Oversizing involves installing an array of solar panels (string or strings of solar panels on your roof) that have greater peak generating capacity than the output capacity of the connected inverter. In technical speak this is often called the Array-to-Inverter ratio, oversizing ratio, overloading ratio or DC-AC ratio.
In the past this ratio has rarely been greater than 1:1 and where it has been deliberately applied in the past it has particularly been at a ratio of 120 per cent-133 per cent oversized.
Oversizing and supersizing to become the norm in 2014
With plummeting panel costs and grid operators starting to restrict the connection of solar to the nation’s electricity grids, oversizing and supersizing will become the normal specification for solar systems installed in the future. Indeed it should already be the case today.
Why did installers oversize in the past?
Generally speaking oversizing, where it has been applied up until now (to about 133 per cent), would increase the annual capacity factor of an inverter without "clipping" the generation produced by the panels. Clipping is where an amount of potential generation from panels is lost because it exceeded the capacity of the inverter to convert it into the alternating current power that could be used in the home or exported to the grid..
For instance a 3kW SMA inverter installed in Sydney might produce 4,436kWh per year at 100 per cent solar to inverter ratio and 5,754kWh at 133 per cent with almost no clipping i.e. same kWh produced per unit of solar panel capacity installed.
Why no clipping with 133 per cent oversizing?
There is no (or very little) clipping or curtailment up to 133 per cent because a solar panel (and hence array) is very rarely operating under the optimum environmental conditions under which its capacity was rated at the factory.
Some of the reasons include the fact that solar panels:
– often operate at high temperatures which make them less efficient;
– they get covered in dust and grime:
– there is energy lost in wiring and at the inverter, and
– the panels rarely directly face the sun.
For the tech-heads there’s more technical detail in the appendix below.*Panels are rated at 25C in an instant flash test but generally operate at 40, 50, 60C-plus with the sun shining on them during our hot summers which causes them to reduce output by upto 20 per cent and beyond.
What is supersizing?
Supersizing is when you take oversizing much further than 133 per cent, such as having a total solar array installed at anything from 133 per cent to 400 per cent larger than the rated output of the inverter (in alternating current or ‘AC’).
*Full full graph, see bottom
Why didn't supersizing occur in the past?
Systems were $12 per watt when I installed my 1.65kW solar system back in 2001 and the aim of the installer and customer was to make sure that every kWh that could possibly be generated was not lost. No system was likely to be installed with partial shading, or facing east or west of solar north, customers would consider regular cleaning of their panels, and certainly wouldn't have sized a system that clipped (curtailed) some of its annual generation.
Why have we entered the age of supersizing now?
With panels at today's prices and rapidly heading towards 36 cents a watt, as well as grid operators increasingly applying limits on the amount of solar capacity that can connect on the sub-network, oversizing allows customers to produce electricity to meet their needs even while (often unreasonable) technical restrictions are being applied by network distribution companies.
1) Distributors capping the size of export capacity they’ll connect to the grid
The main reason is to get around network distributor limits on what they’re prepared to connect to the network, which is based on inverter size. In country NSW, Essential Energy is making it difficult to install systems with inverter output greater than 3kW outside of the major centres.
Customers in country areas often have greater demand than those in the city due to their reliance on electricity for hot water, heating, cooking and water pumping as well as those with light industrial loads such as agricultural properties. Therefore a system with 3kW of solar panels on a 3kW inverter will not match a rural family's requirements. It is usual for these customers to have annual demand in excess of 15,000kWh a year or more than twice the average.
2) Provides steadier (less peaky) level of generation that maximises self consumption
A system that is oversized produces output from the inverter that is more constant over the period of daylight hours, increasing the number of hours it produces at full output versus the number of daylight hours (it’s ‘daylight hours factor’). A 100 per cent sized system might have a production to daylight hours factor of around 34 per cent of the inverter’s capacity, while a supersized system at 300 per cent would have 65 per cent and at 400 per cent would have 72 per cent. At the extreme and upper end a system at 400 per cent would have 80 per cent.
