Introduction
Melbourne has just passed the winter solstice and it’s six months since I realised my long-time dream of having a ‘you beaut’ solar PV system installed on my home
The previous article dove into the specifics of design and components used for the system.
This follow up article focuses on key learnings, analysis of the energy generation and financial performance variation through the summer and winter months, and the financial feasibility for installing a battery system.
But first, a milestone!
The 6.48kWp/5kW AC solar PV system I designed is now delivering 62% of my home's total energy use (directly powered by that great big nuclear fusion reactor in the sky), which I’m very happy about. A special thanks to Greg Kincaid and the skilled Enhar electricians.
With that said, let’s dive into the data and the key learnings.
Overview of The Solar PV System & Constraints
From a solar yield perspective, the house design is not ideal and presents some inherent challenges; The roof is aligned facing East/West and has a steep 30-degree pitch and there’s a ~25m tall gum tree to the North which causes considerable shade loss.
To maximise generation, I modelled various designs before settling on the following layout; 7 panels- 2.835kW on the East facing roof at ~98° azimuth and 9 panels 3.645kW on the West facing roof at 268° Azimuth.
The data shows, unsurprisingly, that the tree causes a significant reduction in our late morning winter generation, especially when it’s sunny. Interestingly, the dip in generation isn’t as prevalent when it’s cloudy weather due to greater diffusion of the solar energy.
Due to the combination of these factors, my house generates ~30% less energy a year than a more ideal North facing roof in a similar location.
Smart Inverter Data
The system uses a Fronius inverter ‘smart meter’ that enables me to see power consumption vs. generation in almost real time (it’s got a delay of ~15s) from my phone.
I chose this inverter due to the excellent data visibility it provides so I can monitor the system is working correctly and to help my family optimise the time/way in which we use energy.
Let’s take a look at what the data shows.
Diving Deeper Into The Data
With 6 months of data at hand, it’s possible to calculate the financial return and create real-world projections on what our energy costs are.
This also gives us an opportunity to quickly consider battery (energy storage) feasibility based on our current tariff structure and whether it would be workable for us to go ‘off-grid’.
The Loads
My family and I live in a relatively efficient all-electric house with induction cooking and a heat pump hot water system (running on a timer to use solar starting at 9am).
The house has split ACs throughout and double-glazed windows.
We are all very mindful of our carbon footprint and actively work to keep our energy use and associated costs as low as possible. With the help of the Fronius app, we have been able to prioritise using solar power wherever possible, especially to power the largest loads.
The AC works as an efficient heater and has seen more use recently to take the edge off in frosty conditions (usually in the mornings or at night) and the heat pump also works harder in Winter.
For transport, I commute on an electric cargo bike and I estimate that I travel ~20km per day on average. The bike is incredibly efficient, using 10-15Wh of electrical energy per km. Whenever possible, I charge the bike with solar power, but this is harder to do consistently in Melbourne on the shorter winter days as sunlight does not extend much past working hours. With solar costing us 5.4c/kWh our ‘EV’ (or more accurately human-electric hybrid) costs ~$0.00065/km worth of electricity! Our ‘naughtiest’ appliance is a 300W projector that gets a fair bit of evening use.
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Winter Generation
I’m quite proud of the load vs. consumption data from a sunny and a cloudy day last week.
While there was a lot of frost on both those mornings, I worked from home and ran the dishwasher and split system heating in the study (in ugg boots and puffer jacket to save on heating).
You can clearly see how dishwasher and induction heater loads are create a spiky in the consumption profile:
Sunday after the Solstice was a great sunny day outlining peak mid-winter performance (note how even on sunny winter days we’re not getting enough solar generation to cover our daily load):
Looking at June’s monthly summary so far:
Our June 2024 solar production only exceeds the consumption on 2 days so far (with 1 week to go in the month), and a deficit of around 80kWh (~30%). May’s data had only seven days where consumption exceeded generation and an excess in production vs. generation of ~70kWH.
Summer Generation
Summer looks much different, and the system produces maximum output for a number of hours. You can see the ‘clipping’ where the solar is stuck on 5kW because of the inverter’s max rating.
As a designer we’re happy to live with these losses since our loads can’t get anywhere close to them and they only make up a fraction of the total generation through the year.
