Recalculating Payback Time

As mentioned in October, I got some new panels, so I thought I’d have a quick run at recalculating payback time.

No doubt the new, west-facing panels, are having a positive effect on electricity generation. As the days get longer in Summer, clear days are cranking out 2-3 times as much as the house consumes!

Of course, its not all gravy, with this last week in December producing two extremely hot days above 36oC (~97oF), followed by two days of rain. The former item consumed a lot of air conditioning, while the latter didn’t generate a lot of PV energy.

I’d love to have a Powerwall 2 to ride all that out! But I make do with having a grid connection.

Payback Parameters

The new panels were $1320 including GST. That takes the total system cost to about $18k at this stage, now including 6.5kW of PV, inverter, Powerwall, and Reposit.

That is an increase of roughly 8% over the original cost.

Generation capacity has increased by 1.5kW / 5kW = 30%.

That figure is a bit fuzzy because the existing panels don’t face the same direction as the recent additions. Also, the inverter I have is limited to 5kW of throughput. So chances are I won’t use all of that capacity across the year. Let’s call it 20% as a “real” figure.

Regardless, the main point is looking at the cost increase over the capacity increase. Extra funding of 8% has allowed my system to gear up by a potential 20-30% in terms of generation.

This shows what anyone experienced with solar will tell you: panels are cheap. Get as many as you can, with the biggest inverter possible.

As I mentioned in my last blog post, I now have data on finalised billing to cover about 70% of the year.

It shows my electricity cost – including daily connection fees – at roughly 71 cents per day.

Summer is yet to be added to this data set. Like winter, the primary environmental control (my ducted air conditioner) will get usage. I also have a pool pump that runs more in summer than winter.

Looking at historical bills, I tend to use about 3% more in Summer than Winter. Whether that still holds true depends on a lot of factors, particularly seasonal variance. Maybe we had a warm winter last year? I can’t remember.

This may not be significant in the face of increased solar generation:

  • Self-consumption can increase with longer daylight hours
  • Export can increase with higher generation capacity


Taking a Stab At It

Alright, so putting my analytical neck on a chopping block, here is what I think will happen.

Disclaimer: I am cheating a little in that I have data from Reposit Power to guide me on the days I know about since the last bill. To balance that out, not all my billing days were time-of-use (was on single-rate until August), so I’m flying blind in other ways.

As I’m writing this, I have realised just how many moving parts there are! This is going to be tricky, so let’s start with actual data.

Looking at the Statistics page of the site, fed by SolarEdge API, I can see my lifetime system statistics. All of which is very interesting to look at, but only two matter; import versus export.

Summary Import and Export – second week of December, 2016

These two factors are Summer in a nutshell. Increased import, possible reduced export. The daily average covers each day in the last week (up to 16th December, inclusive).

If we add the 3% increase I am predicting for summer from my historical billing, it doesn’t change the import much. The lifetime import has only been around 3kWh per day, so 5.47kWh per day represents an increase of 2.5kWh.

With peak tariff, this could be as high as 90 cents per day! Shoulder rate and offpeak rate make it 69 cents and 35 cents respectively.

I also need to factor in that Reposit Power imported a few offpeak kWh on Friday morning, so will call the extra import 2kWh per day. I’ll use the peak tariff rate to established increased costs of 75 cents per day.

On the flipside, I don’t think the 9kWh export figure is accurate due to the amount of cloud we had over the two days. The lifetime average is about 11kWh. To this, we add the 20% of our extra generation capacity to arrive at 13kWh per day.

“Wait a second!”, I hear you cry, “you’ve already had those extra panels on for a couple of months!”

Yes, I agree. However, we’re now hitting peak summer, so given my 20% figure was a fuzzy projection, based on 30% increase in capacity being the limit, I think it still works.

Again, looking at the statistics, you can see the 28-day retrospective is 13.92kWh. So I’m sticking with about 13kWh per day, keeping in mind I’ll be self-consuming a bit more than usual with the pool pump running.

An extra 2kWh per day exported is 16 cents in reduced costs.

Combine the two of those, and we had 75 – 16 = 59 cents per day.

This is extra, over the historical average of 71 cents per day. This brings us up to $1.30 per day for the summer bill.

Remember, I proposed a 3% increase of Summer over Winter, based on historical billing. The “Winter” bill I received back in September was $1.27 per day.

If you add 3% to that, you come out at $1.31 per day, so I’ll be keen to see if I get that close to my estimate of $1.30.

But What About Payback Time?

Very good question. I’ve just added $1320 capital cost to the system, and spent the last couple of hundred words speaking about Summer only.

Using the summer figure I calculated above, and wrapping that up into the billing I’ve received, puts the annual cost of electricity at my house around $310 as I’ve stated previously.

That is a saving of roughly $2k per annum over my old billing from last year. With the system cost now at $18k, that is a 9 year payback on simple calculations.

However, Autumn and Spring are the counterpoint to the Summer and Winter electricity burn for cooling/heating. Those times of year need to be exploited.

If my exports climb by 20% for both those seasonal bills, then the gains per year could be quite gratifying.

The two bills I have for these periods work out to 59 cents per day (March-June using single-rate tariff), and 24 cents per day (August-October on time-of-use).

The other key factor was full operation of Reposit Power with TOU tariffs, with is another massive advantage along with GridCredits. I can import very cheap power for anticipated poor days of PV generation.

Along with the extra generation capacity of my new panels, this shunted the power per day figure down by nearly 60% for Spring over Autumn.

If this holds for next year, and the Autumn power bill goes down to under 25 cents per day, it will mean for half the year I’m paying $44 for electricity. For the other half (Summer and Winter) I might be paying about $240.

An annual power bill of $282 sounds a lot better than the $2300 I was paying a year ago. And shifts simple payback under 9 years, by a few months.

The addition of these panels doesn’t look like a big deal on the face of it, but I’ve been fairly conservative in a lot of estimates.

I haven’t taken into account any GridCredits I receive, or the Diamond Energy Customer Referral fees I might get. So perhaps under 8 years is possible once the whole-of-environment changes are considered.

Christmas 2016

I’d like to thank everyone who has popped in for a look at the blog this year. Thanks for the connections on Twitter as well!

It has been quite an interesting 11 months since I got the Powerwall installed, and I’ve certainly learned a lot.

I have made an effort the last few years to put up Christmas lights, and copped sunburn, cuts and scrapes doing it. But its definitely worth it for the look on the faces of the local children.

I would like to wish you and your families a safe and happy holiday season, wherever you are.

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