Solar PV - Savings on Electricity Bills

When looking at consumption of all kinds, many of us strive to reduce usage to save the planet whilst for others the primary motive is to save money, however, when looking at energy the two are inseparable. Reducing the amount of energy consumed through improving levels of insulation and purchasing more efficient appliances is pretty straightforward, after all, the less energy you use, the lower the bills, but is that basic concept disrupted when looking to install microgeneration technologies such as solar PV.

Saving energy & money in a non-microgeneration environment.

Without employing microgeneration technologies the basic concept is that the less energy we use, the lower the environmental impact and the lower the bills we receive, the only major exception to this being afforded by making use of 'off peak' tariffs where available.

Energy demand fluctuates considerably throughout the day to a generally predictable pattern which is defined by our own aggregated lifestyles - we wake up, do our normal things as we normally do on that day, we go to sleep - new day/new week - repeat. Obviously, this variability in demand requires the energy sector to supply sufficient energy at all times, creating an imbalance in costly generation equipment utilisation. This imbalance shows as a peak during the day and a trough overnight which the energy industry has historically addressed by encouraging demand shifting by offering lower overnight tariffs, however, as these options normally include higher daytime tariffs they invariably make financial sense only when total consumption is high and weighted towards overnight usage.

Saving energy & money in a solar PV microgeneration environment.

When installing microgeneration technologies, the basic concept moves on from just using less energy, introducing the variables of when and how.

Energy Usage Awareness

Through becoming familiar with the capabilities and generation patterns of their installations, users of microgeneration technologies generally become more aware of where and when energy is being used. This often leads to a drive to further increase appliance efficiencies and more effective use of energy which is being generated in-house, initially thought through and planned in detail, but often resulting in a change in habitual behaviour to that apparent pre-installation.

Self Consumption

The basic concept behind grid connected microgeneration technologies is to continually monitor network conditions, replicate those conditions and feed generated power to the grid. To ensure that power flows out of the property, the microgeneration equipment adjusts the voltage to be slightly higher than that currently apparent on the grid. Whenever a demand is created within the property, power will effectively flow from the microgeneration source because the higher voltage prevents energy imports, this being the case until the demand is higher than the level of microgeneration, in which case the shortfall is made-up by importing power from the grid. Effectively, if a microgeneration source is generating it will form the primary power source and domestic demand will always use that source first before calling on the grid to make-up any shortfall.

Passive Saving

Of course, during daylight hours, solar PV will either meet or contribute towards baseload electricity demand required by items and appliance which normally need to be powered all of the time (refrigeration, timers, telephone, router, alarms etc), therefore reducing electricity imports and bills. Although electrical items which form the baseload normally have relatively low power requirements, the fact that they require to be powered all of the time results in a considerable proportion of typical electricity bills, therefore reasonable savings are possible without the need for any specific change in consumption habits.

Active Saving

Effectively the saving potential for baseload is a given, however, the saving potential for high load appliances depends both on usage patterns and weather conditions.

Planning to utilise power generated by solar PV when generation is expected to be high will increase self-consumption and therefore help reduce electricity bills. Of course, the value of the savings will depend on the ratio of microgeneration to imported power, for example, using a 3kW kettle in dull conditions or at night will result in importing 3kW from the grid, whilst doing so under bright cloudy conditions with 1kW being generated reduces power import to 2kW and doing the same in full sunlight when generation exceeds 3kW there'll be no requirement to import from the grid. Obviously, there's both environmental merit and financial benefit to be gained from attempting to shift a proportion of demand to match the microgeneration supply profile.

Example of a day's generation for a UK 4kWp approx South West facing array showing

actual (green) & theoretical curve (blue). Note how late morning light clouds and more

heavily clouded mid afternoon skies effect potential generation and the length of

time the system is still generating around 2kW-2.5kW

Shifting demand is really no more complex than planning to use high power appliances such as washing machines or dishwashers at times when microgeneration is likely to be high, which for PV depends on orientation, but generally allows a window of at least 4 hours for most of the year. Of course, if looking to run multiple appliances at the same time, consideration must be given to the combined power requirements against available generation, for example, running a dishwasher and a washing machine at the same time (parallel) would invariably result in more energy being imported from the grid than staggering use and running one after the other (serial) even when sunshine conditions favour a typical solar PV system.

In order to understand the balance between the power available through microgeneration and further optimise savings, the provision of simple equipment to monitor generation and usage is recommended. Through monitoring both power generation and whole house demand in near real-time on readily visible remote monitors, a level of information becomes available which can be used to optimise self-consumption and provide savings far in excess of those available through basic demand shifting; although, for some, this may be taken to a level which could easily be considered a little obsessive..