Guest post by Josh, Founder of Grid Getter
The One Bad Half-Hour That Sets Your Whole Month’s Bill
There’s a fee on some Australian power bills that has nothing to do with how much electricity you used. It’s based on how much you used at once, during one specific window, on your single worst day of the month. Get that one moment wrong and you’re paying for it for the next thirty days, no matter how careful you were on the other twenty-nine.
It’s called a demand charge, and if you’re charging an EV at home, it’s worth twenty minutes of your time.
What a demand charge actually measures
Most of your bill is usage-based: kilowatt-hours, the total amount of electricity you drew over the month. The demand charge is different. It’s measured in kilowatts, the rate at which you drew power at your single busiest moment, not the total.
Here’s the mechanic. Your smart meter records consumption in short intervals, usually every 15 or 30 minutes. Your network defines a “peak window,” typically somewhere in the late afternoon through evening, when the local grid is under the most strain. Whichever interval inside that window had your highest average draw becomes your demand for the month. Multiply that peak kW figure by a rate, and that’s a fixed charge added to your bill, on top of everything you already pay for usage.
The part that catches people out: it doesn’t matter if that spike happened once. A single half-hour where the oven, the aircon and an EV charger were all running at the same time sets the number for the entire billing period. The other twenty-nine days are irrelevant.
What this looks like in practice
Every network runs it slightly differently, which is part of why so few people understand it. Two real examples:
In South-East Queensland, Energex’s residential demand tariff applies to the window between 4pm and 9pm, every day including weekends. It charges $7.00 per kW per month, based on your highest half-hourly reading in that window. Ergon Energy, which covers regional Queensland, runs a near-identical structure over the same hours at roughly $8.11 per kW per month.
In the ACT, Evoenergy’s demand tariff is priced differently again: 16.6 cents per kW, per day, over a narrower 5pm to 8pm window. It sounds small until you do the maths. A household with an 8kW peak pays 16.6 cents × 8 × 31 days, close to $41 in demand charges alone for a single month, on top of normal usage. The ACT has also made this tariff (or time-of-use) mandatory for new solar installs; there’s no flat-rate option left to fall back on.
Not every state does this at all. South Australia doesn’t apply a kW-based demand charge to residential customers; instead it prices the evening peak per kilowatt-hour through its Time-of-Use and “Electrify” plans. If you’re in SA, the advice below still applies, just aimed at a per-kWh cost instead of a fixed monthly penalty.
Worth five minutes on your own bill: search “[your network name] demand tariff,” or look for a line item priced in dollars per kW rather than per kWh. If you want to see the maths play out against your own numbers rather than someone else’s, Grid Getter’s savings calculator models a peak window, a demand charge and a battery strategy side by side.
Why EV charging trips it so easily
Most appliances that spike your usage do it briefly. A kettle, a toaster, even a washing machine cycl, they draw hard for a few minutes and then drop off. An EV charging at home doesn’t behave like that. It pulls several kilowatts, continuously, for hours.
And the moment most people plug in is exactly the moment their network defines as peak: arriving home from work, somewhere between four and seven in the evening. That’s not a coincidence. Peak windows are set around that exact behaviour, because that’s when everyone else is home too, cooking dinner and running the aircon. Stack an EV charge on top of an ordinary weeknight and you don’t need a bad month to trigger the charge. You need one evening.
Can a battery just fix it?
In principle, yes. If your battery can supply the entire load of your house, EV included, for the whole peak window, the grid never sees a spike and the demand charge never triggers.
In practice, that’s a bigger ask than it sounds. Most home batteries are sized for backup power and shifting solar into the evening, a handful of kilowatt-hours of storage and an output built for typical household loads. Add a car charging at several kilowatts on top of dinner, and you’re often asking the battery to sustain more continuous output, for longer, than it was ever specced for.
A battery with no coordination doesn’t know any of this, either. Left to its own logic it might discharge freely on a quiet Tuesday and have little left on the one evening that actually decides your bill.
Where automated demand management comes in
This is the gap that software like DemandGuard, from Grid Getter, is built to close. Instead of relying on the battery to guess, it watches your grid draw continuously, every 60 seconds, aligned to your own utility’s billing interval. When usage approaches a target you’ve set, usually 10 to 20 percent below the worst peak on your last few bills, it pulls from the Powerwall automatically, before the spike registers. Grid draw stays flat, and the battery does its work exactly when it matters instead of whenever it happens to feel like it.
Owning a battery and owning one that actually prevents demand charges turn out to be two different things. The second one needs something to watch the clock.
A few minutes worth spending
Check your bill for a dollar-per-kW line item. Find your network’s peak window; it’s usually published on their website under “tariffs.” If you’re on a demand tariff and charge an EV at home, try shifting the start time to after the window closes, or look at whether your battery setup, if you have one, is actually coordinated to catch the spike rather than just along for the ride.