Is domestic voltage optimisation all it's cracked up to be?
Posted by James Page on 3 September 2012 at 9:27 am
See recent article: 'Add-on' equipment for solar PV: an opportunity but beware of possible pitfalls
The domestic microgeneration market has seen an increased interest in voltage optimisation (VO) technology. This is either being promoted by voltage optimisation companies, or increasingly as an additional offering by solar PV companies. Proponents of the technology claim high levels of energy and monetary savings. But do their claims always stack up?
To find out, I’ve carried out some technical testing of VO in my own home, with surprising results…
First, the technical bit: the laws of physics dictate that adding solar PV to your house will always increase the mains voltage slightly. As the house is taking less current the voltage drop in the cable from the street is reduced. If you are generating more electricity than you use, the voltage drop is reversed, and the house voltage will then be higher than your neighbours’ – allowing you to export electricity.
The difference won’t usually be more than a volt or two, but it’s of great interest to the electricity network operators, and one of the reasons installers have to notify them of solar installations. The operator is obliged to keep the voltage below 253V so too much solar means they have to turn the volts down at the substation. And if the grid voltage goes over a prescribed value (usually 253V or 264V), your inverter will turn your PV system off.
Perhaps not surprisingly there has been interest among solar customers in Voltage Optimisation. The claim is that by reducing the mains voltage (usually to 220V) there will be significant cuts in power consumption from electricity loads, and therefore reduced bills. But are they all they cracked up to be?
Trials have shown wide variations in savings – in some cases none at all – hardly surprising as electricity consumption varies day to day in an average house. So when our customers started to ask about the technology it was time for me to reach for the screwdriver and test some real appliances in my house.
Different electrical appliances respond in different ways to reduced voltage, which is one reason why estimating the savings is not easy.
Incandescent (filament) lighting will clearly use less energy if the voltage is lowered, but the lights will be less bright. They will last longer, but you may then be tempted to swap some lamps for a higher wattage. A better way to save energy would be to change to LEDs, which are much more efficient and last longer. According to the Carbon Trust there will be no savings by reducing the voltage for LEDs, and in any case the energy used is very small to begin with.
Devices used for heating (cookers and kettles etc) will not use less energy with a lower voltage since the element will simply be on for longer to achieve the same heating effect.
Flat screen TVs use a lot less energy than old TVs but they can still be a significant draw on energy so I tested our LG flat screen TV. The consumption was 54w at both 240V and 220V. I then tried the Dell PC screen. Again there was no significant change in power consumed (25w).
The biggest savings are claimed to be on appliances with motors, so I connected our Siemens A rated freezer (old, but not un-typically ancient) to an accurate electricity meter. Sure enough the compressor motor – when running – did take less power at the lower voltage (75w at 220V and 83w at 240V) but since the motor was running for longer periods it was necessary to measure the average consumption over several days (and measure the ambient temperature). The average consumption at 220V was 14.5w, and at 240V it was 14.7w – less than 2% higher, and probably not significant.
Of course these tests only indicate that voltage optimization won’t help save energy on these particular appliances. Voltage optimisation certainly makes sense on some commercial properties, where a detailed analysis can be made. However, we would urge residential customers to be wary of voltage optimisation technology sold purely on the basis of energy saving in appliances.
From our initial studies, the savings do not arise for most residential end-users, and are minimal where they do occur.
Photo by Tom Magliery
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About the author: James Page is a chartered engineer and is head of engineering at Joju Solar. All views expressed are his own. He stores solar energy under the kitchen floor.
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