because it's a niche market still. Folks ran through the inverter and installation costs (should only be a few thousand, certainly not up to the cost of a whole car!), interspersed with other folks with the expertise to build their own system, and had, for $1-2000 and showing their aliexpress receipts. 

We've just spent a week with Tim and Michelle in Townsville which was great fun - a battery is a no-brainer up there. Lots and lots of sunshine, zero and even negative solar export rates because everyone's doing it, and AC that runs all day and all night. The Aussie government is talking about subsidising batteries (as an alternative to building more power plants!) and like solar, anything the Aussies do creates a more mature market that we Kiwis can tap into.

Anyway, we are getting more than enough return selling our solar back to the grid right now. We now have backup power with the V2L cable that came with our MG - in other words, we can run our fridge and phone chargers off the car for a few days. So there is no point in spending any money on a battery. I will keep watching with interest.

Heating days

My work colleague, with a ducted heatpump system, has discovered 'heating days' as a metric of  electricity usage - especially if a big portion of your usage goes on heating.

This graph shows kWh usage of his heat pumps (the stacked bars) over the last 2 years, and the line is the “heating degree days” (HDD), which is the cumulative number of hours where Wellington was <16C.

The 'Heating Degree Days' line is, as you can see, a good predictor of actual heating energy usage. July 2024 bucks the trend as he went on holiday to Aus for 2 weeks 😊.

'Heating Degree Days' is also a good way of visualising whether efficiency gains have worked - has adding insulation or more efficient heating had an effect on your power bills when you adjust for Heating Degree Days?

https://Degreedays.net is his source, using NZWN as the weather station. Also lots of links to articles on that site to learn more.