Own The PeakEnergy, from the household side
Search
Menu

Free-window calculator

Given your battery, charge rate and a free window, with or without a daily cap, how full does the battery get, what stops it, and how much of the evening does it cover?

Written 26 Oct 2026Last checked 26 Oct 2026Applies to plan shapes from fact sheets as at 15 Oct 2026Stage Working

What this does and does not do

What this does not do

  • Price anything. There are no rates and no dollar figures here.
  • Compare plans or say which to choose. Plans change; check the current fact sheet.

Everything runs in your browser. Nothing you enter is sent or stored. General information about one household's sums, not financial advice.

Three plan shapes as examples

Pick one to fill in its window and cap. The plan terms are from (OVO Energy 2026), (GloBird Energy 2026) and (Victorian Government Gazette 2026).

This applies ZEROHERO's 50 kWh per day. GloBird's web pages also describe it as a billing-cycle average.

Your numbers

This house charges at about 20 kW. Yours depends on your inverter and wiring.

Enter it net of any solar. It uses up the cap but is not stored.

The result

  • –kWh banked in the window
  • –full at window end
  • –evening hours covered

Battery fill through the window
Evening draw-down of the stored energy

Assumptions behind this result

    Your settings are in the address, so you can bookmark or share this case.

    How this is calculated

    Let the usable capacity be C, the starting charge E0 = C × SOC, the charge rate P, the house load h, the window length L hours and the daily cap K (if any).

    1. Time to fill at the charge rate: tfull = (C − E0) / P.
    2. Time until the cap is used up: tcap = K / (P + h). While charging, grid import is the charge rate plus the house load, and the cap counts all of it.
    3. Charging runs for t = the smallest of L, tfull and tcap.
    4. Energy banked = P × t. Stored at window end = E0 + P × t. Percentage full = stored / C.
    5. Evening hours covered = the smaller of the evening hours and stored energy / evening draw.

    Worked case with the Midday Power Saver style preset, 43.5 kWh, 10% start, 20 kW, 0.5 kW house load: tcap = 24 / 20.5 ≈ 1.17 h, shorter than tfull (39.15 / 20 ≈ 1.96 h) and the 3 h window, so the cap binds. Banked ≈ 23.4 kWh. Stored ≈ 4.35 + 23.4 = 27.8 kWh, 64% of 43.5 kWh.

    What one house's free window actually looked like

    On OVO's Basic Free 3, with no daily cap, this is what the house took.

    Free-window charge shapes020406011:0012:0013:0014:00dashed: a steady 20 kW54 kWhmedian 26JunJulAugSepmedian daykWh bought from the grid, cumulative, 11:00 to14:00, one line per day.
    Each line is one day's grid import between 11:00 and 14:00, 10 June to 30 September 2026. The median day took 26 kWh in the window; the biggest took 54 kWh. The dashed line is a steady 20 kW.

    References

    1. GloBird Energy (2026) ZEROHERO Residential (Flexible Rate), AusNet, offer GLO764991MR, Victorian Energy Fact Sheet. Accessed 15 October 2026. Web address of the fact sheet not recorded.
    2. OVO Energy (2026) Victorian Energy Fact Sheet, The Basic Free 3 (OVO731736MR), electricity, AusNet. https://www.ovoenergy.com.au/vefs/Victorian_Energy_Fact_Sheet_OVO731736MR_Electricity_CZ_6.pdf. Accessed 15 October 2026.
    3. State of Victoria (2026) Midday Power Saver Pricing Order 2026 (Gazette S498, 8 September 2026). https://www.gazette.vic.gov.au/gazette/Gazettes2026/GG2026S498.pdf. Accessed 15 October 2026.