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Estimate my solar PV system

CHOICE are launching a new Solar estimator tool to improve the experience of finding a good solar installer.

Our simple tool provides ball-park estimates of savings, payback times and solar system installation costs based on where you live and your household size. We have partnered with SolarQuotes so you can request up to three quotes from recommended solar installers in your area.

We’ve partnered with SolarQuotes because of their long-standing reputation vetting and rating solar installers based on over 40,000 customer reviews.

How do you estimate the size of the solar system I need?
The kW size of a solar photovoltaic system is the maximum amount of energy it can deliver. Our estimate of solar system size should meet the typical energy needs of your household size. Our estimate assumes you have enough roof space to install a system of this size.

How do you calculate my savings?
Our estimate of savings has been achieved taking into account the geographical location of your house, your solar system size and average state-based electricity pricing.

We have used SolarQuotes’ Solar Power Calculator using average state electricity pricing from the Australian Energy Market Commission Residential Electricity Price Trends 2019 and solar feed-in-tariffs for each state region based on CHOICE’s Solar Panel Payback Times resource.

Why is the price range of solar systems you provide so wide?
The main components of solar photovoltaic systems are solar panels and inverters and these vary in quality and price. Our pricing is the approximate total cost of a good quality solar system, fully installed using Tier 1 solar panels. This amount includes a ‘point of sale’ discount from the federal solar rebate. It does not include a further rebate that may be available to people living in Victoria. Request a quote for a more accurate estimate of costs of installing a solar system on your home.

How do you calculate the time to pay back the costs of my solar system?
This calculation estimates the period of time it will take you to completely pay off your solar system based on the energy savings you make in your home and the income you earn from feeding excess energy back to the grid.

Our calculation assumes that you have a north-facing rooftop with no tree shading and that you consume 25% of the solar energy you generate.

We have estimated payback times with SolarQuotes’ Solar Power Calculator using average electricity pricing from the Australian Energy Market Commission Residential Electricity Price Trends 2019 and solar feed-in tariffs for each state region based on CHOICE’s Solar panel payback times resource.

Find out how much a solar system might cost you and what your savings might look like.

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Two questions and 14 months after installation every parameter is within the estimated ranges. Well done! As for others, YMMV.

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Same - a little more pessimistic than my actual results but quite respectably close :slight_smile:

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It says in assumptions that it is single storey “tin” roof. Googled definition of tin and didn’t get steel cladding or sheeting come up.

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A great way to quickly answer some important questions about the real value and benefits of Solar PV.

Compared to how our bills have changed since putting in PV, I think we achieve slightly greater savings than the estimate suggests. Possibly because our usage pattern is more biased to daytime consumption.

As we use LNG for HW and all cooking I found the best match came from selecting a smaller number in the household to reflect our actual pre-solar electricity consumption, per our old bills. Perhaps that may be an alternate input option. EG (one person or less than nnn kWh pa, two persons or between nnn kWh and mmm kWh,…).

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“Tin” is just the colloquial industry term for a steel roof which is a thin steel base wsheet which is coated either with galvanising, zinc-alume, or colorbond. Galvinised in the old days was all you could get, and used to last well in drier climates because the older process gave a thick coating on heavy steel. There days the only real choice is Colorbond or Ainc-alume. Colorbond in the lightest colour (surf mist) reflects more heat. You can see the heat performance of various colours if you go to a Colorbond web site and look at the technical information.

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A couple of points to add:

  1. If your roof is big enough, most people with single phase power (just about everyone) will be offered around 6-6.5kW systems by the supplier. For many suppliers, they will not make money on a smaller system, so competitive has the market become. Such a system will be set to allow a maximum 5kW to go to the grid for most electricity retailers but it means you will get more power on days that are cloudy, raining etc and at the beginning and end of the day.