We’re much more interested in the valuable generation at the margins – at the beginning and end of daylight when there is breakfast, coffee machine etc loads in the morning and usually cooking loads at the end of the day.
Solar energy feels incredibly plentiful in Summer, and the sunny days around the Autumn equinox still produce almost 3 times more energy than mid-Winter ones.
Electrical Tariff & Financial Learnings
We are on a competitive flat energy tariff with Momentum energy in the North of Melbourne CBD. The tariff is ~26.1c/kWh for consumption and 5.4c/kWh feed in tariff that will be going down to 4c/kWh in July.
We don’t pay a demand tariff and have no trickery in place to bid on the NEM/take advantage of wholesale surge pricing etc.
Tallying up self-consumption and exported power gives us savings of $440 so far, ~$830 worth of energy tariff savings projected into the year (after the reduced feed in tariff).
The solar and heat pump combination has been a game changer in terms of low energy costs, our last two bills (April and May) were in the $45-$60 range and we’re expecting ~$500 worth of electricity bills total for the year.
Note, we have no gas connection, so this is incredibly economical, the savings from the solar being worth a principal of $14K under 6% interest.
Return on Investment of a Battery?
Adding a battery to our setup would greatly improve our ‘self-consumption’ - that is, how much of our generated energy we use on site.
The Fronius solar.web website has a tool that lets you simulate this and predicts 90% avg. self-consumption with a 10.2kWh battery bank:
From the half years’ worth of data - reducing the energy drawn from the grid (‘imported’ energy) by 90% equated to an additional $150 worth of savings in energy tariffs along with a reduction in feed in the tariff payment of around-$30. This works out to $250/year.
A battery installation of this size will cost upwards of $12.5K, so based on these numbers, we’re projecting a 50-year simple payback.
6% interest on $12.5K is $750 a year, 3 times the dollar value of the energy we would be saving with batteries.
Under the current tariff scenario, a battery does not make financial sense (not sure that blackout protection is worth it either as we lost power once for an hour or so in the last 6 months).
This is based on our current charges/tariff scenario, and will change as usage, tariffs, ToU export limiting and the advent of solar taxes etc. and we’ll be watching this closely. Ausgrid in NSW recently announced a 1.2c solar feed in charge (the much-feared solar tax), in a state where energy tariffs are routinely in the 50-60c range. This will change the battery economics dramatically.
Without getting into the maths, I’ll be setting my inverter to ‘zero export’ before installing batteries. In my opinion, even with a charge to feed energy back to the grid, it is still significantly more economically viable to leverage solar with zero export than purchase batteries.
How About Going Off-Grid?
As a standalone designer, the next obvious question is, if I disconnect from the grid and suddenly save the connection cost, would that be worth the $1.03/day supply charge saving?
In standalone design – the ‘worst case design month’ gives us the basis for sizing our system and it usually makes sense to allow for some auto-start generator runtime to provide backup power and make up for shortfalls.
Running a loud, polluting diesel generator to make up for any shortfall would not be an option for us in the suburbs. This means we would have to extend the array to reliably generate the required wintertime consumption which would mean adding another ~2.5kW of array/inverter.
We would also need a battery bank with at least 2 days’ worth of backup power ~20kWh and we would also forgo the feed in tariff revenue – these are called ‘spill’ losses due to fully charged batteries in standalone design speak, they would be more than 50% of our total generation in this case, a waste of renewable generation.
Also, this system would not be able to accommodate future growth in energy use – like EV charging, increased demands etc.
A project of this scope would cost upwards of $40K and is unfeasible for a number of reasons. The upfront financial and time/labor costs, scarce resources (batteries use rare earth metals) and energy that would be better utilised elsewhere, such as in EVs, grid scale smarts and storage etc.
By going ‘off-grid’, we would also be reducing how much renewable energy we are feeding into the grid by at least half, which is the opposite of what we need to be doing more of in a climate crisis.
Off-grid power is also notoriously difficult to live with as there is constrained energy availability, issues are common and can result in extended power outages.
These can happen at the worst time as well – for example after a couple of miserable winter days like this one:
Conclusion
I hope this was an interesting insight from a real-world domestic perspective. At Enhar, our passion is enabling renewable energy projects that can stand up to harsh economic realities for our government, business and education clients. We’d love to help take an unbiased look at your project to ensure it makes financial sense as well ensuring it is designed, engineered and installed to stand up to the test of time!
