Heating water is, quietly, one of the biggest electricity bills most households carry. A standard 150-litre electric storage tank eats roughly 20-40% of a home’s total power, running day and night to keep water hot whether you are using it or not. Which is a bit mad when you consider that the sun, a giant free hot-water boiler, is parked over the roof for most of the day.
Going solar for your hot water is one of the highest-return eco decisions you can make at home – payback is usually within 3 to 7 years, and after that you are heating water for free for the remaining decade-plus of the system’s life. But there are four fairly different ways to do it, with four fairly different price tags. Here is how to tell them apart. Prices below are rough installed ranges, converted to US dollars for comparison – actual costs vary widely by country, installer, and roof access, so treat them as ballparks rather than quotes.
1. Passive solar water heaters (thermosiphon)
True to the name, these systems run passively with no pump. An insulated tank sits on the roof directly above a bank of evacuated glass tubes or a flat-plate collector. As the sun warms the water in the collector it rises by convection into the tank. No electricity, no moving parts, nothing to break. Ideal for small households, a 150L to 200L (40-55 gallon) system will typically run you $600 to $1,200 installed for a certified kit.
The trade-offs are real though. You need a roof that can take the weight of a full tank of water. The pipes run exposed, so in frost-prone areas you need a frost-protected model or the tubes will burst on a cold winter morning. And the output drops off sharply on cloudy days and in the depths of winter, so nearly every installation keeps a standard electric element as a backup. You will still use it, just a lot less.
2. Pumped solar water heaters (split systems)
A level up. The tank comes off the roof and lives on the ground or inside the house, while only a slim collector stays up top. A small pump moves water between the two whenever the collector is hotter than the tank. Lower roof loading, better tank insulation, easier to service. Expect to spend $1,200 to $2,100 installed for a 200L to 300L (55-80 gallon) system.
If you live somewhere with an unreliable grid, pay the extra $100 – $200 for a version with a dedicated 20W PV panel driving the pump. That way the system keeps circulating whenever the sun is shining, mains or no mains.
3. PV-to-element controllers (solar-diverted water heater)
This is the sleeper option a lot of installers will not lead with because the margin is thinner. If you already have (or plan to have) a PV array on the roof, a PV-to-element controller swaps your standard AC water-heater element for a dual-voltage element and diverts spare DC power from the panels straight into the tank. Any solar electricity that would otherwise be clipped by the inverter or exported for pennies heats your water instead.
No batteries needed, no inverter needed, no plumbing changes. Just an element swap and a small controller box. Installed, it runs $250 to $500, which is a fraction of the cost of a solar thermal retrofit. If you are adding PV to a home that already has a working electric tank, this is almost always the highest-return upgrade on the list.
4. Grid-tied or fully off-grid solar PV
Going all-in on PV for the whole house is a different conversation. Your water heater stays exactly as it is, but it runs off solar electricity for most of the day. Ballpark pricing:
- 3kW grid-tied (no battery): $3,000 – $4,500. Covers daytime loads, water heater included if you add a smart controller.
- 5kW hybrid with a small battery: $5,000 – $8,000. Rides through outages, covers most household loads.
- 10kW+ off-grid with full battery backup: $10,000 – $15,000+. Whole-of-life grid independence.
If you are going down this road, make sure the installer sizes the system with hot-water loads in mind. Water heaters are thirsty – a 3kW element runs for two to three hours a day – and a PV system sized for lights and plug loads alone will never cope with them. Better: add a smart water-heater controller that only allows the element to run when there is surplus solar on the roof. Now you are doing option 3 and option 4 at the same time.
How to choose
A shorter way to think about it:
- You just want the cheapest hot-water bill possible. Passive thermosiphon. Done.
- Large or growing family, lots of hot-water demand. Pumped split system, 300L (80 gal) tank.
- You already have or are about to install PV panels. Add a PV-to-element diverter. Smallest cost, biggest ROI.
- You want to disconnect from the grid entirely. Full PV with battery backup, plus a smart controller on the water heater.
- You rent and cannot modify the plumbing. A heater timer and an insulating tank blanket. Not solar, but still good for 20-30% savings on water heating.
Size it yourself
Before you phone any installers, it is worth knowing roughly what size of system your household actually needs and what it would pay back in. The calculator below uses typical household hot-water usage (about 50 litres / 13 gallons per person per day) and lets you pick your local electricity tariff and currency. Defaults sit at a middle-of-the-road residential rate – adjust to match your actual bill.
Solar water heater calculator
A rough size and payback estimate for your household. Pick your currency and local tariff below.
Remember, the best solar hot water option for you depends on your needs, budget, and how much sunshine you get. Weigh your options, use the calculator above as a starting point, and let the sun start working for you.
For more ways to lower the footprint and the bill around the house, have a browse through our Energy & Home articles, or find installers and eco-building suppliers in the directory.
Author: C
