When & Why Should You Oversize Your Array: In the first Techie Top Tip of 2019, Jordan Wiley our technical partner’s Technical wizard from UK explains the benefits of oversizing an array.
“Oversizing a PV array in relation to the inverter is an easy and cost effective solution to maximise the benefits offered by solar PV. Put simply, it involves connecting an amount of solar panels that is greater in terms of Wattage than the rated capacity of the inverter; which allows the inverter to operate closer to its maximum output for longer periods throughout each day.”
How To Oversize Your Array & Why, by Jordan Wiley
To understand how to oversize your PV arrays you must first know how to calculate their size. This can be done by multiplying the Wattage of a single solar panel by the total number of panels in the array. For example; 16 x 250W solar panels creates a 4000W (4kW) rated PV system.
To find out the size of an inverter either check the physical label on the unit or the data sheet online. If it’s an inverter that Segen sells then this information can be found under the respective tab on the products page of our portal. You’re generally looking for Rated Power or P AC.
On a typical 4kW PV system you may be tempted to choose a 4000W rated inverter to match the potential output of your array, but there are actually a few reasons why you may want to reconsider.
Take a 250W rated panel as an example; this panel is only rated 250W based on STC (Standard Testing Conditions) and these conditions are rarely met in real environments. Panel output can be affected by temperature, weather, module defects, shading and even local wildlife. You may find due to a combination of these factors your PV system is generating considerably lower than its total panel rating in Watts (and if you don’t notice this, the homeowner usually does). Oversizing allows you to reduce the effects of these factors as the added modules should make up for everyday energy losses.
More Energy at a Lower Cost
Also, by maximising the amount of modules connected to an inverter or using less inverters for a set quantity of modules you will deliver a higher amount of energy for a lower total cost. A recent study by SMA on their design tool showed that matching a 100kW PV array with 3 x STP25000TL-30 inverters only produced 2% less energy annually in comparison to the same system using 4 x STP25000TL-30’s. Showing that an extra 25kW inverter on site only increased annual production by 2%.
More importantly in the UK if you were to fit a 4000W rated inverter you would actually be exceeding the acceptable limit for installing under G83. The primary limitation of any PV system in the UK is what the DNO (District Network Operator) allows you to export back to the grid. Despite the upper limit of the UK domestic system climbing to 10kW’s recently – the rules regarding G83 and G59 have remained the same. As such you cannot exceed 3680W or 16A per phase without having to file under a G59 application.
The G83 limit of 3680W is based on the rated AC output of the solar inverter and not the potential Wattage of the total number of installed PV panels. This is why some installers choose to fit as many panels as possible within an inverter’s acceptable limits so that the system generates consistently higher than a typical system without there ever being any danger of the system exporting more than it should.
In terms of actually sizing the system; most centralised string inverters have a power tolerance ratio of around 125% – which means you can generally oversize by a quarter of the overall capacity. SolarEdge are the leaders in this field and can often be taken up to 155% which essentially means a 3.6kW inverter (3680HD Wave) can have up to 5.75kW’s of PV modules connected.
A common question for such an array is what is happening to the excess energy that has the potential to be generated during optimum conditions? And if the inverter does not have the ability to generate the total amount of energy – is this not considered wastage?
The graph below demonstrates why this is not the case;
Make better use of all energy produced
You can see 2 curves on the power generation graph (above) – let’s assume the lower curve represents 4000W’s of solar connected to a 4000W rated inverter and the upper curve represents 5000W’s of solar installed on a 4000W inverter. During optimal conditions the 5000W system produces more power than the inverter can generate energy from, this phenomenon is known as ‘clipping’ and doesn’t damage the inverter in any way. This simply refers to the situation when additional power may be available from the PV modules but the inverter is operating at maximum capacity. Although it’s true that the area above shaded in red is energy that will not be generated by the inverter and could technically be considered ‘wastage’, the area in green represents the points of the day where the inverter is generating more than it usually would due to oversizing.
Remember – there is no danger to oversizing a PV array providing you ensure that the total voltage and amps connected to the inverter do not exceed the device’s limits.
To Conclude…
There are many benefits to installing an oversized PV array. You can make better use of the inverters rated AC output and lower the cost of the electricity delivered. Also, the inverter costs will be reduced and the system owner can benefit from a maximisation of daytime energy. So, when done intelligently and with the right equipment, an oversized system will in all likelihood increase or at worst equal the energy from a system that is not oversized. Why not try it on your next system?
Need any more technical information? Call the technical department on +44 (0) 333 7720526 and look out for next months TTT which discusses Bi-Directional metering.