Solar and batteries are great. But they’re not a get-out-of-jail card for bad houses.
Today I saw someone ask online:
“Should I spend $20k on double glazing or $20k on solar + battery and run the air-con 24/7 when necessary?”
If you even have to ask, the answer is simple: don’t build a crap house and try to plaster over it with panels and a battery.
Build the Cake Before You Add the Icing
If your house leaks heat in winter and cooks in summer, solar and batteries won’t save you. You’ll run the air-con flat out, chew through energy, and still feel lousy.
Build it right – insulation, airtightness, double glazing, orientation – and suddenly you don’t need so much cooling or heating. You also don’t need a monster air conditioner or a giant battery. A smaller kit means lower bills and less stuff to break.
And insulation and windows don’t stop working if a capacitor blows. They don’t need an app or a switch or any mental bandwidth. They just keep performing, day after day, year after year.
Houses should last a century. Yet we build them like throwaway shells and hope a solar battery system will cover the cracks.
This Is About Health Too
Living in homes that regularly drop below 15 °C or climb above 28 °C is linked with worse health and wellbeing. And it’s miserable.
You may be able to keep the temperature stable with bags of air conditioning, but most people are not going to run it 24/7. Even with a giant solar battery system, frugality and practicality will leave the house too hot or too cold much of the time.
A house that effortlessly stays warm in winter and cool in summer keeps you healthier and happier. And it’s not only about you: when you build, you’re also shaping the lives of the people who’ll own the place after you. Old-fashioned, maybe, but I reckon you have a duty to pass on a house that’s good for their health and happiness, too.
Solar and Batteries Still Shine – Later
Once the house is right, solar and batteries are brilliant. Pre-heat or pre-cool during the day, ride out peak prices at night, shrug off blackouts.
But don’t get lazy. If you’ve only got the money for one or the other, spend it on the house. Solar and batteries are easy to add later, often with no upfront cost and payments cheaper than regular electricity bills.
Phase Shift is a weekly opinion column by SolarQuotes founder Finn Peacock. Subscribe to SolarQuotes’ free newsletter to get it emailed to your inbox each week along with our other home electrification coverage.
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I disagree.
Our 1960s bungalow has plenty of affordable energy efficiency upgrades like ceiling insulation, external shading and thick curtains and only needs mechanical air conditioning maybe 20 days per year.
We actively manage airflow. Our smallish solar arrays and smallish battery see us about 8000% net positive over a year according to my whole house meter.
Zero need for Passive Haus standard in Temperate Adelaide. Double Glazing is still an expensive option in Australia. If building new, design for minimal glass and ensure correct orientation for solar gain.
I think you are correct.
The “double-glazing” example is a red herring. A more realistic question is whether you should spend $5000 on roof space insulation before you buy a $10,000 solar system. Of course the answer is get the insulation first.
In most of Australia, single glazing is adequate. Insulating the roof space is easy, and the single best way to improve energy efficiency. I think it can decrease energy to heat and cool a space up to 40%.
We have one window facing east and the other west in our lounge room. I shut those widows in the morning and pull the blinds if the forecast high is over 25. Even on days where it reaches 33 that room stays below 25.
Most of us are stuck with what we have. As you state, building from scratch (and renovating) are opportunities to properly orient glass. Wall insulation is useful, but difficult to retrofit.
In most of Australia, window orientation, insulation and thermal mass will do 90% of the job…
Yes, our first job as new home owners around 1981 was insulating the ceiling.
Strong memories of stinking hot work even at night and poking batts into the far ends with poles with nails in them.
I think total cost was $500. The house only cost $31k
The place was an energy efficiency disaster but most changes were relatively affordable.
But you get to a point of diminishing returns for further improvements, where solar and now storage can offer better value.
Hi Rod,
It still blows my mind that so many houses were built back in the day with zero insulation.
When wall insulation was introduced there were some dodgey pricks going around sticking a piece behind the power points.
If anyone pulled a wall plate off to check, there was fluff there behind they power point… but nothing in the rest of the wall!
I insulated the walls when we built in 1976, did the work myself, the salesman that sold me the R1.5 batts thought I was crazy. Did the roof later.
Hi Finn,
I totally agree.
This is a very good piece of advice.
I wonder why it is not enshrined in Australian standards?
Who gets to benefit from building cheap, leaky houses and buying expensive air conditioners to run?