3) Avoiding unnecessary incremental cost on the networks
Supersizing provides more opportunity to generate without significantly increasing incremental costs for the network. The crude modelling capability of the network distributors (which just happens by coincidence to favour them economically in terms of bringing forward any solar related upgrades that would be required) currently treat solar systems as though they are capable of generating at full inverter output anytime of the day. So actually producing at full output gets better value for you and the community at large. Even if they used modelling in the future that took into account solar panel array size, the incremental increase in requirements to accommodate solar inverters that are oversized would be far less than if they were not.
4) Provision additional power capacity for daytime activities where network capacity is constrained.
Supersizing provides additional power capacity to consumers who would otherwise have to pay for a network upgrade that might involve lines and transformers that could run into the hundreds of thousands of dollars.
Won't it cause the inverter to fry or cause safety problems?
The simple answer is no, but in the appendix below is a more detailed explanation for the tech heads. The addition of small and inexpensive fan (like computers have) to extract heat from the inverter ensures no noticeable difference.
What about the Clean Energy Council rule that prevents oversizing above 133 per cent
There used to be a rule of thumb in the previous Clean Energy Council guidelines that said the inverter shouldn't be less than 75 per cent the peak rating of the array. This was because the solar industry was small, and anyone installing solar wanted to make sure their super expensive system, with super expensive panels, would supply every last watt hour it could generate to the grid.
At the prices that panels were coming in at in the early 2000s you could think that it would have been prudent for a solar customer to get a shammy out every morning and polish their panels for a maximum full days power production. Fast forward to 2013, panels are wholesaling for $0.50-$1.00 a watt, that's between 1/16 and 1/8 of the price at the turn of the century.
It’s time for the CEC to update this rule to reflect the times and this includes changing the way government rebate certificates or STCs are calculated to reflect the actual output of a system. Why should a 200 per cent oversized system which faces solar north and produces more kWh/kWp be banned/ not accepted for STC rebates, when a 100 per cent sized system facing due east or due west would produce less kWh/kWp?
In excess of a million households (out of a total of 8 million) have solar and with that will come some minor grid issues with integrating lots of solar. On this basis the distributors, whose model is threatened by solar, are putting restrictions on the size of systems that are allowed to connect to their networks. They're doing this for either real technical reasons or politically driven agenda based reasons. Either way until these issues can be worked out with available technical solutions (at the customer end or at the grid end) and/or political solutions can be found, supersizing allows a customer to generate from 133 per cent-215 per cent of the output of an inverter rating at less than the cost per kWh of an unsubsidised solar system purchased and installed in 2011 – i.e. taking a $2 a watt unbsubsidised benchmark system today and turning it into an equivalent of a $3.40 a watt system in the 400 per cent supersizing example.
Matthew Wright is the executive director of Zero Emissions Australia, a not-for-profit volunteer based energy and climate research organisation.
Reasons why panels produce less than their rated capacity:
– Panels are rated at 25C in an instant flash test, but generally operate at 40,50,60C with the sun shining on them during our hot summers which causes them to reduce output by up to 20 per cent and beyond.
– Energy is lost as heat in wiring
– Energy is lost in conversion from DC to AC in the inverter
– Energy never produced due to solar test conditions being 1000w/m2 when available solar radiation at the surface of the panels is more likely to be 800w/m2 in real world conditions striking the panels.
– Aerosols dust and other airborne particles can reduce panel performance further
– Energy lost due to dust, dirt and grime accumulating on the panels.
– Undervoltage losses, early and late in the day when the voltage of the array is low and the inverter has not hit is design point for powering up or is running in sub optimal low light low voltage conditions.
– And most importantly energy lost due to co-sine losses, otherwise known as the "projection effect" where in fixed solar arrays (which is the majority) the angle of the sun is very rarely perpendicular to the panels. (Depending on roof pitch usually only around noon at each of the equinoxes.
