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Photovoltaic Cells v Solar Hot Water Panels

By Jonny Ritson     overspong@gmail.com

'Jonathan Ritson is a freelance sustainability consultant, please contact him for advice or help on green projects' 




Picture this- you’re up to your ears in loft insulation, every draught has been excluded, every window double glazed (or triple if you’re really going crazy), the boiler is the most efficient money can buy and is rarely used thanks to all the other work you’ve done on the house… you’re feeling happy. Content. But a nagging doubt remains- could you do more to save energy?

It could be time to harness the sun to do something more useful than make the coleslaw a warm, wasp attracting mess at the summer BBQ.  Any eco-minded person knows the available options but their relative pros and cons are often lost in a maze of government schemes, supplier offers and general clamour to part you with your cash.

So, which is going to be better; Photovoltaics (PV) or Solar Hot Water (SHW)?

First off you might want to check if your house is suitable for any solar technology. The best energy savings are achieved with South facing roofs at a 30o incline with no over-shading, although you can still use the technologies even if you don’t have this. It’s simple enough to check, but remember that tree next to your house is going to grow over the next 30 years.

Solar Panels

You might also want to think about your location in the UK- if you’re basking in sun-drenched Cornwall you’re going to make more savings with solar than in the far flung corners of Scotland. Check your location with tools such as the free PV-GIS (http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php) to tell you the average amount of solar energy where you live.

Each technology has its merits - you either plump for free electricity with PV or free hot water with SHW- careful consideration of your needs is going to be required before you’ll know which will be best for you.

From a carbon reduction point of view PV tends to win out. Although it produces less energy, (700kWh vs. ~1,300kWh for a typical* installation) your PV panels offset electricity, which has a much higher carbon factor than the gas saved by SHW (just think of all those coal-fired power stations that we use to produce our electricity). In your first year you’re likely to save around 370 kgCO2 with PV compared with 260 kgCO2 for SHW.

But aren’t we getting rid of all those coal-fired stations?

The future carbon factor of electricity is expected to fall significantly as we move to more renewables and new nuclear providing energy for the national grid. Gas isn’t expected to change as the methane you’re burning now will be identical to the methane you’re burning in 30 years (although the DECC haven’t addressed the fact that extraction will become more difficult as reserves dwindle)

At current DECC estimates SHW could potentially save more carbon over 30 years but until we actually get rid of the coal-fired power stations  there are only rough guesses available of how much CO2 will be produced per kWh of electricity in the future.

That brings us to cost: SHW panels are significantly cheaper than PV, as long as your heating/hot water system is capable of adapting to pre-heated water. However at present PV again wins out, this time for two reasons.

Firstly, electricity is more expensive than gas. A lot more expensive. Calculations for Building Regulations use a price of 3.1p/kWh for gas compared to 11.46p/kWh for standard tariff electricity, although you’re likely to paying more for both from you energy provider.

 Secondly, you’ll receive a 21p/kWh premium for electricity produced via PV under the Feed in Tariff (FiT). Added on to this is a 3.1p/kWh export tariff if you don’t use the electricity yourself or an 11.46p/kWh saving if you do, giving potential savings of 32.46p/kWh. This government support is guaranteed for the next 25 years and increases in line with inflation- a handy boost that will yield around an 8% return on investment. That’s 8%, linked to inflation and backed by the government - try getting that out of your bank.

SHW suppliers are still desperately waiting for the Renewable Heat Incentive (RHI) to kick in and offer similar returns- it’s expected to become available in autumn 2012. RHI is currently available for non-domestic installations, offering 8.5p/kWh, giving us an idea of where the domestic subsidy will be placed.

The final thing you need to think about it the usage patterns of your house. Both technologies are running at peak output around mid-day when most people are at the office rather than running a bath. For PV this is not such a problem – you’re still getting your FiT and export tariff even if you don’t use the electricity. For SHW there’s no such luck- you’re going to be producing the largest amount of hot water at mid-day in summer, exactly when your heating and hot water demands are at their lowest.

So where does that leave us? In most situations PV is going to provide a greater return on investment, although the gap will be reduced once the RHI becomes available. In terms of carbon saved PV is winning at the moment but in 30 years time as the national grid gets cleaner it could be eclipsed by SHW.

SHW is not to be completely discounted- in some scenarios daytime demand for hot water can increase the savings. Any building which has constant hot water demand can benefit with examples of successful installations for commercial kitchens, schools or any buildings with changing/shower facilities for their users. For the domestic user though, the meagre savings of around £50 a year mean there are better options out there.

The Future

Green technologies are ever changing and solar is no different. Photovoltaic-Thermal (PV-T) systems are becoming available which offer both PV and SHW combined. This has dual benefits as PV efficiency actually decreases as the panels get warm- by using water as a coolant PV-T systems manage to increase the electrical efficiency of the PV cells whilst providing free hot water… a win-win situation.

 Photovoltaics (PV)

Take up for the technology has been slow as prices are still high and it is yet to be recognised in the government’s Standard Assessment Procedure (SAP), which is used to check compliance with Building Regulations. Once the technology is adopted into this assessment method expect to see it being increasingly incorporated into new-build developments and prices falling as production volumes increase.

*I’ve assumed a 1kWp PV array and a ‘large’ SHW panel by industry standards, both with 10% losses.

 

 

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