• What are Photovoltaic panels, and how do they generate electricity?
  • Can solar panels provide sufficient power to meet the energy needs of a typical house?
  • Do solar panels provide enough savings to justify the expense of their installation?
  • Can photovoltaic solar panels be incorporated without compromising the elegance of a new or existing building?

The solar panels are called photovoltaic (PV) because they turn light (photo) into electrical current (voltage). This process is made possible by wafer-thin cells composed of crystalline silicon. Tiny amounts of phosphorous and boron in the cells turn the silicone into a semi-conductor. When sunlight hits the cells, an electrical charge is created.

The photovoltaic panels convert the sun's energy into Direct Current (DC) electricity. This current travels through cables down to the basement where it will be converted to standard Alternating Current (AC) used by electric lights and household appliances. No moving parts are needed to generate electricity through solar energy, and the installation is virtually maintenance-free. The electricity produced in this way is absolutely free. It does not cause pollution and cannot contribute to smog, acid rain, or global warming. The energy is clean, silent, and unlimited.


About 10 m2 of photovoltaic panels are needed to generate a kilowatt of electricity. Depending on the estimated energy consumption of the building, it is perfectly feasible to install enough panels to generate more electricity than the building would consume. If average domestic consumption is approximately 2kWh per month per m2, and average generation capacity is 15kWh per month per m2 of panel, then about 0,15m2 of panel would be required for each square metre of building. For example, a 200 m2 house would require about 30m2 of photovoltaic panels, equivalent to the footprint of double garage.

So, what happens at night or if it rains all day?

Some solar energy systems store the collected energy in a battery for use when the sun isn't shining. But a battery installation large enough for normal house would be expensive and space consuming, and should only be considered an option when it is more economical than a new connection to the main grid (ie for isolated rural buildings with no existing connection. Instead, excess electricity generated by the solar panels is stored in the main grid for future use.

This is done via a bi-directional electric meter. When excess energy is generated, the meter spins backwards. The unused energy goes to the regional electricity distribution system (the grid). The building that generated the excess power is credited. At night or in cloudy weather when the solar panels cannot generate sufficient power for household use, the meter spins forward. Power is drawn from the grid. The difference between power generated and power used is called net metering.


The viability of a photovoltaic installation depends on the following factors:


Design criteria

Economic criteria

Environmental criteria

The location and orientation of the generator panels will affect the efficiency of the installation, although photovoltaic panels are less susceptible to factored losses than thermal panels. Ideally they should be south facing, inclined to maximise solar absorption, and free from shadows. They can be incorporated in roofs, facades, or laid horizontally, without losing too much generating capacity.

The design of the house should facilitate the incorporation of the panels. A well designed sustainable house with plenty of natural light and low energy domestic appliances will have a lower electricity consumption than a profligate house with air conditioning.

The economic criteria may be either self sufficiency, maximum return on investment, or in the case of existing buildings, the reduction of running costs. Self sufficiency can be relatively easily achieved, exemplified by the example quoted above. Here in the mediterranean, a well designed house with a few photovoltaic panels can easily generate as much energy as it consumes. However, in order to maximise the return on investment, it is also desirable to balance the costs of the installation with the revenue generated by the power that is returned to the grid. Here in Spain, and in most other European countries, net metering permits net generators to be paid for the difference between electricity generated and electricity consumed, usually at a premium. In Spain, for example, the bonus for generated electricity is 5 times the tariff for electricity consumed. Therefore a building that generates more electricity than it consumes will ensure a rapid return on investment, and will generate cash as well as kilowatts. The objective, therefore, is to design an energetically self-sufficient house with a generating capacity that would permit a return on investment within 6-7 years.

The environmental advantages of photovoltaic generating systems are self evident. For each kW of electricity generated, there is a saving of 20 barrels of oil, 300kg of acid rain emissions and 7,2 tons of CO2 over 10 years, which far outweighs the carbon footprint left by the manufacture and recycling of the panels.


Photovoltaic panels can be an ugly imposition, or they can be elegantly integrated into the design. Which is why it is advisable to consult both with an architect and an installer before deciding where and how to install the panels on an existing building.

Most photovoltaic panels are only a few inches thick. The latest generation panels can be installed instead of roof tiles, or may be unrolled like a carpet over complex roof forms. The panels are unobtrusive, and are mounted several inches above the roof or wall surface, leaving space for air to flow under the panels. Without proper air flow, solar panels reach extremely high temperatures, which reduces their efficiency. Photovoltaic panels may be used to substitute roof or wall finishes. They may be bolted to retaining walls away from the main building. The ingenuity of the designer of the building will determine whether the installation is elegant or not, as the panels themselves should not shoulder the blame.


5 tears ago, a domestic self sufficient photovoltaic installation had a return on investment period of 12 to 15 years, which considering that the usual guarantee period for the panels was 12 years, was not really a viable proposition. As energy prices have increased, and will undoubtedly continue to do so for ever, the efficiency of the panels has increased greatly and manufacturing costs have decreased, the current situation is far more favourable. In Spain, the cost of the installation has reduced considerably, with a 10% tax rebate on solar installations, a 20% grant available for the panels themselves, and the availability of interest free loans for the remaining overall cost of the installation. Photovoltaic generators are also obligatory on larger commercial, industrial and residential developments.

In 2007 we designed a solar energy installation for an existing house which we were converting at the time, using both photovoltaic and thermal panels. The thermal panels were designed to supply 100% of the domestic hot water, 80% of the heating requirements, and guaranteed a pool temperature of 27º for 10 months of the year. The photovoltaic installation was designed as follows:

A 3kW 30m2 installation mounted on the roof with two inverters and net metering.

Max. generation: 580 kWh/month - July

Min. generation: 246 kWh/month - December

Annual total: 5,070 kWh

Cost: 26,000€

Grant: -5,200€

Tax rebate: -2,080€

Total Cost: 18,720€

Annual surplus 2,200€

Return on investment: 8,5 years ( assuming 2007 tariffs)

Environmental benefit over 10 years:

60 barrels of oil

600kg of acid rain emissions

21,6 tons of CO2

That's why.

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