Knowledge: Solar PV
Home affairs
An immersion PV controller is the answer to getting the most out of domestic solar
systems, netting your customers hundreds of extra pounds a year. Bob Morris,
Apollo Solar Electric, explains
A
round half of the annual energy
generated by a domestic solar PV
system is exported to the grid; for
a 4KWp solar PV installation this
is energy which could be worth over £250 per
year at today’s prices.
The export tariff pays the generator 3.3p
or 4.6p per unit for 50 percent of the total units
generated on the assumption that half of the
generated units will be exported, but this can
still be less than one quarter of the cost of
buying the exported units back.
Since much of our energy consumption
will be at times when no energy from PV
is being generated we will always need to
buy the exported units back in the form of
electricity, gas or oil.
To get the most from a microgeneration
system it is important to use as much of the
generated energy as possible at the point of
generation, i.e. in the home. The problem with
this is that at the times when a PV system is
producing the most energy, consumption in
the home is often at its least.
Using as much as possible of the energy
being generated sounds simple enough but
it’s more diffi cult in practice than many people
imagine. Many appliances do not present a
static load, their power requirement constantly
varies. In addition the energy generated by
the PV array is also constantly varying, and
even with a careful eye on generation and
the use of household appliances it is almost
impossible to closely match supply with
demand.
The answer lies in automatically storing
the excess energy produced during the
day until it can be used later. Every unit of
energy that is stored rather than exported
will avoid the cost of repurchasing it later.
A further bonus is that in the majority of
cases the export payments will still continue
to be received as these are based on units
generated and not units actually exported.
One obvious method that comes to
24 | www.renewableenergyinstaller.co.uk
One direction: Diverting surplus PV generated electricity to an immersion heater will prevent it being
lost to the grid and being ‘bought back’ later, says Bob Morris of Apollo Solar Electric
mind for storing excess PV energy is to use
batteries. Battery storage has the advantage
that the energy can be stored until needed
and can then be used for any purpose such
as lighting or powering appliances. There
are however signifi cant disadvantages with
battery storage solutions including size and
weight of the batteries, system complexity,
battery life expectancy of around fi ve years
and cost. The cost to implement a worthwhile
10-15KWh of battery storage would be several
thousand pounds.
For many homes however there is an
alternative energy store already available
at no cost – hot water. All homes need hot
water and 75 percent of British homes use a
hot water cylinder to store hot water. A 200L
hot water cylinder at 65°C can store around
10.5KWh of energy.
By using an intelligent immersion
PV controller that can monitor PV energy
production together with household energy
demand, any excess energy can be captured
and stored in the form of hot water until
required. The cost of fi tting an immersion PV
controller is just a few hundred pounds and
could have a payback of less than two years.
At current FiTs levels an average 4KWp
PV system can be expected to generate
around £880 per year in FiTs Revenues
and electricity cost savings. Fitting a PV
immersion controller system enables the
household to almost double the usage of the
onsite generated energy, adding up to £280
per year to the income and savings from the
PV system. For an all electric household an
increase of over 24 percent.
Every unit of energy that is
stored rather than exported
will avoid the cost of
repurchasing it later