Custom calculators › PV water heaters
PV water heater calculators
A customer picks their tank, their array and their household, and the tool works out how much of their hot water the sun actually pays for. Below is a working example on generic equipment. It is not a lookup table or a spreadsheet formula: each time you change an input, our server runs a fresh AS/NZS 4234 annual simulation of that tank against that city's weather year, and answers in about a second.
PV to a resistive element in the tank, so the roof carries panels and the tank carries no collector loop. This is not solar thermal: no flat plate or evacuated tube collector, no pump, no glycol. If that is the system you sell, the same engine models it, and the calculator would be a different build.
1. The household
2. The system
3. Money
What is doing the calculating
Most calculators for this equipment divide an annual energy figure by a solar fraction someone picked. This one simulates the tank.
A real annual simulation
87,600 timesteps of 0.1 hour through a 20-node stratified tank, with conduction between nodes, buoyancy mixing, per-node heat loss and thermostat hysteresis at the sensor's own height. The draw schedule, the seasonal load curve and the cold mains temperature are AS/NZS 4234's own, for the climate zone selected.
The same engine as the submissions
This runs the AS/NZS 4234 engine EnergyAE built for scheme modelling, ported so it answers fast enough for a web page. An automated test holds the port to the original: the tank physics agrees to twelve decimal places, and the largest disagreement anywhere in the result is under one part in a million.
Stratification is the point
A tank charged in the middle of the day spends most of it part hot and part cold, and the answer depends on where the sensor sits in that gradient and how much usable water is above the draw port. A single-temperature tank model cannot see any of that, which is why it gets the shoulder seasons wrong.
What a build for your brand adds
The example above runs generic equipment. Four things separate it from a tool you would put in front of your own customers.
Your real tanks and controllers
Volume, standing loss, element rating, and the actual heights of the element, the thermostat and the draw port for each model you sell. Those positions move the answer more than the tank size does, and they are already in the data we hold if we have modelled your range.
Your control strategy
The example runs a daily solar window with a comfort boost outside it. If your controller does something else, a diverter that modulates on surplus, a time-of-use boost, or a heat pump instead of an element, that logic goes into the engine rather than being approximated around it.
The customer's own house
The five Australian and two New Zealand zones here are the reference locations AS/NZS 4234 tabulates. A build can run any location with a weather year, take the roof's real tilt and orientation, and net the element against the rest of the household load instead of assuming the array is free to it.
Embed, branding and leads
One line of embed code on your own site in your typography and colours, or a hosted page under your domain. Optional lead capture on the result, delivered to whichever inbox or CRM you use.
Two inputs that move the answer
Tank temperature is the first. Storing hotter banks more of the day's sun, so a higher target lifts the solar share, and it also raises standing loss and pushes the delivery temperature further above what the tempering valve will pass. Both effects are in the simulation, so the trade-off shows up rather than being argued about.
The heating window is the second. Start too early and the element burns grid electricity before the array has woken up. Start too late and the tank never reaches temperature before the draw begins. The example exposes both under the control detail panel.
The number nobody quotes
The second chart is spilled solar: energy the array generated that the tank had no room for. It is usually the largest number on the page, and it is the one that tells a customer whether a bigger tank is worth more to them than a bigger array.
A calculator that reports only the solar fraction cannot answer that question, because the fraction goes up when you shrink the array. This one reports both.
Want one for your range?
Tell us which markets the tool needs to cover and send your tank and controller data. We will scope a build against them and come back with a fixed quote.
Request a quoteThe calculator above is a demonstration. The tank, element, array and installed-cost figures behind it are generic residential placeholders rather than any product's specification, the design basis assumes the array's output is available to the element with no competing household load, and no feed-in credit is applied to spilled solar. Figures are estimates and are not a specification, a performance guarantee or a recommendation for a specific installation.