Solar PV vs Solar Thermal - What's The Best Way To Heat Your Water?

Did you know there are two fundamentally different ways to generate solar energy, and therefore two fundamentally different types of solar panel? Find out more in this video about solar PV and solar thermal for water heating applications, plus Finn discusses solar heat pumps and PV diverters.

You can read more about solar hot water options in the Good Solar Guide:

[atanner]

Good to see the videos are back. Hopefully your vodcast will be back soon also.

[markimbesi]

In having small grid tie pv and solar thermal, the thermal is more efficient and takes up far less space and activates earlier and stays on later. Living in Canada with the cold and snow the thermal also is better at shedding the snow and performs better in low sunlight conditions.

[markw.]

Excellent clear summary of options. Many thanks Finn. So many of these vids tell me little more than how to suck eggs i.e. the bleeding obvious!

[barryhamm]

I installed top mounted solar thermal hot water a couple of months ago at a cost of around $4, 800. What prompted me was when a cylinder of LPG hit $150 which with cylinder rental would have seen me paying around $700 pa for hot water. I have plenty of PV panels so using a heat pump or replacing my LPG system with a simple resistance electrical hot water were possibilities.

My research indicated that a top mounted solar thermal hot water system needed some replacement valves every five years at an installed cost of around $400 and if done it would have a life of several decades. A heat pump has many more moving parts and hence a shorter life of around 10 or so years. A simple resistence heater again has a life of 10 or so years, plus using electricity, even solar would eat into my exports currently paying 20 cents per KwHr.

A solar thermal system wasn't the cheapest option but I am hopeful that over the long it will prove to be a reasonable cost options and will preserve my solar refunds. Much of course depends upon future FiT, but fingers crossed they will stay high and my solar thermal will continue to make economic sense.

[vijgai]

We just installed a 270 L iStore heat pump & 6.6 kW of solar. Plus also got induction cooktop & a heat pump dryer (Bosch).

[thesnakeanddragonreptilebr]

I’m undecided, I wanted a Heat Pump system but all of the reviews I’ve seen for most models suggests that they are horrendously unreliable and typically fail within 5 years. So I am going back to the idea of solar thermal. So my question is on low solar days can I not use electricity to heat instead of gas?

[alfredlow]

I would appreciate an update on whether the advice in this video is still relevant in 2023.

[FernandoFCadenas]

Brilliant ideas as always. Greetings from Spain

[centralplumbing]

You can also heat water using a 12/24v DC element(Dernod-AMZN) directly from load output, and/or a low wattage 120v element run off of your inverter. My system: 40A Epever mppt charge controller, Outback FM80, a 40 gallon dual element water heater, a 300w 24v DC upper element and a 750w 120v AC lower element(Dernod cartridge heater-AMZN) with a 1''x1/2'' brass bushing.(2) 325w panels connected to (2) 100A 12v batteries in series running off of the Epever. (6) 325W panels on the FM80 connected to a 5k inverter run thru a (5.5k MOES-AMZN) utility/inverter AC transfer switch powering the 750w AC lower element, and selected AC loads(and washing machine/ LPG dryer). lower AC element controlled by thermostat and a timer from 9AM to 10PM(10PM cutoff is to insure some hot water for morning showers). Upper element controlled by charge controller's programmed load output 9AM to 5PM. Between 10AM and 3PM 300w DC element can produce over 400w of power, up to 3.5kw a day. AC element thermostat usually shuts off by 3PM. On cloudy/rainy/laundry days hot water is automatically guaranteed by utility power. Payback? 5kw> daily electricity savings. 2 years now no glitches, 4 person adult household. Do not waste money on solar water heaters of any kind.

[thefutureaboriginal]

Sorry. A better question would be is it worth replacing a natural gas instantaneous water heater with an electric water heater when you have a large amount of excess solar?? It might be better to ask this question if/when I need to replace the hot water system

[stephendoherty]

So to heat your water and have PV solar you need ro pay more for an 8kw system and an extra $1000 for a PV to water scavenger option whereas a solar thermal system needs just a 4kw system and you could add an heat pump ($2k) to augment it for non solar days. Note most lower tariffs from elec utilities are usually just overnight and perhaps weekends. This presumes air heating of your home and domestic water heating uses up more money than running your household electrical appliances and a heat pump can run as an aircon unit in reverse if you need cooler air

[kevindouglas]

Just get a roll of pex pipe and put it in a box. Paint it flat black. I priced it today. It was $100 total.

[cheekyeve]

Great information I wish I (and my installer) knew this when I bought into Solar. One this you didn't cover Finn is how much energy does it actually take to heat say a 100 litre tank before it's hot enough to use and does it take that constantly or just top it up? Thanks.

[David-lrvi]

The even cheaper option is to just use a simple timer to control the hot water. I just had 8kw of solar installed and got my existing old school electric water heater disconnected from the off peak tariff and put on a timer.

On a sunny day even at this time of the year in South East Queensland I’m generating about 5-6kw of solar energy during the peak hours of the day (around 10am until 1pm as my system has an easterly bias). That’s well and truely enough solar to offset the hot water system, my pool pump plus anything else I happen to be using in the house.

On rainy days like it was today the system only generates about 2kw at the peak of the day so not offsetting 100% of the hot water demand or general house usage but exports should compensate for the minimal times that the system isn’t generating enough electricity and I have to draw electricity from the grid.

A solar PV diverter is quite expensive and most people would be better off spending that money on more solar panels (assuming you’ve got the roof space) as the extra panels would probably have a better payback than the solar PV diverter.

[markstemmett]

You could also try a mmpt pv to ac geyser controller from Microcare in South Africa. 4 x 330 w panels give you about 1, 2 x 5, 5 kW of power per day. That’s a 2, 5 year payback

[isrnmn]

As an immigrator from Israel, I'm totally shocked.
Solar thermal is a mandatory requirement in any house in Israel. the cost is up to 2K (including the installation), How do we get to 4k in Australia. someone is ripping us off.

[mkzhero]

Say WHAT now? Here in Israel, a thermal (non electric) water heating panel and system, enough for a household (depending how much people you have and how much you want it to store) will cost you 800-2000$, and considering that in such hot areas as here its effective almost all year long, and these last 5-20 years and require little to no maintenance, and can be far better recycled, its far more cost effective.

[neildelgado]

So where do you get the power of back up water heater? And how the smart solar controller works?

[fastbike]

This video could do with a refresh - 6.6kW system ? Seems pretty small.

[tubeyouser]

Regarding the diverter, wouldn't solar PV provide energy for the entire household, which would include an electric hot water system anyway? Or is the point to dedicate a portion of the solar energy to the hot water system only?