I'm debating between installing a solar water heater vs. a heat pump for supplying hot water to my house. The house is in Bangalore, India - we have about 300 days of sunshine in a year. My estimate is that we'll need about 150litres of hot water a day, mostly between 6:30AM and 8AM (just after sunrise). Whichever system is selected would be installed in the terrace and exposed to sunlight.

A few things I'm concerned about:

  • Availability of hot water on cloudy days and early in the morning - is the insulation of the typical solar water heater good enough to keep the water hot overnight?
  • Power consumption - with solar water heaters, the power consumption is pretty much zero. What kind of power consumption can I expect with heat pumps for heating up 150litres of water a day?
  • Reliability: what kind of life can I expect for a solar water heater vs a heat pump. Intuitively it feels like solar water heaters should be more reliable since it has less electronic parts. What should I consider when installing these in the terrace to protect them from damage?
  • How do these compare on maintenance cost and effort?

Since these will be installed in the terrace and there will be about 30ft length of pipe to bring the water down, I'm concerned about the water in the pipe cooling down and the cold water having to be drained before the hot water from solar heater/heat pump in the terrace comes out through the taps. This could mean a lot of water being wasted. Is there any way around this: for example, are insulated pipes available/a good idea in this case?

  • Are you planning to run the domestic water directly thru the collectors, or were you figuring on having the collectors on a hydronic loop feeding some sort of heat exchanger? Mar 14, 2020 at 12:13
  • Also, does the heat-pump water heater you are looking at as an alternative option support use as a hydronic heating source? Mar 14, 2020 at 12:14
  • It doesn’t take full sun to get water hot, I live in Oregon USA we have about 300 cloudy days a year and heat our pool with solar. The problem for residential usage is storage and possibly the water not hot enough so a on demand electric water heater may be needed to bring the temp up to normal hot water standards or you may end up with Luke warm water in the am and hot in the evening depending on your tank size and insulation.
    – Ed Beal
    Mar 14, 2020 at 14:08
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    @ThreePhaseEel: I have no idea about India, but where I am (Germany), hot water for use nowadays is basically required to have a heat exchanger for hygienic reasons. (You could use a tank, but that would need periodic heating for hygiene reasons and that is difficult to guarantee with solar tank). Also, here we use solar-thermic panels as in SolarMike's answer, but in southern Italy I've seen plenty houses having a big black tank on their roof - but they may stem from times when a) hygienic and b) comfort considerations were different (lukewarm shower is much better than cold shower...). Mar 14, 2020 at 23:26
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    @cbeleitesunhappywithSX the idea behind what I'm thinking is that we flip the shell and tube in the OP's HX around, with solar loop/heat pump water (or glycol) in the shell and domestic water in the tubes. This provides hydraulic separation "for free" compared to what the OP is doing with solar loop water in the tubes and domestic water in the shell, and also has less risk of Legionella growth involved with it Mar 15, 2020 at 3:05

3 Answers 3


I see 5 questions:

1) Solar hot water in the morning and on cloudy days. Your usage pattern is worst-case since maximum solar energy availability is soon after your usage. So you will need a storage tank that can keep the water hot until you need it. Lots of variables here so your best bet is to consult an insulated tank supplier in your area. As far as cloudy days, you're either going to have to upsize the tank considerably (probably not feasible) or plan on installing a backup hot water heater.

2) Power consumption of heat pump based hot water heaters. These can be 2-3 times more efficient that a standard resistance heating water heater. The formulas for calculating energy to raise a liter of water 1 degree are straightforward and you can do that yourself. But keep in mind that most of these units also have resistance backups when demand exceeds capacity of the heat pump and that can reduce your savings. Also remember that these effectively cool the room they are in and that can impact your heating costs.

3) Reliability. Heat pump units are more complex that solar units and it's likely will require more maintenance and not last as long. But depending on the solar unit you choose, there may be reliability and longevity issues with that as well. Check with your supplier for their warranties. Also keep in mind that you will have a pump with the solar unit that may need maintenance.

4) Maintenance costs. See above but it's difficult to say without specifics and even then it's a lot of guesswork. You need to rely on the data provided by the supplier of your specific equipment and, as you know, your experience may vary.

5) Insulated pipes. In either case YES, you will want to insulate the hot water pipes to conserve as much of the heat energy as possible.


Photovoltaic plus tankless water heaters plus counterflow heat exchangers (there are youtube videos how to build inexpensive shower heat exchangers) would be also an option. Heat exchangers may cut the needed power by 25 to 45%.

Rentability depends on the relation of the feed-in vs. feed-out tariff, and of course of the installation and procurement costs. Maintenance costs should be much less compared to solar heaters and heat pump. Life expectancy is also much higher, warranty for PV modules is 20 years or more. In many regions there are special subsidiaries for PV, maybe in Bangalore as well.

The PV can also bridge power outages/emergency situations for basic needs during day time like radio, TV, cell phone loaders, computers, routers, fridgerator, etc.

  • What is your reference that direct solar heating of water has a lower life expectancy than solar electric?
    – rjt
    Mar 14, 2020 at 15:06
  • According to a short google search, the maximal warranty period for (very expensive vacuum type) thermal collectors is the same as the standard 20 or 25 years for nearly all mainstream cystalline photovoltaic panels, but only 10 years, often 5 or even 3 for the mainstream average thermal collector.
    – xeeka
    Mar 14, 2020 at 16:33
  • @xeeka In India, 2 types of solar water heaters are available commercially - a flat plate collector model (typically more robust) and the vacuum type one you're referring to. The flat plate collector style heaters are more expensive, but also much more durable.
    – Rohith
    Mar 16, 2020 at 3:40

You can do this without any electrical parts at all.

Use a solar thermal tank and Thermosyphon flow from the solar thermal panels to the tank.

enter image description here

The panel arrangement in B will tend give a lower higher temperature compared to A but more hot water...

With thermosyphon flow all the pipe runs have to be sloping, up for hot and down for cold.

I had a 1280 litre tank that the sun heated to 70 degrees C in one sunny day in Switzerland, so in India you should not have an issue.

The tank can easily get to above 90 degres C so you need a temperature limiting valve on the delivery. If 1 litre of water leaves the tank at 80 degrees C then it is mixed with nearly a litre to get it to 46 deg C... So you can have ample amounts of hot water. The 1280 litre tank will supply hot water on the 4th day if it started hot and had no sunny days in between.

I have not shown all the parts of the circuit just the main concept.

Take care with solar water systems - I have seen flat plate collectors produce steam on even mildly sunny days when they were being commissioned...

  • Is there a reason you aren't using a reverse indirect for the tank here? This'd let you simply have a manifold or LLH off the tanks that any number of heat sources could plug into, while maintaining hydraulic separation between the thermosiphon-driven solar system and the circulated flow from the heat pump. Mar 14, 2020 at 22:52
  • @ThreePhaseEel I have bought solar tanks from specialist tank manufacturers that can provide a separate coil for each heat source... and they can be finned for improved heat transfer...
    – Solar Mike
    Mar 15, 2020 at 4:30
  • "The 1280 litre tank will supply hot water on the 4th day if it started hot and had no sunny days in between." Wow! Didn't think insulation can be that good!
    – Rohith
    Mar 16, 2020 at 3:43
  • @Rohith if the tank is at 80 deg C at the bottom that is a lot of warer especialy when reduced to 45 deg C for use. A tall tank with properly fitted insulation does help.
    – Solar Mike
    Mar 16, 2020 at 6:48

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