Probably there is no single answer and there is a combination of tradition, builders’ reluctance, politicians’ incompetence, peoples’ ignorance, etc.
Nah I’m going to disagree with you for once Finn. Solar first! Why?
1. It’s easy.
2. It’s got the best payback time of any improvement.
3. Someone else does it, leaving you free to get on with job #2.
You’re 100% right with the icing vs cake analogy but if time, money and mental energy are limited (and who doesn’t have these problems!?!) then just whack a decent solar system on your roof, with our without batteries, and get get cracking on sealing those cracks. 😁
Double glazing is new enough in Australia that we old folk take a while to realize how good it is, even in our climate, milder than Europe. When I owner-built a few years ago, it was the 6-stars (now 7-stars) regulations which compelled me to install double glazing, as well as all the insulation that I had my head around. (My old-fashioned perspective was to just rely on lots of solar, and the wood heater’s cozy warmth in really cold weather.)
But, Ooooh, it’s niiice to just flick on the RCAC on a cold night, using a mere 0.7 kW for toasty warmth, instead of setting, lighting, and stoking the fire – then waiting an hour or two for it to bring the temperature up.
I grumped about the interfering regulations, while building, but I’ll forgive them now, given the extra comfort. And the resulting surplus solar is going into the BEV now, 25 kWh needed to bring it back up to 100%. (Should be done just after lunch, as it’s sunny today.
Double glazing also cuts nuisance noise – very nice.
Actually the real issue seems to be the experts who can’t or won’t explain whether double glazing, or any of the other options, are worth buying over regular glass. You can spend more to get double glazing sure, but if there’s no real benefit – according to the experts, then why bother wasting the money?
Old folk can ask questions, but if the experts won’t support the idea, then they won’t see the point of investing in it.
Yes you can try to do your own research, but if you don’t come across clear evidence of why X is better than Y for your climate, only evidence that X is far more expensive than Y, there’s no point to spend the money.
John Alba: – “You can spend more to get double glazing sure, but if there’s no real benefit – according to the experts, then why bother wasting the money?”
Where are you getting your information from, John?
Heat transfer through double glazing is DEMONSTRABLY slower compared with only single glazing, per the simple demo in the video at: https://www.youtube.com/watch?v=ygUwWiml7Qg
Double glazing reduces recurring energy bills, noise pollution, and condensation, and can increase property value, but has a higher upfront one-off cost.
https://selectwindows.com.au/double-glazing-myths-debunked-and-why-its-essential-for-every-australian-home/
I should probably qualify that experts – from memory, the builder, the glass seller, and the glass manufacturer.
Hi John,
You have to remember that the builders realised years ago that windows are cheaper than bricks, lintels, insulation & framing, so they love to spruik large floor to ceiling windows as a benefit…
John Alba: – “I should probably qualify that experts – from memory, the builder, the glass seller, and the glass manufacturer.”
How long ago are you talking about, John? Multi-decades ago?
Double glazing in Australia still remains a niche market compared to other developed countries, though its popularity is steadily increasing due to rising energy costs, new building codes (minimum 7-star energy rating), & growing awareness of its benefits.
Why is double glazing still considered niche?
• High initial cost;
• Past reliance on lower cost energy to solve heat & cold issues instead of improving a home’s thermal efficiency;
• Perception as an optional upgrade rather than as a standard feature in new builds;
• Lack of regulation in Australia for mandatory double glazing in new builds, unlike in many other countries;
• Until recently, a smaller market means fewer local manufacturers & lower economies of scale, which keeps product prices high.
John,
There is a ton of information showing the real benefits of double glazing.
Even 10 years ago when I started my research followed by windows retrofit, there was plenty of information.
I challenge anyone to show me, based on research and not hearsay, that in Australia there is no need of double glazing.
My solution was to use uPVC metal reinforced frames, with tinted outer glass (for summer, sun) lowE inner glass (for winter internal heating retention) and 18mm gap filled with argon.
The windows have 2 types of rubber seals, are installed with expanding foam around the frames, so there no unwanted air movement inside or outside the frames.
This is the maximum thermal comfort, but there is huge noise reduction from outside.
I give my windows 10/10 and my older sliding aluminium single pane windows, 2/10 (where 0/10 is no windows at all).
Thanks you all so far for the perspective, and I am in two minds regarding installing double glazing (“DG”).