  2. Most installers will quote on a cheap or at most middle of the road 6.5kW system, as they know people buy pretty much on price, as they can’t see the differences. However, if you are good at maths and analysis, you may find that a top of the line system which costs more but has a better warranty will pay back faster than the standard models calculate, and at less risk to the customer. Like a small difference in interest rates can compound over 25 years, so can a small difference in panel performance. That is because they have A. longer and more useful warranties, and B. handle cloudy days, C. high temperatures, D. age and E. hail better than the cheaper ones. Multiply it all out and they can outperform over 10 or more years.

If you go to top of the line, you will be limited in your choice of supplier as many companies won’t stock or quote on them (even though they might put a logo on their website) because they know the bulk of the market buys on price. They will try to talk you out of them but that is really up to you, your needs and how long you are staying in the house. Top of the line is not for everyone but has more advantages than the salesperson will tell you if you do the maths and apply a few probabilities to the the future.

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It has its limits though, a system costing twice as much is unlikely to pay itself off anywhere near as fast as a cheaper, but still reasonable quality system. 10% extra output might sound good, but with twice as many panels for the same price you get a lot more than 10% extra output!

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Very simple estimator for use as a guide.

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I was just having a dig, for a doc aimed at public consumption personally I would say “steel clad” or the like.

Anyway whilst I’m in the evaluation phase of a PV system I put together a spreadsheet to try and represent my circumstances and understand the operating cost outcome.

Came out with 2k/year ahead in operating costs using 5kW inverter/6.6kW panels with spreadsheet, 1.2 - 1.4k/year and 5kW system with estimator tool. We only have electrical energy (except bottled gas for hotplates) and that would skew the numbers from having gas for heating as probably modelled in the estimator tool. Interestingly, I plugged in some numbers for 8kW (basically scaled 5kW numbers up by 8/5 for generation) and it returned no significant improvement. I guess this is because the assumed numbers for potential to be replaced by PV don’t exceed generation.

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Gordon, your view is the typical one that the industry advances. Many experts agree with you. I would agree on the example you gave, except I think you are setting up an example that would not apply in real life to prove your point.

I prefer to let the maths speak, not opinions, and do a real cost/benefit analysis over a long period. I intend to stay for my retirement so used 25 years. Someone else could choose differently. With install being two years ago, I don’t have all my scribblings and spreadsheets from that time, but I worked things out just using known data, with no assumptions, based on 2018 dollars. Of course you can then play with assumptions if you wish but I think it just makes you confused and who knows what the future holds?

In my case I compared two quotes from the one supplier. (I also tried some other mid tier panels from others but don’t want to complicate the story.)

Prices are mid-2018 prices and panels. They were identical quotes as far as mounting, labour, inverter and more. 20 top of the line panels (total 6.54kW) cost an extra $1040 over a 20 good quality well known and well performing mid tier panel system (total 6.1kW). It is hard to get panels exactly comparable in output if you use top panels as they tend to have better outputs through design for the same size panels, but that is a solid bonus right away from day one.

So from day one for 25 years, for my extra $1040 I got:
A. An extra 0.44kW x 20 in ideal conditions, but also extra power performance in cloudy conditions. This in particular has proved very useful as even on the cloudiest days we get enough output to power everything.
B. Degradation over 25 years of minimum 87% versus 83.6% (from warranty)
C. Less reduction in power/voltage and amps for each degree rise in temperature and less reduction in output with low irradiance (e.g.clouds) These differences are a fraction of a percent in some cases but are all one way so when you add them up and compound them over 25 years they make a real difference, and do so even in the shorter run given the initial output is higher.
D. Wind rating of 7500Pa versus 4000Pa: useful if cyclones or storm happen as they are getting more powerful these days.
E. Warranty on product and performance 25/25 versus 12/25. Also any panel failure is replaced free of charge on site on the top of the line panels for 25 years, but labour/travel is extra on the cheaper panels for the latter portion of the warranty.
F. The top panels have different construction. They do not use glued-on front placed aluminium busbars over the top of the silicon like most panels. This improves performance of the silicon and prevents micro-cracks forming which are an unrecognised source of micro-cracks which reduce performance in hail hit panels of almost all brands even though the glass is intact and the panel apparently OK on visual inspection. Most people say panels are seldom affected by hail but no-one actually knows!! It certainly can destroy whole panel systems if the storm is bad enough. While you may say, ‘insurance covers it’, it is a hassle and there can be hidden costs.
G. Identical power tolerance on both panels i.e if panel was rated at 327W, it would be putting out between 327W and 327W + 5% when new.

