Solar's hot, even when the sun is not

On the cloudiest winter's day in Melbourne, a 15kW rooftop PV system can still produce more power than a household's daily demand. And with the race on to improve efficiency, things can only get better.

On the cloudiest day in the gloomiest weather, when I check my solar system I find it is still generating and exporting clean renewable energy into the grid. My solar system, like all rooftop solar systems, generates even when it's cloudy. That's because solar technology is able to produce electricity under diffuse light conditions.

Generally speaking, in the darkest, cloudiest hour on the gloomiest day, your solar system will be generating as much as 25 per cent of a normal clear day output. On a day with light cloud cover, your system could be achieving as much as 50 per cent of a normal clear-day's hour of production.

On the worst day in winter, the sky covered in thick clouds with only nine hours of daylight in Melbourne, my 100 square metre 15 kilowatt solar system still produces more electricity than my all-electric household's entire daily demand; and in summer on a cloudy day I'm still exporting significant excess electricity to the grid.

One of the reasons that cloudy day output is surprisingly good is that on cloudy days the surface of your solar panels are usually cooler, meaning that the solar cells themselves perform better. There is less light to cause the photovoltaic effect, which is how the photons of light get converted into electricity. But the superior operating temperature compensates considerably for the lower light conditions.

Today, solar panel efficiency is rising (in watts per square metre): the average panel would be 14 per cent efficient in optimal conditions, while SunPower, owned by French oil company Total (the market leader by efficiency), markets very efficient (albeit expensive) single layer monocrystalline panels at 18.5 per cent efficiency. Then comes Sanyo, who have a multi-layer panel which combines a conventional silicon wafer (what you see on most solar rooftops) with an amorphous layer. Today Sanyo are selling panels that have an 18 per cent efficiency, and are predicting they can rise to 30 per cent efficiency in the not too distant future, using this multi-layer technology.

Whether Sanyo or another technology wins out, within a decade we can expect solar panel efficiencies to double and costs to continue to decline significantly as a result of increasing efficiencies.

So in the future, when I swap my 15 per cent efficient solar panels for new 30 per cent efficient units, effectively I will be producing half the best output of today's panels even on a cloudy day. And in the future, anyone who wants the same performance on cloudy days as on clear days today will only need twice the roofspace, and in the worst clouds they'll be producing the same amount of electricity as in the best clear-sky conditions today.

Furthermore, my house and shed is 170 square metres – a fairly representative roofspace for the 6 million average Australian detached homes. And on top I have 5kW of rooftop solar panels facing north, 5kW of Solar facing east and 5kW facing west. My system can be producing significant electricity production in the early morning and well into the early evening.

The total annual production of my system in Melbourne is 18,000kWh for the year. For comparison, my all-electric house (heat pump hot water, induction cooktops, two LCD televisions, laptops, six heat pump heater/air conditioners and a water pump) consumes 4,000kWh in a year. Much of the 18,000kWh of production occurs on cloudy days as, according to the Bureau of Meteorology, Melbourne experiences 180 cloudy days per year. So the fact that rooftop solar produces when it is cloudy is an important one, that needs to be noted by anyone who is considering a solar system or planning solar for a 100 per cent renewable energy future.

Today, an average 170m2 house can produce more than 18,000kWh of annual electricity. There are over 6 million stand-alone houses and if all of those households had the same amount of rooftop solar as my house we'd (theoretically) be producing half of the nation's on-grid electricity demand. Of course, to achieve 25-50 per cent penetrations of PV we'd need to load shift much of what we and many of our businesses, such as water authorities, do at nighttime to daytime, pre-heat and cool our houses during daylight hours (sunny or cloudy) and we'd have to curtail some amount of production in summer. And of course in just 10 years time we'll be able to double our rooftop solar systems output, just by flipping the panels and inverters to new cheaper more efficient ones.

Oh and this is eminently doable – In Germany they installed 3,000MW of rooftop solar in December on their Christmas holidays when temperatures were sub-zero. That's equivalent to 200,000 Australian houses with the same size solar system as mine.

Rain hail or shine, solar will power on.