We (Canberra) have two windows that get precious winter sun, one East and one West. So in winter they do a fabulous job of heating the living areas of the house. But if I double glaze these windows, what happens to the solar gain? No search of the web or books wil give me a straight answer.
On the other hand, we have a kitchen “garden” window that faces south. There was always a draught near that window in winter, so as a cheap fix I used thick flexible olastic that comes on a roll and is used as a plastic tablecloth for outdoors meals.
I biult a frame out of flyscreen aluminium framing that fitted exactly in the garden window frame. Sealed around the edge with the foam strip used to seal doors. It’s very easy to fit and remove seasonally, and works fairly well. But as a sound deadener, it’s amazing. Noisy traffic to inaudible (almost). I think the flexible plastic is the key.
Jeff,
Use clear glass for both panes of glasss for north window. The addition of the second pane of glass has minimal effect on light transmission, so you will continue to have precious sun in winter.
However if you want to retain that precious internal heat in winter, as much as possible, use for the internal pane of glass, low E glass. It contains metal oxide which helps to bounce back into the room the heat trying to escape out.
The first layer of glass is MUCH more effective than each subsequent layer. The usefulness of subsequent layers depends on several variables…
A house that climbs above 28 is a poor house?
I ‘ve never seen a house that didn’t in Qld. A usual week of summer days between 35 and 40 will take any house over that. Insulation only delays heat transfer, it doesn’t stop it .
I do not think that is what Finn meant. He was talking about comfort and health, not solar performance. That said, thermal performance of the average Australian house leaves much to be desired. You only need to delay heat transfer until a few hours after sunset.
Insulation, or avoiding leaks, is mostly about delaying heat exchange. It will never absolutely prevent a house from dropping to single digits, or hitting 30°C+, well maybe unless you live somewhere like Coober Pedy.
Wide eaves delay when the sun will hit your house, walls over glass offer better insulation possibilities, but at the cost of views. Tinting helps reduce glare, heat, and UV to varying degrees, also helps with privacy, while external shutters can be dropped in more extreme weather to act as another layer of insulation. Internal blinds, drapes, shutters etc do likewise.
Once all the build options are exhausted however, you’re left with the likes of fireplaces, heaters, fans, and AC.
I have a mate who built his second house spending vast sums on insulation, double glaze and passive house standards in south Australia, and ended up coming to the conclusion that he still need to invest in heating and cooling,…ultimately all the extra expense in the build cost only limited those costs.
Clearly air tightness and small glass areas and ceiling insulation are very worthwhile for the costs ( or limiting the build costs.) but then put your money into big solar and electrify the home as you say.
Your mate doesn’t understand the definition of a Passivhaus and the fact that Passivhaus and passive solar are not the same thing.
A Passivhaus will usually require some form of forced heating or cooling…even if you incorporate aspects of passive solar into the design…eg thermal mass, shading etc. The latter are not part of Passivhaus design.
My Passivhaus in The Adelaide Hills requires heating during the winter….accomplished using thermal mass plus two 7kW reverse cycle air conditioning units. No forced cooling during summer months is required due to effectiveness of two internal rammed earth walls combined with the HRV ventilation system (night time purge mode removes heat built up during day within RE walls).
We bought a 1987 brick and tile house that was horribly leaky, sun exposed with mega temps topping 38 inside 40 deg days to avg 9 to 16 deg in winter. Over 6 years of constant graft as a DIY dude have spent + $120k (all on wkds/hols) fixing it. The improvements include: extensive seasonal shade sails on E/W/N sides. Big patio with louvres to be able to shade or catch sun, painted the roof most reflective white coating, Cat 5 insulation, bubble wrap on windows. This was serious graft and the family life suffered heaps. Also budget was a major strain ontop of everything else. Next week we do the final stage of double glazing most windows. In my opinion it is possible to get most of the way there/have insig power bills by spending <1/3rd as much and hardly anytime/graft. Put in a 48Kw sigenergy or similar battery and totally max out panels. We went for 24kw of panels and 15kw invertor. With one step have your cake and eat it. All the stuff we did is unaffordable to most.
Couldn’t agree more – we spent $12k on 13kW of panels and a 15kW/42kWh inverter/battery package and it just blows away the insulation and leakage problems in our house, along with reducing our bill down to the daily supply charge and providing 63A of backup capacity to make our unreliable power supply a thing of the past.
