How HotWaterWise calculates

thermal kWh/day = litres/day × 4.186 kJ/(L·K) × 39 K ÷ 3600

The model uses a 21 °C cold-water planning reference and 60 °C hot-water reference. The resulting 39 K lift is annualised over 365 days. Electric input is thermal kWh divided by electric efficiency or heat-pump COP. Gas input is thermal kWh × 3.6 MJ/kWh divided by gas efficiency. Cost then uses the selected variable usage rate in $/kWh or $/MJ.

Planning values are 90% for electric storage, 55% for gas storage, 78% for instantaneous gas and COP 3.0 for a heat pump. They are planning assumptions, not ratings for a particular product or property.

Efficiency is the share of input energy delivered as useful hot water: 0.90 means 90%. Heat-pump COP compares useful heat with electricity input: 3.00 means 3 kWh of heat per 1 kWh of electricity. The gas-boosted solar value is the share of hot-water demand met by solar collectors: 0.70 means 70%. Its remaining gas demand uses a separate 80% booster-efficiency assumption. These planning numbers have no kWh or MJ unit; the table names the input-energy unit used to calculate running cost.

Shower and bath volumes are modelled as 40 °C mixed water and ordinary tap use as 30 °C mixed water, then converted to the equivalent thermal demand at 60 °C. The shower planning flow is 9 L/min. A bath uses the130 L planning allowance. Ordinary taps use 5 minutes × 9 L/min once per household, not once per person.

Appliance label water is total appliance water, not automatically water drawn from the household hot-water service. Household Usage therefore asks separately whether each appliance uses a connected hot inlet. A connected dishwasher contributes 11 L per cycle, the rounded unweighted registered-model median. A connected washing machine contributes 20 L at 60 °C equivalent per warm/hot wash, derived from the median warm-minus-cold labelled energy of eligible dual-inlet registrations without internal heaters. Neither value is an Australian household average.

“No” contributes zero. “Not sure” uses the same arithmetic as a confirmed connection but is visibly marked assumed. Cold washes and appliance-internal electrical heating are not counted as household water-heater demand. Each appliance contribution is cycles/week × planning litres/cycle ÷ 7, rounded to whole litres/day before summing. If no Household Usage result is supplied, cost calculators use 50 L/person/day. If a valid Household Usage total is supplied, it takes precedence without adding appliance demand twice.

Current defaults include 6 shower minutes per person/day and 1 bath/week; the form allows these values to be changed.

Solar thermal roof collectors and a storage tank, with a gas-fired booster that tops up the water when solar contribution is insufficient.

booster input MJ = annual thermal kWh × 0.3 × 3.6 ÷ 0.8

The disclosed model assumes 70% of thermal demand is met by solar collectors and 30% by a gas booster operating at80% planning efficiency. The 70% figure is a solar thermal fraction, not gas-appliance efficiency or rooftop-PV self-consumption.

The calculators do not ask whether the household has rooftop PV. Household rooftop-PV self-consumption is excluded from electric cost and payback estimates because the available inputs cannot quantify it reliably. No fixed percentage or tariff discount is applied. Rooftop PV is not used to reduce modelled gas consumption or gas costs. Actual electric costs and savings can differ. Rooftop PV remains distinct from solar hot water.

A water heater on a separate Energex controlled-load/economy circuit cannot directly use the household’s rooftop PV under that documented wiring arrangement. A heater on the household’s general circuit may use PV only when operation overlaps available generation, but HotWaterWise has insufficient inputs to quantify that overlap.

• General electricity: 28.5 ¢/kWh.
• Controlled load: 20.5 ¢/kWh.
• Mains gas planning default: 3.4 ¢/MJ.
• Mains gas tariff “not sure” representative: 3.6 ¢/MJ. An unknown gas supply is not assigned this tariff.

Controlled load and general electricity remain distinct selections. Custom rates are used only with their matching custom mode. The calculations exclude fixed daily supply charges, account fees, connection/disconnection costs and future tariff changes. The Electric vs Gas result is therefore a variable-energy-cost comparison, not a total-bill comparison.

