How are PV systems rated?

Solar photovoltaic (pv) systems comprises of two main components, the panels and the inverter.

Systems are rated as the total of the maximum outputs of all of the modules, e.g. a standard 4kWp system could build from 16 x 250w panels:

16 x 250W = 4,000W = 4kW

These panels are then wired into an inverter that has its own rating.  A 4kW system would typically be matched with a 3.6kW rated inverter.  This is as per the manufacturers’ recommendations. The lower rating helps ensure the inverter runs closer to its peak efficiency for more of the day.  The inverter will have the optimum power delivered over the year’s variety of weather conditions. An overrated inverter may potentially perform better in the (very rare) “peak” power conditions but taken across a typical year it would likely under perform.

The inverter efficiency is very important regardless of which panels are used – it determines the amount of DC power from the panels that will be converted to AC, usable power in your home or business. Using a quality inverter will ensure not only a better yield but the device will last longer saving money on future repairs or upkeep.

Panel “efficiency” relates to how much of the light energy falling on the panel can be converted to electricity. A more efficient panel will therefore take up less space to achieve the same wattage output. Where space is not an issue a low efficiency panel might be preferable as they are usually cheaper. What is more important is the yield a system will deliver over a year in a variety of conditions. Yield is measured in kWh (kilowatt hours) and a prediction of the annual yield of each system we design is provided in the detailed documentation you will receive upon obtaining a free survey from us. At Sunlight Future we look to optimise systems so that the yield is maximised as through the Feed-In Tariff scheme you are paid for every kWh you generated (your electricity company isn’t going to pay you based on how efficient or expensive your panels are!).

Why does everyone talk about 4kW solar arrays?

Customers often enquire about installing 4kW solar arrays or are under the impression that this represents the largest or best solar PV arrays available. So, where does this magic number come from and what does it mean?

The main reason why companies often promote prices on 4kW of solar is that, under the old Feed-In Tariff scheme, 4kW systems were the largest system that could qualify for the highest tariff band, earning the owners the highest payments per kWh generated.  This usually equates to the greatest financial return per pound invested. That said, the Feed-In Tariffs run for systems rated up to 50kW and in general as the systems get larger the cost per kW drops.  It is part of our service to help customers find the sweet spot – balancing the size of their investment with the energy generated and the projected financial returns.

Under the new Feed-In-Tariff regulations the payments per kWh generated from a 1kW system is worth the same as from a 10kW system, so there is no longer a benefit of installing exactly 4kW of solar. However, around 4kW is still a useful system size as this equates to roughly 16A (amps) placed on a single phase electricity supply. Anything above 16A per phase needs permission from the Distribution Network Operator (DNO), such as Western Power Distribution or SSE, whereas below this threshold systems can be installed and the DNO simply needs to be informed of the connection.

Many, if not most of our domestic customers don’t have enough roof space for 4kW unless they opt for pricier (more efficient & smaller) panels.  Because of this we always quote for several system variants so that you can really see the cost-benefit of different rated systems, built with different components. We often send quotes out for systems as small as 1.5kW that still offer a great investment.