Model your real hot water use — showers, baths, household size, your city's climate — against your gas and electricity rates, including any free or off-peak window your heat pump can soak up. Everything recalculates live.
How much hot water your home actually draws each day.
When you draw hot water — and when the heat pump reheats — decides how cold the air is at heating time. A heat pump is far happier topping up in warm midday air than at 6am.
Cheaper power, but pre-dawn air is cold — so the heat pump uses more electricity for the same hot water. The calculator weighs both.
Gap between the day's high and low. Bigger inland (Canberra), smaller on the coast (Sydney, Darwin).
Cold-water inlet temperature sets how hard the heater works; air temperature sets your heat pump's efficiency. Both move month to month.
Your two units. Defaults follow the AS/NZS 4234 standard (a 4-star reference gas heater) and conservative real-world COP. The modelling knobs sit under Advanced — leave them unless you know your numbers.
It's printed on the unit. 4★ is the standard's reference baseline.
Datasheet COPs are lab figures and not comparable between brands — to pin a real one, find your model in the Clean Energy Regulator register and use its STC count for your zone.
The ambient temperature the datasheet COP was measured at.
From a recent bill, or the Victorian Default Offer (ESC) / Energy Made Easy (AER) reference prices. Set how the heat pump is scheduled under Daily rhythm (02); here you set what each unit costs.
What on-demand heating pays.
Controlled load / overnight.
What you'd pay to switch, after STCs and any state rebate (Solar Victoria / Victorian Energy Upgrades).
Usually $0 — but if you'd otherwise export it, set this to your feed-in tariff (the income you give up).
NGA 2025, set from your state — tracks your city.
NGA natural gas combustion (~51.5 kg/GJ).
Winter is the real test — colder inlet water and a less efficient heat pump. This is where the gap narrows or holds.
Reading it: annual estimates, not a quote. Air and cold-inlet temperatures are modelled monthly from city climate norms. The electricity supply charge is excluded since you pay it either way. On emissions: solar self-consumption counts as zero, but on Victoria's coal-heavy grid (0.78) a heat pump run entirely on grid power can emit more than efficient gas — so the CO₂ result hinges on your solar share and falls every year as the grid cleans up.
Everything in Advanced is pre-set from these. The visible fields are yours to fill from your own home and bills.
COP claims aren't standardised between brands, so datasheet figures run optimistic — the STC count is the fairer like-for-like.