Today is the winter solstice. The darkest day of the year. If you’ve got solar on your roof, this is about as bad as it gets.
Which makes it the perfect time to ask: Is your system pulling its weight when you need it most?
Averages Are For Amateurs
It is likely that your existing solar was designed based on average generation. That’s a novice move. Or as my hero Nassim Nicholas Taleb put it:
“Never cross a river that is on average four feet deep.”
Averages hide the bad days and solar is all about the bad days. Design for those, with a bigger system and the rest of the year is a breeze.
The Temptation To Under-Size Your Solar Array
Feed-in tariffs are dropping, falling to just 1-3c per kWh from July 1. Understandably, some solar owners are wondering if it’s even worth having a big system anymore. Why oversize when exports are practically worthless?
And it gets worse: export charges are already here with some local electricity networks (SA Power Networks just joined the party). For now, most retailers absorb the networks’ charges, but that won’t last. Sooner or later, households will likely have to pay for exporting some of their excess solar.
But the answer to lower value exports isn’t smaller solar arrays. It’s smarter solar inverters.
Cover your winter needs with panels, and make sure your inverter can be set to zero export on command – protecting you against future export charges. More importantly, make sure your installer is someone who’ll help you configure that when tariffs change. Spoiler alert: it probably won’t be the one with a retired cricketer in the ads.
What About Batteries?
Batteries should not be sized to minimise solar exports. They should be sized to minimise grid imports overnight. And tonight your home’s overnight load, especially with heating, is likely at its highest.
So measure it. Track your usage from sunset to sunrise. Assuming you’ve already done what you can to address your home’s gaps, glazing and insulation, that’s the number to bear in mind when sizing a battery. And if you don’t have consumption monitoring to look at? Come on. It’s 2025. Get it sorted.1
An example of seasonal estimates of bill changes before and after solar, using our Solar & Battery Calculator.
Common Pushbacks Against Bigger Systems
1. “Mr Frugal on Facebook gets by fine with 6.6kW.”
Great! He’s got the right size system for him, and maybe lives in a super efficient home. That doesn’t mean you should copy him. Audit your usage, not a Facebook stranger’s.
2. “I’ll just use off-peak day tariffs to charge up cheap from the grid.”
Maybe. But those super-cheap day rates often come with sky-high peak rates. You’re giving up energy autonomy for a tariff carrot that can vanish without warning.
3. “I’ll just game Amber.”
Go for it, if you’ve got a giant battery and time to babysit wholesale prices. But if you want stability and sanity, there’s a better way: design your system to work well without needing to game the system. It’s a lot less stressful.
Get Started Now (But No Rush To Get Quotes)
With the battery rebate kicking in, installers are flat out. So you won’t get a system change done before July 1, when most tariffs flip. Instead of rushing, use the next month or two wisely as your solar audit window.
Look at your last 7 days of generation. Shift loads to the daytime. Track your overnight usage.
Ask yourself:
- Have I addressed the gaps, glazing and insulation in my home?
- Have I shifted all my time-agnostic loads to the day?
- Do I have enough panels for winter?
- Do I have the right battery size in mind?
- Can my system adapt when tariffs change?
In Most of Australia, Even Mid-Winter Solar Works
That’s the beauty of it. Even in the depths of winter, the sun still shows up.
So, design for the darkest days. And build yourself a solar setup that earns its keep year round.
Footnotes
- To measure overnight consumption without monitoring, take a meter read at sunset and a meter read at sunrise. The difference is your overnight usage. ↩
About Finn Peacock
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Having a Powerwall 2, and those now being discontinued, I still wrestle with what options we might have if we want to increase battery storage down the track when any remaining stock of Powerwall 2 is gone. For now we don’t really need a second battery, due to a well-sized system, insulated home, and use of low rates overnight and in the middle of the day to top up the battery in winter if needed.
Any tips as to what options Powerwall 2 customers might have for upgrades, when the stock of those runs out?
Yep, my first winter with my solar and battery, so watching things closely with lots of interest.
I never realised the sun dropped so low in the northern sky before I got solar!
Interestingly enough, my generation is a little higher now than a month ago, as the sun is making a bit of an appearance under the canopy of the trees to the north of me for an hour or so late in the day. before that I was losing most of my generation by about 11:30am due to those same trees. Still more than enough to fill the battery by about 10:30am though.
I would like SAPN to take local voltage readings at night to prove solar is the culprit here.
I think our small 2.6kW system should be fine. It behooves those of us still on the SA Premium FiT, to export as much as possible. My highest days are exports of around 17kWh in Summer.
My latest ‘Plan’ has the import price higher than the export price for the first time, so that changes the incentive to export a little.
I disagree.
😉
Batteries should be sized to maximise grid exports overnight.
🏆
Looking at consumption between “sunset” and “sunrise” to size a battery may not be enough.
I’d look at consumption from: late afternoon when the solar system stops producing much (due to e.g. panel direction, shade from trees or neighbours), to mid-morning when solar is producing again.
My solar system didn’t produce much between 4pm yesterday and 9am today.
Up here in FNQ we get plenty of sunshine all year long except for periods of very wet weather.
Today there is steady drizzle at my place and at 11am we need a light on.
My solar output from 12kW of panels is 0.8kW and we are managing to export 0.2kW. If sunny output would be about 9kW at this time of day.
Long periods of heavily overcast weather is another factor in solar output estimates that rarely gets a mention. No way a battery would recharge today.
Hi Finn
I thought if flexible export was set up and it was working right you would not have to pay for exports.
A bit rough calling us early adopters “novice move”. We literally all were novices.
Anyway, I have 2 seperate solar installs and a single Powerwall 2 battery. Total of 8KW Inverters and 9.6KW panels. The small system, a 3KW ZEN (rebranded SMA inverter) was installed in 2012 on my garage at a total cost of $10,000. It was the most I could afford.
The second and larger system is a 5KW SMA inverter with 6.6KW panels in a East/West orientation on my house. Total cost being $6,500 and installed in 2017. It was the largest practical size I could fit on my house roof, the North facing section is unusable due to the presence of a very large brick chimney (non functioning).
Many of us are limited by budget. Some of us are on fixed incomes (retired) and some of us have our homes filled with adult children on the autism spectrum who live on their gaming PC and enjoy online social activities all hours of the night.
Yep a high energy home. It is what it is.