Solar PV – A Retrofit Case Study
Specification:
- Detached house of 140 sq meters
- Occupants: 2 Adults and 2 children
- Household annual electricity usage: approx 2000kWh
Roof mounted PV array – eight 165Wp modules
Calculation of the annual electricity requirement for this household
A typical energy efficient 140 m2 house uses approximately 2,000 kWh of electricity annually for everything apart from heating and hot water. Approximately 75% of this is consumed during the daytime tariff period and approximately 25% during the nighttime tariff period.
In addition to this most houses with boilers use electricity for hot water outside the heating season… typically for 30 weeks in the year. The electricity requirement for this in an energy efficient four occupant 140m2 house is approximately 12 kWh per day… or 2,520 kWh annually. Approximately half of this is consumed during the daytime tariff period and half during the nighttime tariff period.
The total cost of the above electricity today (July ‘09) is approximately €900 per annum.
The figures for a similar but inefficient house will be approximately double the above.
Main components of the system
A typical planning exempt PV system consists of nine 165 to 180 Wp modules (each c.1.6m x 0.8m) - i.e. c.12m2 in area; one inverter and one EMMA controller. The cost of the all of these components including the mounting system for the modules is in the region of €11,500 including VAT. The cost of installation is typically €1,200 for the roofer plus one day for an electrician.
Ground mounted PV array – ten 165Wp modules
Integrated PV tiles – twelve 52 Wp tiles
The modules are fitted first. This usually takes an experienced roofer one day, comfortably. The roofer normally connects the module to module cables and leaves the end cables in the attic. The inverter and EMMA controller are then fixed to the wall beside the “fuse board”. The inverter can be fitted in the attic. The system is then connected up by an electrician. This usually takes an experienced electrician less than one day.
The inverter must be certified as being compliant with the Irish requirements set out in the European Standards for grid-connected micro-generators…. known as EN 50438. A complete NC6 form must be submitted to ESB Networks four weeks before the inverter is connected to the grid. The inverter and EMMA controller must be installed by a properly trained electrician.
ESB Networks compliant inverter
EMMA controller – this matches the load to the PV output
Import-export meter
To get paid for exported power you need to apply to ESB for an import-export meter. Some of these are being installed free. Others cost a couple of hundred Euro. The simplest thing to do is to wait for your smart meter to be fitted free in due course as this can double up as an import export meter.
A properly placed planning exempt PV array will generate in the region of 1,600 kWh of daytime electricity annually. With EMMA control all of this will be used to displace imported electricity. This system will reduce the cost of imported electricity by approximately €300 per annum or 33%.
PV arrays are very long life, fit and forget systems. They are not cheap. The best way to assess their value is to look at them as a long term investment as follows:
The initial investment is approx.€13,500. This installation generates the equivalent of a before- tax revenue stream of c.€500 per annum (the amount of income you would need to generate before tax to pay for the displaced electricity) and is therefore the equivalent of an initial taxed return of 3.7% per annum. This is significantly more than the best return available at the moment for similar long term investments (e.g. Building Society Deposits)… which is c.3%. The great thing about the investment in PV is that it:
a) Gives a better initial return than most comparable investments
b) Gives a return that will increase in line with increasing electricity costs
c) Substantially improves the BER rating of your house… and it’s value
On top of this you will be substantially reducing your carbon footprint and reducing related carbon tax payments if/when they are introduced.
If you are building a new house, a properly designed PV system is the most economical way to comply with the Building Regulation renewable energy requirements.
Roof integrated PV modules – twelve 180 Wp modules
Tim Cooper - Director CGE Cool Power Ltd