I cannot easily find the spreadsheet I used but allowing for the various data differences and better change coefficients for low irradiance, temperature rise, and lower time degradation, then these differences although often less than a fraction of a percent per degree meant that the dearer panel differences when added over 25 x 365 days (i.e. added on top of each other per year) quickly overtook the mid-tier panels in cost within a few years assuming current electricity value at replacement cost. If you convert the return into dollars and compound it as an investment rate you will get an even better result.

As I come from a somewhat finance oriented background where the benefits of compounding over a long period are known, I think most people in the industry and those who create calculators fail to take all issues into account on the basis that “it is all pretty much the same so we’ll just round it all off and assume dearer panels are not worth it.”

The fact that you can pay the panels off sooner, have better outright performance over a long period, AND less risk and repair costs, makes the case for top-tier panels much higher than it is stated within the industry because no-one takes the trouble to use real panel figures compounded over 25 years. But to each his own, and every site is a little different… Whoever you want to believe. I am confident enough to believe my own figures and make the decision accordingly.

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Fair enough if that analysis works for you, and that is the important thing, as not all circumstances are the same for everyone :slight_smile:

How does the outcome change if you spend the extra $1040 on panels oriented in way to deliver ~5kW to the inverter all day long (IE 4kW to the west at say 45deg and 4kW to the east at 45deg)? Fronius make an inverter that can take 8kW of panels and output 5kW to the grid, and I know someone with this sort of set-up.

In Australia the average time people stay in a house is just 8 years, rather than the 25+ you (and I, 29 so far!) intend to stay.

Performance warranties are an interesting subject, and sadly most people have no real idea how their systems are performing, as they have no accurate monitoring. Is your installer still going to be around in 25 years? Is Sunpower (I know they make 327W panels) or any other manufacturer?

BTW as an aside, a 327W panel will put out 327W under test lab conditions, not in the real world for much of the time. The panel rating parameters are determined at a cell temp of 25C, 1000W/m^2 and atmospheric absorbance typical of mid US latitudes, 1.5 atmospheres worth. For the majority of Australia, panels always run hotter than that when solar radiation is 1000W/m^2. In summer radiation levels are typically above that in the middle of the day, and I’ve measured up to 1700W/m^2 with some cloud edge effect contributing to the radiation levels.
Certainly lower temperature derating coefficients (lowers voltage, but increases current, for a net power decrease) are an advantage for most in Australia for most of the time. In very cold climates it is a disadvantage.

I think large hail will affect expensive panels to a similar extent to less expensive, but as mentioned earlier, most will not realise it as they do not have sufficient monitoring.

I trust you have monitoring, so that you can pick up any issues well before the next electricity bill arrives. In my experience many do not even twig that there’s an issue then, they just think solar power isn’t very good!

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The estimated usage is no doubt based on people not being home during the day.

Perhaps there might be a variable for the number of people at home during most days, for example stay at home parents with kids, or retired folk etc.

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I agree with all you said.

Yes I have a monitoring system. Where I live there is a fair bit of cloud on sunny days. I notice that although the system is clipped to 5kW export, that with the clouds that a lot of the time it is working very close to 5kW because the graph is very zig zag compared to those in articles. I try to use as much of my power on site so having 6.5 or 8kW is good for example it allows my air con to idle and has enough to heat my hot water. Guilt free living! The $$ earned from the retailer are just a welcome bonus for me.

If the panels are orientated differently (and I have had that on a previous house), then it does not change much. There are so many variables that you have to look at for each situation but assuming no shade and reasonable suitability for solar in terms of shade, latitude etc, the figures can be quite close to due North whether you do 2 x 45º or 2 x 90º. There are various graphs on line (sorry no link) which can show you the difference in output at various angles, and maybe some apps. Again, too long since I set it up to remember what was useful and what not. It can work, but would not where I live now. It all depends.

