Residential HPWH Annual Simulation Tool | Knowledge Base

Overview

An annual performance simulation tool for residential heat pump water heaters, implementing the AS/NZS 4234 simulation methodology. The user selects a product and climate zone and receives the annual energy consumption, zone energy factor, and estimated certificate entitlements under SRES (STCs), VEU Activity 44 (VEECs), and other applicable schemes.

This tool serves two primary use cases: internal EnergyAE consultants scoping certificate entitlement for a new client product, and manufacturer clients checking their own product’s zone ratings and estimated scheme value before committing to a registration pathway.

The underlying Python simulation engine (AS/NZS 4234-based) is already built. The developer’s task is the web wrapper around it.

User Stories

As an EnergyAE consultant, I want to enter a product’s test data and a target climate zone and immediately see the estimated STC and VEEC entitlement so I can give a client a quick scoping answer without running a full TRNSYS deck.
As a manufacturer client, I want to run my product across all seven AS/NZS 4234 climate zones in a single batch so I can see which zones are most valuable and prioritise my registration targets.
As a consultant, I want to see the certificate calculation workings (zone energy factor, annual kWh, comparison factor) so I can sense-check the outputs and explain them to a client.
As a user, I want to save a simulation run against a product record so the results are accessible when I revisit the product later.

Context: AS/NZS 4234 Climate Zones

The standard defines seven climate zones corresponding to different Australian and New Zealand locations. Each zone has a standardised ambient temperature profile, solar radiation profile, and draw-off schedule. Alastair to confirm the exact zone definitions and confirm whether NZ zones are handled separately. Summary for spec purposes:

Zone 1: Hot humid (Darwin, Cairns)
Zone 2: Sub-tropical (Brisbane, Townsville)
Zone 3: Warm temperate (Sydney, Perth)
Zone 4: Mild temperate (Melbourne, Adelaide)
Zone 5: Cool temperate (Hobart, ACT fringe)
Zone 6: Cold (Canberra, elevated inland)
Zone 7: Very cold (alpine areas)

STC entitlement is calculated from the zone energy factor. Higher zones (colder climates) yield higher zone energy factors and therefore more STCs, but products must achieve a minimum zone energy factor to qualify.

Inputs

Product parameters

These can be sourced from the product database (preferred) or entered manually. Required inputs are a subset of what the 5125.1 analyser extracts:

COP vs ambient temperature curve (table of data points at minimum 3 ambient conditions)
Rated heating capacity vs ambient temperature
Tank volume (litres)
Boost element power (kW) and position
Setpoint temperature (°C)
Thermostat cut-in differential (°C)
Insulation heat loss coefficient (W/K)
Refrigerant type (informational)
System type: integral / split with separate tank

Simulation settings

Climate zone(s): single zone or batch across all zones
Draw-off profile: AS/NZS 4234 medium household (default) or large household
Control mode: HP-only / HP + boost
Off-peak boost operation: yes/no toggle (applies boost only outside morning and evening peak hours)

Scheme selection

User selects which scheme outputs to calculate:

SRES (STCs)
VEU Activity 44 (VEECs)
ESS HEAB (ESCs) — Alastair to confirm whether residential products are eligible
NZ EECA — separate calculation methodology, Alastair to confirm requirements

Simulation Outputs

Zone energy factor and annual performance

Per climate zone:

Annual energy delivered to water (kWh)
Annual energy consumed by HP compressor (kWh)
Annual energy consumed by boost element (kWh)
Zone energy factor (annual energy delivered / annual energy consumed)
Number of days boost element was activated (diagnostic)
Number of cold water delivery events (setpoint not met)

Certificate entitlement

STCs (SRES)

Zone energy factor for each selected zone
Comparison factor vs reference water heater (derived from zone energy factor per AS/NZS 4234 methodology)
STCs per unit (calculated from comparison factor and zone multiplier)
Dollar value estimate (STCs × current market price — user-entered or auto-fetched)
Alastair to confirm exact STC calculation formula and reference heater parameters

VEECs (VEU Activity 44)

Annual energy savings vs baseline (kWh/year) calculated per VEU Activity 44 methodology
VEECs per unit (energy savings × activity factor)
Dollar value estimate
Alastair to confirm the VEU A44 calculation method as it may differ from the AS/NZS 4234 simulation output — the VEU methodology has its own defined calculation steps that may use test data differently

Other schemes

ESS HEAB: ESCs per unit if applicable
NZ EECA: entitlement per unit if applicable

Results presentation

Results shown per zone in a summary table
Bar chart showing STC entitlement by zone (helps prioritise zone targeting)
Certificate value summary table: certificates per unit × volume input = total certificate value
Toggle to show calculation workings (zone energy factor derivation, comparison factor steps)

Batch Mode

User can run a single product across all seven climate zones in one submission. Results displayed as a zone comparison table and map visualisation showing entitlement by zone.

Volume scenario: user inputs an estimated annual deployment volume (units sold per zone) and the tool calculates total certificates and total dollar value across all zones. Useful for client business case presentations.

Integration with Product Database

“Select product” at the top of the input form pre-fills all simulation parameters from the product record
After running, user is prompted: “Save results to product record?” — stores zone energy factors and STC per zone against the product
Run is logged with input parameter snapshot so historical results are reproducible

Out of Scope (v1)

Commercial HPWH annual simulation (different standard and methodology — separate tool)
Solar-assisted HPWH or solar pre-heat configurations
NZ climate zone simulation (add after AU zones are validated)
AS/NZS 4234 Appendix compliance report generation (separate output tool)
Automated STC price feed integration (manual price entry in v1)

Data Model (indicative)

residential_simulation_runs
  run_id
  product_id (nullable, if linked to database product)
  user_id
  created_at
  input_parameters (JSON: product params, climate zone, control settings)
  output_per_zone (JSON array: zone, annual_kwh, zone_energy_factor, stcs_per_unit, veecs_per_unit)
  scheme_selection (array of schemes calculated)
  volume_scenario_units (nullable, per-zone deployment volumes)
  output_summary (JSON: total certificates, total value per scheme)
  saved_to_product (boolean)

Acceptance Criteria

Open Questions

Does the existing Python simulation engine implement the full AS/NZS 4234 annual simulation methodology, or only the time-series performance simulation? Alastair to confirm scope before build begins.
VEU Activity 44 uses its own calculation method for VEECs that may not be identical to the AS/NZS 4234 simulation. Alastair to clarify whether the engine outputs need to be post-processed to match the VEU methodology.
Are residential products eligible for ESS HEAB, or is that commercial only? Alastair to confirm.
Should NZ EECA entitlement be calculated in v1, or deferred until the AU methodology is validated?
What is the reference water heater specification used for the STC comparison factor calculation? Alastair to confirm from the CER guidance.
Should the volume scenario deployment volume be saved as part of the run, or is it a transient scenario overlay that does not get stored?