Hmm that seems like an extraordinary price for that system. Did you use solar quotes? How many quotes did you get. What State are you in? Who did you end up getting the system from? Thanks in advance for any reply.
I’m in NSW, I got a couple of quotes that were priced similarly but with different brands like Neovolt and ESY that were short on inverter capacity. My chosen installer was Aussie Solar Batteries.
Hi Beau,
Be sure to leave a review. With such a new company we’ll be interested to know how things go in the long term.
https://www.solarquotes.com.au/installer-review/aussie-solar-batteries/
It seems nobody studies the solar sine law at the solar angle. I disagree with 13 kW panels and a 15 kW / 42 kWh inverter/battery pack. 13 kW solar PV power? Endless mistake! You would need twice the grid-connected solar energy and four times the off-grid solar energy to operate for up to 17 hours.
I failed to mention that I already had a pre-existing 13kW PV system, so 26kW in total. Secondly, even 26kW of panels isn’t enough on cloudy days, so I simply charge from grid during my free tariff period between 11AM-2PM. I haven’t imported any energy other than a little bit of background draw when the inverter doesn’t match up the import/export bias perfectly.
Remember, I’m connected to the grid, so I have access to everybody else’s PV systems too!
For anyone interested bang for back energy improvements in my opinion for a free standing leaky 1987 brick and tile home in Perth are:
1 bubble wrap on windows 3 degrees straight away
2 paint roof high reflective white easily 5 degree cooler in summer and keep cooler longer for less AC use
3 insulation – thicker the better gave 15% improvement in energy use changing from 1987 batts to cat 5 batts
4 shade sails everywhere easily another 3 to 5 deg in summer plus usable outdoors and remove each winter for solar gain
5 go big on panels and large stack of batteries so you can wave good by to bills and keep warm cool anytime you like
6 double glazing will cost as much as 1 to 4 plus a bit of 5. The big advantage beside energy efficiency is not needing to draw curtains in peak and trough temps. Natural light can really improve liveability.
In my opinion if not handy and chasing a quick fix doing 5 could be enough for most and has a very short timeframe to implement/start paying off.
A double glazing retrofit can be very hard in these economically straitened times. If I were still in my nearly 40 yo single glazed build at 600 ft altitude in The Dandenongs, I’d install honeycomb blinds – perhaps the translucent variant for daytime use too. Fitted between the window reveals, and sliding up or down, they provide substantial insulation without double glazing. There’s an advertisement or two in Renew magazine, most issues. (Including the one in newsagents now.)
Even that house had ceiling and wall insulation, last century. The benefits accumulate.
With no aircon, I was glad that I had designed it upside down; bedroom downstairs, living upstairs – much easier to sleep in the naturally 5°C cooler downstairs, tucked in against the hillside.
Sadly, solar wasn’t an option there, nestled deep in a tall stand of Messmates, shading everything year-round. So high tech isn’t always a quick fix. Congealed sunlight, firewood, 10 tonnes annually, was a green fix, though.
Hi Erik, i have used honeycomb blinds before and they definitely work well. It was in our previous much smaller and better built brick and tile house. We had semi transluscent ones which had 4 layers giving 3 air gaps. The glass we had was typical WA 3mm float and sliding windows so low efficiency. In summer they are great but you are effectively eliminating the windows. In winter every morning i had to raise them and use a sqidgy plus big sponge/bucket to remove the condensation build up. I usually harvested about 400ml per morning this way in order to eliminate likely development of mould. Cost was about $800 to $1000 per window. By comparison modern +6mm laminated eglass replacement was about $350 to $450 per window. We had several wand type blind mechanisms fail within 3 years and had to replace them unfortunately. If getting thes i recommend getting the cheaper and less complicated blind mech as there are reliability issues with the wand ones.
On the honeycomb blinds, I went for the “top down, bottom up” style. Again, reveal fit in the bedrooms.
One bedroom faces East up a street, so before the blinds we had to close a roller blind when dressing etc.
Now it is SO different! Lower the top rail to get sun and privacy!
Why are you still grid connected Beau? Guessing it costs a bomb to disconnect?
Rod,
You are a lucky one to live in a 1960 bungalow so well insulated that doesn’t need air conditioning more than 20 days per year, in your temperate Adelaide. Maybe you use a space heater in the cold winter nights and forgot about it.
