Solar Water Heater vs PV Geyser Calculator

Two solar routes to hot water

Water heating is often a household’s second-biggest energy use, and there are two quite different ways to hand it to the sun. A solar water heater (SWH) is thermal: evacuated tubes or flat plates heat water directly into an insulated tank, at 3–4× the efficiency of PV per square metre of roof. The PV route adds panels to run your existing electric geyser — less efficient per square metre, but it shares hardware with the rest of your solar system, works in any season the sun shines, and produces general-purpose electricity whenever the geyser is off.

The physics is small; the habits are big

Heating 150 litres by 25°C takes about 4.4 kWh — modest — but repeated daily it is ~1,600 kWh a year, real money at any tariff. An SWH quietly delivers most of this with no moving parts and 15–20-year lifespans; its weakness is cloudy spells and winter, when the backup element runs (the ~70% annual solar fraction in the math). The PV route needs the geyser to heat when the sun is out — a timer or a smart switch — otherwise the panels export cheaply at noon while the geyser burns grid power at night.

Which wins?

On pure payback, the SWH usually wins where hot-water demand is steady and roofs are tight — it extracts more heat per square metre and costs less than the equivalent PV capacity. The PV route wins when you are already installing a PV system and can simply size it ~1 kW larger, when roof space is plentiful, or when you value flexibility (the same panels charge an EV or run the AC in seasons you need less hot water). In South Africa the same comparison applies to geyser conversions; a full savings model can fold either route into your whole-house numbers.

A worked example (India)

150 L/day raised 25°C at ₹8/kWh costs about ₹12,700 a year on a grid geyser. A ₹28,000 SWH saving 70% pays back in ~3.1 years. The PV route needs ~1.1 kW (₹60,000 at ₹55,000/kW) but offsets the full annual energy — payback ~4.7 years, and the panels keep earning after the tank is hot. Both beat the grid; which is ‘better’ depends on your roof and whether you are installing PV anyway.

Frequently asked questions

Per square metre of roof, yes — thermal collectors convert 50–70% of sunlight to heat versus ~21% for PV, so an SWH usually has the faster payback for dedicated water heating. But PV wins on flexibility: the same panels serve any load, all year.

Litres × temperature rise × 1.16 Wh. A typical 150 L at a 25°C rise is ~4.4 kWh per day — about 1,600 kWh a year, often 15–25% of a household bill in geyser-heavy homes.

Cloudy spells, monsoon weeks and cold inlet water in winter force the electric backup element to top up. Annual solar fractions of 60–80% are typical; the calculator uses 70% as an honest middle.

Yes — the simplest route is a timer so it heats during solar hours, or a dedicated PV-geyser controller. Without that timing, midday solar exports for a small credit while the geyser burns full-price grid power at night, and the savings evaporate.