Solar Panels

Solar PV panels convert SUNLIGHT into ELECTRICITY.

These are different from Solar Thermal panels which use the Sun's HEAT to provide HOT WATER.

The electricity from Solar PV panels is in direct current (DC) form. For use in the home this has to be converted into alternating current (AC). This is done using an Inverter.

The inverter directs the power to the household appliances. If there is not sufficient power from the panels to meet the needs of the house at any given time, the house will draw the extra needed from the Grid.

Solar PV generates ELECTRICITY

Solar THERMAL provides HOT WATER

Components of a home solar energy system

Solar PV panels: These convert sunlight into direct current (DC). They do not need heat. On a cold day in January if the sky is blue, the sunlight will be sufficient to generate close to 50% of the amount generated on a hot July day.

Of course, most days in winter are overcast so the average output in winter tends to be up to 20%. General advice is to maximise the number of panels, in all directions.

Inverter: The Inverter is an essential piece of equipment in a solar PV system. This converts the DC from the solar PV panels into AC to power the home It also directs the flow of the electricity in the system. An inverter has a limit of about 5kW AC which can supply the home or get exported. A hybrid inverter has an added DC capacity of between 3kW and 5kW. This allows the inverter to charge the battery from excess solar power or from the grid.

Battery: This is an optional item and is generally recommended by ~90% of solar owners of larger systems. It stores electricity which can be used when the supply from the panels dips or stops. It can also be charged from the grid overnight at cheap rates for use during the day when electricity is expensive.

Great if you have a Heat Pump. You can charge the battery from the grid at night rate and use this power to run the heat pump during the day, cutting the running costs by maybe 30%.

The inverter must be a hybrid to accommodate a battery.

A basic battery is, typically, 5kWh but general advice is to have a big enough battery to cover one day's usage. This could vary from 10kWh to 30kWh.

EDDI (Water diverter): This is an optional item. These are intended to use solar energy that might otherwise be exported to the grid to heat the water in the immersion. So effectively the water is being heated instead of the household being paid for the export. There is evidence that water diverters accelerate to life cycle of the immersion due to the increased on/off cycles.

System Design

There is no one best system. Systems should be designed to suit priorities of the household.

Factors include size of household, orientation of house, type of roof, annual electricity usage etc. Your contractor will give you a design based on your requirements. You will find that different contractors will give significantly different designs. Get at least 3 quotes. Look for independent advice on Irish Solar Owners Group Facebook page.

System Size and performance.

System size or potential output is quoted in kiloWatts peak (kWp). This is the maximum output it will achieve in near ideal conditions. Systems will get close to the peak output around midday on sunny days in the summer months. Different panels have different output so the number of panels does not necessarily indicate system size.

Typical system sizes vary from 3kWp to 10kWp, though larger or smaller systems have been installed.

Irrespective of the size of the system installed the payback period is typically 6 to 8 years. Some households have chieved payback in 5 years, but that is not common. Larger electricity users tend to get a better payback.

Orientation

Solar PV panels facing South will get more sunlight for a longer period during the day. Therefore, you will get maximum output from the panels throughout the year. For panels facing East and West the norm is to divide the panels 50:50 in each direction, but this is not always possible. Those facing East will get the benefit of morning sun, those facing West will get evening sun. While the combined output will not match the South facing panels, they can have an advantage of producing electricity over a longer stretch through the day, with peaks in the morning and evening (see the graph below).

This can result in a higher proportion of the solar energy being used as it is being generated. South facing panels tend to over-produce in midday and under-produce in morning and evening.

Solar energy which is used by the household as it is being generated is worth the full market value per unit. If you use say 20 units of electricity from solar PV panels you will save by not having to buy those 20 units from the grid. This can be referred of as displacement or substitution but more often it is called Self-Use.

The Green line in the graph above shows the possible combined output in the mid-summer for 8kWp system divided east/west. As you can see, the peaks in output occur when the electricity is needed most. So more of the electricity is used in the house and less exported.

The RED line is the capacity of the inverter so EAST/WEST panels tend to remain under the upper limit.

The next graph shows the comparison between 8kWp South facing and 8kWp East/ West facing. As you can see, the South facing system produces mainly around the middle of the day when it is not needed so much and less between 7am and 9am and between 5pm and 7pm when the home uses electricity most

Over-production is generally exported and is worth about half price. If a battery is installed more of the solar energy can be Self-Use.

Solar energy you USE YOURSELF is worth TWICE as much in your pocket as energy you EXPORT.

East/West facing panels tend to have higher proportion of their output as Self-Use than South facing panels. So, the Value gap between both systems is not as great as you might first assume.

Annual Usage

Self-Use is the amount of electricity generated by the solar PV panels that is used by the household. Households with high consumption of electricity tend to achieve higher levels of Self-Use and thus higher savings. So households with Heat Pumps and Electric Vehicles gain more.

Equipment Life Expectancy

Householders are often worried about how long the equipment will last.

The "life-expectancy" of solar panels is about 25 to 30 years.

The inverters are usually guaranteed for 5 or 6 years, but will probably last closer to 10 years.

Batteries are guaranteed for at least 10 years but may average up to 15 years.

Return on Investment for Solar PV Panels

The cost of the system will almost always be paid back within 8 years, and in some special cases it can be as low as 5 years. For most households the payback will be between 6 and 7 years.

Shading will impact the performance of solar PV panels. When panels are connected together in a string, shading on one panel impacts them all. If possible, all panels should be placed so as to avoid shade, particular from 10am to 6pm. If this is not practical individual panels may have optimisers. This makes individual panels independent of each other.

Clipping

Larger solar PV panel systems tend to have peak capacity which is up to double the capacity of the inverter. The inverter will limit the output from the panels. The output from the panels is "clipped" to the capacity of the inverter. This occurs mainly on bright days from May to September and is more pronounced in South facing panels. The easiest way to limit clipping is to have a battery to take the excess output. Your contractor will calculate the appropriate battery size. As an indicator an 8kWp system might need 20kWh battery.

Grid power outage – Changeover switch

It is likely that with increasing number and ferocity of storms in the coming years that the grid will go down for periods over the winter. It is possible to continue to use your solar panel system when this happens if a changeover switch is installed.

However, as power outages are more common in winter it is not likely the solar power will be sufficient to power the home. This is where batteries are important. In the winter batteries should be fully charged at night so that they can act as a back-up when the grid goes down.

Ask the contractor to include a changeover switch in his quote.

Where to go for advice?

The Irish Solar Owners Group Facebook page is a goldmine of information and advice. It has over 85,000 members from all around the country who are happy to share their experiences. It is very well run, and includes members with very high levels of expertise who are very helpful.

Post any quotes on the Facebook page and members with experience in this field will give feedback on the contractor, the design and the price.