For 45 kg LPG bottles, annual gas input is divided by 2,232 MJ per nominal full bottle (45 kg × 49.6 MJ/kg, gross propane energy). The system efficiencies and the 70% solar-thermal / 30% residual gas-boosting split are unchanged. The resulting annualised bottle count is not rounded up to purchases or deliveries.

Enter the usual ongoing price for one 45 kg refill or exchange from a bill or supplier. Exclude one-off introductory promotions or credits, separately itemised delivery, and annual bottle rental or service charges. A bounded Greater Brisbane price check on 19 September 2026 did not establish comparable public ongoing prices, so no indicative bottle price is supplied. No delivery, collection, rental or service cost is assumed.

A confirmed separate delivery fee is divided by the bottles in that order, then multiplied by annual hot-water bottles. Collection and delivery with no separate charge add nothing; unknown delivery is excluded and labelled unknown. Annual rental/service charges remain separate whole-service costs. Hot Water Cost and Heat Pump Payback include them as savings only when their amount is confirmed and LPG would be discontinued. Electric vs Gas keeps its usage-only difference and shows potentially avoidable fixed costs separately. Unknown charges and cancellation costs are excluded. Other bottle sizes, bulk tanks and unknown gas supplies cannot produce a guessed gas estimate.

Sources: DCCEEW energy conversion (2024, Table 9).

Evidence and volatile defaults are reviewed quarterly and when a material tariff, program or source changes. Stable technical evidence receives an annual deep review. Values are not refreshed continuously.

annual saving = current-system usage cost − heat-pump usage cost + confirmed avoidable LPG fixed charges (if applicable)

payback years = full installed cost ÷ annual saving

If Hot Water Cost supplies a valid submitted current-system cost, that amount takes precedence over a new estimate of the current cost. The default installed-cost benchmark is $3,875 equipment + $600 installation = $4,475 before incentives. It is not an incremental cost versus another replacement and is not a quote.

Payback is not shown when annual saving is zero or negative. Signed annual differences are preserved. Net positions subtract the installed cost from the signed annual running-cost difference × year and are shown automatically after 5, 10, 15 and 20 years. These are fixed planning horizons, not predicted equipment life, warranty or guaranteed performance. No discount rate, inflation, tariff escalation or financing is introduced. Rebates and STCs are not applied automatically. Use a current net installer quote if an incentive is reflected in it.

No Queensland heat-pump rebate is assumed. Eligible systems may receive federal STCs; verify current eligibility and assistance on official Queensland and Clean Energy Regulator sources.

• Queensland Government — Home energy savings: Current Queensland household energy information and any active assistance programs. No general Queensland heat-pump hot-water rebate is assumed in the calculator.
• Clean Energy Regulator — eligible solar water heaters and heat pumps: Eligible solar water heaters and air-source heat pumps may create Small-scale Technology Certificates (STCs). Eligibility and certificate value depend on the approved system and installation.

• Australian Government — Hot water systems: Consumer guidance states that heat-pump water heaters use about 30% of the energy of conventional electric hot water. HotWaterWise combines that comparison with its disclosed 90% electric-storage benchmark to derive the effective COP 3.0 planning value; it is not a product rating.
• Australian Government Energy Rating — Heat-pump water-heater Decision RIS: April 2026 evidence on Zone 3 performance, product variability, typical system efficiencies and purchase/installation cost context. Product and household outcomes vary materially.
• Energex — Economy tariffs: SEQ guidance on Tariff 31/33 availability, dedicated circuits and retailer-dependent rates. The selected tariff remains a planning input; households must confirm equipment eligibility and wiring.
• Australian Energy Regulator — Understanding your energy bill: Distinguishes usage rates from fixed daily supply charges. The calculator models hot-water usage charges and excludes fixed supply charges.
• Queensland Government — Energy and water info: Queensland household energy-efficiency information and current assistance links.
• Energex: Electricity network information for the Energex distribution area. Retail usage rates depend on the household retailer and plan.
• Ergon Energy: Electricity network information outside the Energex area; not used to claim council-specific retail prices.
• AEMO: Australian Energy Market Operator — NEM wholesale and retail market publications.
• Australian Energy Regulator (AER): Federal energy-market regulator — retail price monitoring and consumer guidance.
• Australian Government — YourHome water heating: Independent guidance on water-heating technologies, efficiency, sizing and installation considerations.