As far as using expensive panels in any situation, I think it is the same answer for any issue including your assumed ownership of the house. Collect information you can trust, get the panel’s data sheets and any extra background, and if you can do a spreadsheet do a side to side comparison of medium v high priced. Or use pencil and paper. I can’t remember the date my current system paid off the extra cost of buying the top panels but it was surprisingly low like 2-3 years. So the top quality panel decision made itself, even if I was not staying 25 years. But if you have lots of unknown variables like shade at 4pm from trees in summer which will vary the rest of the year then it becomes hard to model.

I think the point is to compare like with like i.e. I know a 327W panel probably won’t be actually 327 and it will depend on the temperature, but if both brands you compare are 0-5% guaranteed on the data sheet, then you can leave this variable out and spend your effort on other differences. The more you play the more you learn what matters. If others do the work using rules of thumb you learn nothing.

Having built a whole house and done a number of things differently to standard based on my own analysis, I’m simply wanting to get across the message that industry rules of thumb are just that. You will often find a better solution if you have the energy & time to look. What drives the retailers 98% of the time despite what they tell you to get the sale, is their own convenience and profit. Most don’t want to carry 2 types of panel because they will sell 95% cheaper one so the other is not worth it to them. The very fact they will sell you the top panel without laughing at you is a good sign. Anyone who laughs at you, walk away.

I have found again and again that industry rules of thumb are there for the convenience and profit of the industry, not the customer, when I take the trouble to work it out. But I realise most people can’t be bothered and want an ‘expert’ to tell them.

Yes, there is no guarantee that a panel supplier or installer will be there in 25 years, but I ended up going with an installer that I believe has the best chance of getting close to that. He has already had panel manufacturers go broke and he has done the right thing. I always try to use a ‘triangulation’ of information to assess people from all angles and it generally works out well. Nothing is certain, except that some people you can tell won’t last. The best way to tell is how they react if things go wrong. In my case I did have a problem which was not their fault (Clipsal part failed) and they were there in a flash even though I live 150km away. And they had a smile. 50%+ of people in building if they create a problem the first thing you hear is an excuse why its not their fault, then you have to argue until you win, then they fix it with bad grace. Sometimes you are left with it. It happens. Not my guys. They rose to the challenge in a difficult unusual install, and also at every step of the way.

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Yes whether you are home is an absolutely crucial point, especially given the difficulty projecting future paybacks using feed in tariffs which change every year.

I think you need to do a bit of scenario creation if you are out all day to see at what feed in’s it takes forever to pay back. Just assume the B’s will always try to screw us!!

But if you are there all day you can obviously shift your usage as much as possible to the day with washing, dishwashers, hot water etc.

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Articles showing those super smooth bell curves are just theoretical, not even close to reality for most of the time. I log at 1 sec intervals and most days there are all sorts of spikes and dips in the output.

Apart from component quality, a really big issue I’ve observed (I’m always looking at PV installations when out and about) is that there are a large number of poor installations that suffer from shading when they really should not. TV antennae can reduce the output of a string of panels by ~20%, similar to vent pipes, and satellite dishes are worse, yet arrays of panels are mounted where they will suffer shading from them, with no attempt to move the offending shading structure.

One of the worst installations I cycle past regularly is at a pub 21km away where they have 3 rows of panels. For 6 months of the year the front row shades the bottom of the 2nd row, up to all day for a period around the solstice, so there would be minimal output from the 2nd row when it is sunny. It would perform better in cloudy weather with the large diffuse light source of the sky. The 3rd row also suffers a bit of shading from the 2nd row in winter. All it would take would be to lay the 1st row back quite a bit, and the 2nd row back a little, and they would produce significantly more output for the half of the year that self-shading impacts the array, and more in summer due to the more face-on solar radiation.
All installers and system designers are supposed to have a decent knowledge of these things, yet there are huge numbers of installations that appear to be done by installers who have no clue. :frowning:

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