The Province of Alicante is one of the sunniest places in the world but even today there are a lot of houses still using mains electricity for water heating.
A Solar Water Heater installation has been a legal requirement for all new properties built in Spain since 2006. In Europe, Greece and Israel lead the way, but here in Spain, Solar Hot Water has not had the impact one would expect especially when we hear the often quoted “…with over 300 days of sunshine every year …”.
One could argue that government policies in recent years have been less than helpful with about turns on feed-in-tariffs, reduced incentive payments and the debacle of the so-called “sun tax” on electric systems, prime examples showing complete lack of cohesion and no real direction. Having said all that, Solar Hot Water systems are highly efficient heat converters and still make economic sense.
How much can I save if I invest in a Solar Water Heater?
The first question to ask is how much does the electricity cost to heat your domestic hot water now? I’m going to offer a simple example and I’m going to have to make a few assumptions I’m afraid. Mains water temperature in Alicante is typically 15-20oC. The powers-that-be advise that the minimum temperature for storing domestic hot water is 60oC. Assuming I use 80 litres per day, and I want to raise the temperature of the water by 40oC.
(Next is the science bit so skip down the page if maths gives you a headache.)
kWh (Q) required to raise the temperature of 80 litres of water by 40oC
Q = mc(ΔT) in joules or watts/second where:-
m = mass of 80L of water (1L = 1000gms)
c = specific heat of water 4.184 joules/gram
ΔT = temperature difference 40oC
Q = 80*1000*4.184*40 = 13388800 watts/second
divide by 3600 = watts/hour = 3719
divide by 1000 = kW/hour = 3.719
So with a 2kW heater element it will take 1.86 hours to get the tank up to temperature which is 3.719 kWh. I am paying 0.167€ per kWh for electricity so if I used an electric boiler it would cost me 0.62€ per day or 226.69€ per year. The water temperature will drop because of heat lost through the tank insulation causing the thermostat to cut in, so we’ll include 20.00€ per year for heat losses, grand total 246.69€ per year.
Every household is different so what you pay could be more, it could be less, if you know how much water you use and your own temperature setting, use these to make your own calculation.
The second question is how much domestic hot water will I get from a solar water heater?
First of all, I know its stating the obvious but … solar water heater performance is dependent on the heat from the sun (it’s called Insolation). Insolation is the amount of solar-generated heat and is measured in kilowatts per square meter per day (kW/m2/day). And yet another glaring fact, it is hotter in summer than in winter! I’ll focus on the winter months only because the rest of the year there is an abundance of sunshine and solar hot water is really, really hot!
Heat generated by the solar water heater during the winter months
The 10 year average data for Alicante shows average insolation levels to be 2.17 over the winter months.
Assuming a collector area of 1.8 m2 and 95% efficiency, the average amount of heat generated per day = 1.8*2.17*0.95 = 3.71kW which is what we need to maintain the water temperature. In reality, some days there will be enough heat generated, other days there will be less and this is when it is necessary to use the standby immersion heater. In practice this means I have to switch on my immersion heater for approximately 20 days per year which will cost about 12€ altogether.
A good quality Solar Water Heater costs around 2100€ fully installed, so using the simplistic scenario outlined above, the Solar Water Heater would pay for itself in about eight years. If you take a proper view of payback, you must factor in real future energy price increases, take account of inflation etc. so depending on individual circumstances the payback time could be much faster.