Description
Solar-direct PV water heating uses a small dedicated photovoltaic array connected directly to a hot-water cylinder’s heating element through a simple controller/modifier (sometimes called a PV diverter or solar-direct/hybrid controller). It converts sunlight to electricity and dumps that electricity straight into the resistance element as heat — there is no battery, no whole-house inverter, and minimal wiring. It is distinct from (a) whole-house grid-tied PV, being smaller and simpler, and (b) solar-thermal collectors, which heat water directly with the sun rather than via electricity. Several products are commercially available in NZ (e.g. REFUsol, Sun Flux, Easy Warm HOT PV / EnaSolar). OT_038
How it works
- A small PV array (sized to the cylinder, not the whole house) feeds DC/AC power to the cylinder element through a controller that maximises the energy delivered (the REFUsol controller is quoted at >99% efficiency; one off-grid/hybrid solar controller at up to 96%). OT_038
- The cylinder acts as the thermal store; surplus sun heats water during the day for later draw-off. When sun is insufficient, the normal grid/element supply tops up to the set temperature.
- Because it bypasses battery and inverter losses and avoids whole-house PV complexity, it is cheaper and simpler to retrofit — but it can only heat water (a single end-use), and only when the sun shines.
Performance
- Summer: strong — even a small array with a large hot-water draw-off needed only ~one-third of the grid electricity of a resistance reference cylinder; the best spring/small-draw-off case (REFUsol) used only ~12%. OT_038
- Winter: markedly weaker — low irradiance means a larger array is required to make a meaningful dent in winter hot-water grid demand; otherwise resistance top-up dominates. It is best understood as a summer/shoulder load-shifter, not a year-round standalone supply. OT_038
- Reference point: the electric-resistance baseline it offsets is 8.4 kWh/d (~3,070 kWh/yr) of grid electricity for hot water. OT_038
- Compared with a CO₂ heat-pump water heater (36–46% of resistance grid electricity year-round, effective COP ~2.5 — see OT_038), solar-direct PV wins in summer but the heat pump is the more consistent year-round performer.
- PV vs solar-thermal on a scarce roof (IEA SHC Task 69, 2025). Per unit area, solar-thermal collectors (~60% efficiency) out-yield a PV+heat-pump path (>50% solar-to-water, COP 3 × PV 16.7%) for raw thermal — so solar-thermal wins where the only goal is thermal-per-m². But PV electricity “can be used to meet other electrical loads and would not just serve the hot water system”, so where electricity is versatile (+ a heat pump multiplies each kWh) the model claims scarce roof for PV first (its V0.7.4 ruling), solar-thermal filling leftover roof. PV2Heat also suits “unreliable grid service, high connection costs, or low up-front capital” — the off-grid case. OT_095
NZ context & suitability
- ~73% of NZ household hot-water systems are electric storage cylinders (BRANZ Pilot Housing Survey, White 2020), so the retrofittable target market is large; water heating is ~31% of NZ residential energy. OT_038
- Best suited to northern/sunnier NZ regions and summer-peaking demand; southern/winter-peaking communities would need a larger array or a heat-pump pairing.
- Recognised as one of the six standard NZ domestic water-heating families (“dedicated photovoltaic, with electric back-up”) in BRANZ’s water-heating overview, and grouped with heat pumps and thermal solar as a low-operating-emissions option (vs fossil gas highest). OT_041
- Regulatory status (2018). As of the trans-Tasman E3 Policy Framework for Hot Water Systems, direct-PV water heating had no MEPS, no energy label, and no product measurement standard at all (“Nil”) — and heat-pump and solar water heaters likewise had no MEPS/labelling. E3 flagged PV-direct as a newer technology needing an appropriate measurement method to be developed. This dates the technology’s NZ regulatory immaturity and explains the scarcity of mandated per-model efficiency data for it. OT_161
Cost
No NZ installed-cost data is yet recorded — BRANZ SR488 is an energy-performance study only. Cost-benefit and payback figures are required before this technology can be costed in the NI model (see Research targets).
Research targets
- Manufacturer / installed-cost data for NZ solar-direct-PV water-heating products (REFUsol, Sun Flux, EnaSolar) and a NZ cost-benefit vs heat-pump and resistance — needed to add this technology to the model’s ④ cost cells.
- Winter/low-irradiance sizing for southern NZ regions, to determine whether it can be more than a summer load-shifter in a self-sufficient design.
Connections
Links to
Referenced by
Sources (3): OT_038 · OT_052 · OT_095
EDT domains (1): D01: Renewable Energy & Storage Systems