PV Energy Management in a post-FIT Environment

PV system owners are becoming increasingly aware that their systems generate the most energy at a time when household demand is typically at its least, i.e. during the day when the sun is shining and the occupants are out at work. This results in surplus energy being exported to the grid. With a 4KW PV array if all the energy generated were to be used in the home it could be worth £500 or more per year.

In a post FIT or minimal FIT environment PV owners are looking at the value of the energy they are exporting and changing habits to use more energy when production is at its highest.

While this is generally the right thing to do the problem with most solar PV installations is knowing exactly how much power is being generated, used and exported at any given moment in order to know when is the best time to use power. To help solve this problem there are a number of products available which give the householder much more visibility of system performance and if there is surplus energy which can be used.

Using as much as possible of the energy being generated sounds simple enough but it’s more difficult in practice than many people imagine. Most appliances do not present a static load, their power requirement constantly varies as heaters, motors and pumps cut in and out. Add to this the fact that the energy generated by the PV array is constantly varying, and it can be seen that even with a careful eye on generation and the use of household appliances it is almost impossible to closely match supply with demand.

The only practical way to recover as many of those valuable export units as possible is by using an active energy management system such as the Apollo GEM. Such a system will accurately monitor micro-generation energy production together with household energy demand and then ensure that any excess energy that would have been exported is put to some useful purpose or stored for later use. Since every home requires hot water, and hot water stores energy, it makes a lot of sense to use the surplus energy to produce hot water. The Apollo GEM active energy management system will capture surplus energy as it is generated which then stays stored in the form of hot water until we are ready to use it.

A typical 4KWp solar PV system in the UK can generate 3800-4200 units per year however much of this energy is generated in the daytime when it is not required and on average around half will be exported to the grid. If all of this surplus energy could be usefully used this would be worth around £280 at today's energy prices. The GEM system is able to recover much of this surplus energy and store it as hot water. In practice not all of the available surplus can always be used due to the fact that during the summer months more surplus energy is produced than is needed to fully heat the water resulting in some exports, albeit greatly reduced.

In order to further reduce exports, in addition to diverting excess PV energy to a water heater, GEM can work in a threshold mode which allow excess energy to be diverted to other appliances if the water is fully heated and surplus energy is still available.

The table below shows example savings with the GEM system heating a 150L hot water cylinder using data taken from a real installation in the UK.

With a larger hot water cylinder more of the peak summer energy can be utilised resulting in further savings as shown in the following table.

In Summary, around half of the energy generated by solar PV systems is exported to the grid; this is energy which is could be worth £280 pa at today’s prices. The GEM system can be installed with a new PV system or retrofitted to existing systems. With the lower feed-in tariffs now on offer, the additional energy and cost savings obtained from installing such a system are significant and could boost the overall return from a 4KW PV system by 50%.