The Switching Question · Part 7 of 7
Free Hours, a Full Battery, and the Number 48
In short
- On 25 October the house drew 49.93 kWh of free electricity in three hours against a 48 kWh battery, because the house kept running while the battery charged.
- Three caps, side by side: 24 kWh a day on the Midday Power Saver, a 24 kWh fair-use test on the OVO plan, 50 kWh a day on ZEROHERO.
- The four questions: what could my system do, what did each electron cost and what is the cap, what share would I capture, which plan suits which season.
- 49.93 kWhfree-window import, 25 October
- 57 kWhthree hours of charging into the battery
- 24 kWhdaily free cap, Midday Power Saver
On 25 October my house took 49.93 kilowatt-hours (kWh) of free electricity in three hours. My battery holds 48. For a long time I thought 48 was the number that decided what a free window was worth. It isn’t. The number that decides it is in the plan’s small print: the cap.
This is one household’s arithmetic: a 19.4 kW solar array and a 48 kWh battery in suburban Melbourne. Your roof, your habits and your numbers will differ. It is personal experience, not advice.
What a free window is worth
A free window is a block of hours in which the retailer charges nothing per kilowatt-hour for the electricity you draw from the grid. With a battery, the obvious use is to fill it inside the window so that the evening runs on stored energy instead of purchased energy.
Four things limit what that is worth. The window has a length, and the battery can only charge so fast: at the rate my battery charges, three hours of charging can put about 57 kWh into it. The plan usually has a cap, a number of kilowatt-hours per day above which the electricity is charged at the ordinary rate. The battery has a size, and on any given morning it has only so much room left. And the house keeps running through the window, with hot water, cooking and heating, and that draw is free too.
My battery is six modules rated at 48.36 kWh in total. I call it 48. The usable figure is somewhat lower, because the inverter keeps a reserve; I have planned around about 43.5 kWh, an estimate rather than a measurement.
Three caps, side by side
The regulated plan. Victorian retailers with 1,000 or more domestic customers have had to offer the Midday Power Saver since 1 October 2026. Its free period is 11am to 2pm, and its fair use cap is 24 kWh per free period at each connection point, which resets daily. Electricity above the cap is charged at no more than the excess rate in the Order’s schedule. It is open to domestic customers with a smart meter (Victorian Government 2026).
I have not been on this plan, so this is arithmetic and not experience. On a day when my battery needed more than 24 kWh, everything past 24 would be charged at the excess rate. Under a 24 kWh cap, my house can’t fill its battery for free. For my house, on that plan, the cap would set the limit, not the battery.
OVO. From June to October 2026 I was on an OVO plan with a free window from 11am to 2pm. Its fact sheet listed no cap, but its Fair Use Policy, in the version dated 3 September 2026 (OVO Energy 2026), treats use routinely above 24 kWh a day in the window as excessive, and the plan can be ended with notice. In practice the plan billed me $0.00 for the free window throughout, including days when I drew 34 kWh and 30 kWh in June and July. On 63 of the 113 days of meter data I have from that plan, my free-window import was above 24 kWh. It averaged 25.5 kWh and peaked at 54.0 kWh, above the size of the battery.
GloBird ZEROHERO. On 20 October 2026 I moved to GloBird’s ZEROHERO plan. Its free window runs from 12pm to 3pm every day, and the fact sheet reads “$0.00 for the first 50kwh/day”; the excess is charged at 27.50 c/kWh (GloBird Energy 2026). A GloBird app screen words the same limit as “50 kWh/day (averaged over your billing cycle, which is typically 28 days)”, which is a different rule from a per-day limit. I model the stricter per-day reading until GloBird tells me in writing which one applies.
Here the cap, 50, sits just above the battery, 48, so I expected the battery to run out of room first. On 25 October it didn’t. The battery was down to 12.7 per cent at 11:58 on a cloudy Sunday after a full export evening, and the free-window import at the meter was 49.93 kWh. In those three hours the battery took in about 44 kWh and the house used about 8.5 kWh, with the panels adding under 4 kWh on a dull day. On the per-day reading of the cap, that day was within a kilowatt-hour of it. If the cap is averaged over the billing cycle, as the app says, the day is well inside it so far: the average over the first six days on the plan was 14.2 kWh a day. That average is kind to me, because the first four days were sunny enough for the roof to fill the battery. The four days before 24 October had no free-window import at all, and 24 October was 35.45 kWh.
So what is 48 for?
The number 48 is the most free energy I can store, not the most I can use. The most I can use in a window is the smaller of two things: the cap, and what three hours of charging can take in. Within that, the battery takes whatever room it has left that morning and the house takes what it runs.
Under a 24 kWh cap the cap is the limit. Under a 50 kWh cap the battery is the bigger limit on most days, but on a dull day with the house running, the cap can bite first. The line that decides what a free window is worth is the one about the cap, and the line after it about how the cap is measured: per day, or averaged over a billing cycle.
The method, in four questions
These are the four questions I now ask of any plan. The cap belongs to the second.
One: ask what your system could do, not what it did. My own bills understated what the panels could produce by something like 40 per cent across a summer, because they recorded a household deliberately holding back its exports. Judge a plan on what you would generate.
Two: cost-price the electron, and read the cap. Every unit of energy you sell cost you something to supply: nothing if it is sunshine going straight to the grid, a few cents of battery wear if it sat in the battery first, more than you will be paid if you bought it, stored it and lost some to round-trip losses. For a free window, add the cap and how it is measured, then compare it with your battery’s spare room and your daytime load.
Three: measure your capture fraction. A plan you have to operate, chasing wholesale prices, has a high ceiling, but you bank only the share your forecasting and automation actually catch. In my modelling the operated plan got ahead of a simple set-and-forget plan once it caught about 60 per cent, and below that the simple plan quietly won.
Four: respect the seasons. This house is two businesses sharing one meter, a generator from roughly October to April and an ordinary customer from May to September, and the plan that suits one half is the wrong plan for the other.
Answer those four for your own roof, your own battery and your own habits, and the plan choice mostly makes itself. My answer is built around a 19.4 kW array, a 48 kWh battery and a willingness to let the house act as a small power station for half the year. Yours is built around your numbers. On any free-window plan, the first thing I would check is the cap line, and then I would compare it with the room in your battery and what your house runs at midday.
References
GloBird Energy (2026) ZEROHERO Residential (Flexible Rate), AusNet, energy fact sheet, release 1 July 2026. Accessed 17 October 2026. App wording quoted from a screen of the GloBird app, “How ZEROHERO Works”, transcribed 19 October 2026.
OVO Energy (2026) Fair Use Policy, updated 3 September 2026. https://www.ovoenergy.com.au/terms/fair-use/. Accessed 15 October 2026. Plan terms from the OVO energy fact sheet for the plan, release 15 April 2026, https://www.ovoenergy.com.au/vefs/Victorian_Energy_Fact_Sheet_OVO731736MR_Electricity_CZ_6.pdf, accessed 15 October 2026.
Victorian Government (2026) Midday Power Saver Pricing Order 2026, Victoria Government Gazette No. S 498, 8 September 2026, cl. 2(2), 7(1)(a), 12 and Schedules 1 and 2. https://www.gazette.vic.gov.au/gazette/Gazettes2026/GG2026S498.pdf. Accessed 15 October 2026.