Source
https://www.branz.co.nz/pubs/research-reports/sr-488/ — original source (opens in a new tab; the file is not redistributed)
Summary
BRANZ Study Report SR488 (Rupp et al., 2025) experimentally compares promising low-carbon residential water-heating technologies against the standard NZ electric-resistance storage cylinder. Two technology families are tested: solar-direct PV water heating (a small dedicated PV array driving the cylinder element via a simple controller — no battery, no whole-house inverter) in three sizes, and a CO₂ heat-pump water heater. Systems were installed and instrumented at BRANZ and run across seasons with small and large hot-water draw-off schedules. All reduced grid electricity versus the resistance reference, but by amounts that depend strongly on technology, system size, season and draw-off. The CO₂ heat pump was the most consistent (36–46% of reference grid electricity year-round); the solar-direct-PV systems excelled in summer (~⅓ of reference) but fell away in winter. The report is energy-performance only — it carries no cost figures.
Key claims
- claim: "BRANZ SR488 (Rupp, Jaques, Lanner & Pollard, 2025) experimentally compared three solar-direct-PV water-heating systems (REFUsol, Sun Flux, Easy Warm HOT PV/EnaSolar) and one CO₂ heat-pump water heater against an electric-resistance storage cylinder reference, installed and instrumented at BRANZ and tested across seasons with small and large hot-water draw-off schedules."
source_location: "Abstract (p.ii); §2 Experimental equipment; §3 Results"
- claim: "The CO₂ heat-pump water heater was the most consistent performer, requiring less than half — 36–46% — of the grid electricity of a traditional electric-resistance water heater regardless of the time of year."
source_location: "Executive summary (p.1)"
- claim: "Under the winter test the CO₂ heat-pump system used 12.0 MJ/d (3.3 kWh/d) of grid electricity to deliver ~30 MJ/d of heat — i.e. ~39% of the reference system's energy, implying an effective coefficient of performance of about 2.5. (The report states it was not designed to formally measure COP.)"
source_location: "§3.1.5 Spring/winter comparison: Heat pump to Reference"
- claim: "The electric-resistance reference cylinder used 30.3 MJ/d (8.4 kWh/d) of grid electricity for hot water — about 22 MJ/d of measured draw-off plus ~7.9 MJ/d (2.2 kWh/d) of cylinder standing losses at maintained temperature."
source_location: "§3.1.2 and §3 reference baseline"
- claim: "Solar-direct-PV water heating performed well in summer — even a small array with a large draw-off needed only about one-third of the reference grid electricity — but was markedly less effective in winter, where a larger array is required; the best spring/small-draw-off case (REFUsol) used only ~12% of reference grid electricity."
source_location: "Executive summary (p.1); §3.1–3.2"
- claim: "Context: water heating uses around 31% of NZ residential energy (EnergyConsult 2020); 73% of NZ household hot-water systems are electric storage cylinders (BRANZ Pilot Housing Survey, White 2020); there are ~1.9 million NZ households (Stats NZ 2023)."
source_location: "§1 Introduction (p.2)"
- claim: "Ripple control — the long-standing NZ hot-water demand-management technology — gives distributors the ability to cut off up to ~15% of NZ's annual peak demand, and about half of NZ households have it."
source_location: "§1 Introduction (p.2)"Neobiome Intelligence relevance
- D01 — hot-water (DHW) heat-pump duty (complements CR_014). SR488 supplies the water-heating heat-pump performance that CR_014 (space-heating SCOP) does not: a CO₂ heat-pump water heater delivers hot water at 36–46% of resistance grid electricity year-round (effective COP ~2.5 in winter) OT_038. The resistance DHW baseline of 8.4 kWh/d (~3,070 kWh/yr) cross-checks HEEP’s 29%-water-heating share (≈3,310 kWh/yr) — two independent NZ sources agreeing on the hot-water load.
- New ④ technology option — solar-direct-PV water heating. A cheap, retrofittable PV→cylinder system (no battery/inverter) that cuts grid hot-water electricity to ~⅓ in summer; introduced here as the technology page solar_direct_pv_water_heating and a candidate addition to the model’s heat-supply set alongside heat pumps and solar thermal OT_038.
- ⚠ Scope limits (what it does NOT provide). No cost figures (energy performance only) → not a cost-cell source. Not solar-thermal collectors → does not fill the
solar_thermalyield/cost gap, which remains with BRANZ SR188 (RT_204) and CR_016. Winter performance of solar-direct-PV is weak, so it is a summer-load-shifter, not a year-round standalone.
Research targets
Documents to retrieve
- Manufacturer pricing / installed-cost data for solar-direct-PV water-heating systems (REFUsol, Sun Flux, EnaSolar) and CO₂ heat-pump water heaters — needed to turn SR488’s performance results into the cost-benefit cells the model requires (SR488 itself has no costs). (New — see RT below.)
- BRANZ SR188 (2008) Performance of Solar Water Heaters in NZ — the solar-thermal collector yield primary (RT_204); distinct from this PV-driven study.
Research gaps
- A NZ cost-benefit / payback for solar-direct-PV water heating vs heat-pump vs resistance at community scale — SR488 supplies the energy side; the cost side is missing.
- Winter / low-irradiance performance sizing for solar-direct-PV in southern NZ regions (the technology’s weak season), to know whether it can be more than a summer load-shifter in a self-sufficient design.
Connections
Links to
Technologies (1): Solar-Direct PV Water Heating
Referenced by
Sources (7): CR_014 · OT_041 · OT_084 · OT_095 · OT_105 · OT_162 · RD_028
EDT domains (1): D01: Renewable Energy & Storage Systems
Technologies (1): Solar-Direct PV Water Heating