Also it’s amazing your 8000% net positive, maybe you can share your secret sauce.
As about glass area and house orientation, there are strict regulations, you can’t just choose to use airplane size windows and orient the new house at any angle to your street.
There are a ton of studies and decades of experience in passive housing contradicting your personal disagreement.
Heatpump AC living end. No need to cool the rest.
Misters on it for Summer.
Trombe wall and bay window on the North
LED lights. DC fans.
Solar HWS
Zero southern glass.
NatHERS is not very strict and doesn’t mention orientation.
Imported 98.8kWh last year
Exported 3524 kWh
Import Export Annual HWS
2005 1039.7 1486.2 811.1
2006 1099.1 1421.8 1486.7
2007 1777.3 1238.2 650.2
2008 1739.0 1989.1 725.2
2009 1632.8 3085.4 757.0
2010 1580.6 2970.3 674.3
2011 1244.9 3065.9 532.3
2012 693.4 3479.1 394.4
2013 683.5 3480.6 413.8
2014 640.7 3518.2 408.5
2015 723.0 3328.0 666.3
2016 722.9 3225.8 442.7
2017 800.9 3235.5 472.1
2018 672.1 3267.5 198.2
2019 716.5 3296.0 0
2020 643.5 3212.1 0
2021 545.7 3232.2 9.1
2022 585.0 2956.9 145
2023 95.2 3523.8 39.1
2024 98.8 3646.8 39.6
Hard disagree on this one. Building good new homes is one thing I agree with (good luck with that!), but a comprehensive building retrofit can be literally an order of magnitude more expensive than a good sized solar and battery package and still not provide the same comfort and energy cost reductions that renewable energy tech can.
I already know the outcome of the building retrofit vs solar/battery choice that will be made by millions of Australians – the cost-effective one that provides superior material comfort will win.
And people forget that solar gives great shade.
Fill your roof and feel the coolth
Yep, and my goodness is my roof filled – 26kW of 440W Jinko goodness up there gives me the warm and fuzzies!
Beau,
There’s no need for luck to ensure good thermal performance in new homes. The regulatory requirement for “7 stars” means ample wall and ceiling insulation, double glazing, etc. are now unavoidable requirements. Even half a decade ago, when I began an owner-build, the then 6 stars requirements compelled the same.
At the time, I thought the regulations intrusive, but now, living in the greater comfort and easier heating, the benefits are clear. (It was down to 7.6°C yesterday evening, even here in late Spring, yet 0.7 kW of battery powered RCAC gave 25°C indoors, avoiding the need to light the wood heater.)
Hi Erik
That’s great to hear, seriously. Things have come a long since we built our house in 2012 – we have R1.5 insulation in the walls, R3.0 in the ceiling and single glazed (single pane? is that the correct term?) windows. I put another layer of insulation in the roof recently and that mildly reduced heat loss during the winter.
Aside from that the builder installed a solar thermal HWS to meet BASIX requirements – what a piece of junk that was. The pump assembly failed just outside of warranty at 5 years, so it became a regular resistive element HWS powered by coal after that. Ripped the whole thing out in 2023 to fit PV panels and a new 400L resistive HWS powered off a timer and PV.
Your setup sounds great by the way.
Bill and Rod,
The windows in Australia are not adequate.
First is the aluminium frame, secondly is the gap at the bottom where the rollers are and thirdly is the single glazing.
Only someone who hasn’t experienced uPVC frame, double glazing, tinted glass on the outside, lowE glass on the inside, can talk against it.
Yes, it’s expensive, but it’s worth it on the long run.
Insulating the attic doesn’t cost $5000, it is much cheaper, unless you live in a mansion.
You cannot re-orient the windows and the house and cannot add thermal mass.
Wall insulation retrofit is harder but not impossible.
Buying solar and battery should come last.
I am currently importing less than 100kWh per year with very little spent on energy efficiency upgrades.
Spending more money to save next to nothing makes no sense.
It will be different for every house. When we bought (a 15yr old place), we got an Energy Assessor in before we actually moved in.
Our order ended up being:
– Replace downlights with LED IC4 rated ones
– Underfloor and roof insulation
– Replace single sheet perspex skylights with Velux double glazed with honeycomb blinds
– Solar
– Heat Pump & Induction (got off gas at this point)
– Secondary glazing on upstairs windows
– Double glazing for main living sliding door & window
– Honeycomb blinds for main area
– Secondary glazing for downstairs bedroom & study
We survived a summer without this and upstairs got to 40C on regular occasions, now just leave the AC on 21 all day and back off to 25 when the sun goes down and still have zero summer electricity bills.
I got a quote for adding secondary glazing to my 1970’s home.
Main bedroom window $4,400
Bed 1 $1,30
Bed 2 $3,479
Kitchen x2 windows $2,557
toilet $410
Bath $410.
Total cost $12,387.
This did not cover the 2 patio doors off the lounge as they were so expensive they were not worth quoting on and doors being old.
Whilst i have insulated the roof, walls would still leak as well as floors.
I went with the Sigenergy 40kWh battery for $19k instead.
I agree Finn.
Based on my personal experience having lived in 5 homes, the types of thermal and energy related improvements that can be installed depend on where in the house acquisition process that you are.
1. Firstly, if starting from scratch, location, location, location is the first step. Choose a block of land, house design or existing house with orientation and features that maximise thermal benefits eg building orientation
2. Secondly, for a new house, include in the build all the improvements that cannot be easily installed later, for financial or practicability reasons eg wall insulation
3. Thirdly, if not already installed previously, install and fit all items that are more easily retrofitted to an existing house, such as roof insulation
4. Install solar panels to maximise power generated and used by the property
5. Then if justified install a battery, perhaps with point 4
In our current house, we did points 1 to 4 and now power bills are so low a battery is unnecessary.
Thanks Ian, great perspective.
Thanks Finn – excellent topic.
We too have lived in >10 houses, in cold & hot places, forested & barren. We did a 3 month house exchange to north Finland in winter – & learned lots abt life, building, economy. Biggest thing – manage the water vapour in walls & roof.
Location & build standards matter HUGELY. Small is good.
2 yrs ago we got a block at 950m elevation in NSW Central West. First built a Colorbond shed with a lean-to, as temporary accomm. No skylights & 2 single glaze windows in lean-to. Put 10 mm closed-cell foam blanket (reflective) under all roof & wall tin, & brushes over roller doors. Great result.
In May we had a small (140 m2) 3 Br modular home clad in dark Colorbond put on stumps. Double glazed, minimal glass to south, & worked with builder to boost wall and ceiling insulation. Superfloor system (fibro-32 mm polystyrene-fibro laminate) a gamechanger. House remarkably temp-stable.
Abt to do solar, battery likely later.
Still amazes me that many modern houses dont have eaves, probably the most cost effective year round addition any home can have. Glad who ever built my late 70s house had the foresight, insulated roof as well, not to mention north facing with large roof for solar panels.
A quick AI check confirms my recollection that NatHERS considers eaves. It suggests that up to 800 mm eaves are good up north, but they’re of less use down here in Victoria and Tasmania. I don’t recall how they figured numerically when I built 5 years ago, but a house without them is frightfully ugly in my eyes. Window awnings might ameliorate the summer solar gain problem, but not the appearance.
Here I also added a narrow solar-designed porch over the north-facing windows, to cut off direct sun in Spring – some weeks back, as the shadow is half a metre below the windowsill now.
Hi Finn here is my experence, my house is brick and tile 48 years old windows all facing east, west and south. I moved here in 1999 and the house was very cold in winter, electric hot water in the ceiling and gas ducted heating.
first I fixed the roof insulation, next I fixed the drafts etc, I then moved onto the electric hot water, replaced hot water with gas all done in the first 12 months.
Next I replaced the light globes with led iights, my electric usage kept going down from about 7kwh per day to 5kwh. I then manafed to get some insulation batts into the cavity wall by moving back the tiles on the roof and pushing batts down to the noggings.
The living room windows need to be replaced so it was double glazing no dramitic changes but small improvements on everything i had done.
In the last 2 years I have changed to air pump hot water air conditioner for heating and cooling when required, this year I installed a 6.1 kw solar pannel and 5 kwh battery, this give me free hot water
Depending on how many people in your household, and the size of your hot water tank, you can have free hot water just with the solar system.
Just takes the ability to switch the HWS off and on depending on solar output.
It helps if your power company has monitoring that you can log in to.
My power company has this facitity, but it is for the previous day. But it does show NETT power used per hour of the day. So I can easily see what I did yesterday (or any previous day) and the effect is has on my consumption.