Why inverters can handle oversizing
Solar arrays are controlled by an inverter’s MPPT (Maximum Power Point Tracker). MPPT's really are capable of doing three important jobs:
1. Finding the optimum voltage. To get the combined voltage and current to achieve the maximum output of the inverter and tracking this point through different environmental conditions including accounting for angle of incidence of the sun (co-sine losses/projection effect), temperature variations and low light conditions/cloud cover.
2. Safety. Safely shutting off the solar array in case of a fault condition such as overheating. Most of the safety functions are handled by increasing the voltage towards a point called Voltage Open Circuit or VOC (and as that is the ordinary functioning of the inverter it isn't even considered a safety function). This is the point where the voltage is so high that the solar panels work so inefficiently that they lose all their potential production as heat at the cells on the roof. Therefore no power (the current is not flowing) is transmitted along the electrical cabling and able to reach the MPPT. This occurs by pushing the voltage towards VOC this is the smart electronic MPPT equivalent of turning off the solar array.
3. Current limiting. Through setting the voltage above the MPPT but not too close to VOC so that the panels output (current and voltage) will drop and the correct maximum amount of electricity will flow to the inverter for DC-AC conversion and delivery to the electricity grid.
All these scenarios are the matter of course for an MPPT and are what it is designed to do. In the case where no oversizing is used, an inverter may average 33 per cent capacity factor (versus daylight hours) over its lifetime versus 72 per cent for a 400 per cent oversized inverter.
To provide an example of the SMA's Sunny 3000/4000/5000tl-21 models, let’s look at their heat and heat dissipation which is a major contributor to the life expectancy of any electronic equipment.
At 100 per cent sizing ratio the 3000tl version of the inverter would average 50 watts of heat at the heat sinks across potential operational daylight hours, while at 400 per cent oversizing the unit would average around 95 watts. This is not going to fry anything.
For comparison the 5000tl with the same heat sinks would average 66 watts with no oversizing and would average 158 watts at 400 per cent oversizing.
SMA have an optional fan which according to a major distributor in Melbourne has been purchased by less than 1 per cent of customers. Running properly sized (as provided by the manufacturer), these fans have a substantial effect on thermal performance and design life, meaning oversized and supersized systems with a fan could last longer than non oversized systems without that option present. So provided these fans are fitted, supersizing should present no problems at all.
But don’t take my word for it, here’s what the manufacturers say about oversizing:
Enphase. In a paper published by Enphase, the leading manufacturer of micro-inverters, titled "Bigger is Better: Sizing Solar Modules for Microinverters" it states that, "annual losses to inverter saturation increased geometrically with module size , and at a certain point, the marginal losses would begin to exceed the marginal gains. This crossover typically occurs above 140 per cent DC-to-AC ratio."
SMA Germany. According to field experiences over decades, in regions all over the world in every kind of weather condition, SMA confirms that our string inverters (Sunny Boy, Sunny Mini Central and Sunny Tripower) are completely compatible with power ratios (AC Power / PVGenerator Power = kWAC/kWp) lower than 70 per cent.
When using a power ratio outside the range recommended by Sunny Design, the electrical limits outlined in the technical data for the inverters (e.g. maximum DC-Voltage) cannot be exceeded. In addition, the installation guidelines described in the installation manual of each inverter need to be complied with.
Generally, SMA still recommends following the Sunny Design specifications to best ensure SMA customers an optimal technical and energetic system design.
If the electrical limits given in the technical data information and the installation guidelines have been complied with, using a power ratio lower than 70 per cent does not affect the SMA warranties and SMA warranty extensions.
SMA Solar Technology AG
Director Service Marketing & Sales Service
From SMA USA
In Germany, 1000V PV array voltages are common place even in residential applications.
You cannot damage an inverter by putting too much power into it. For example, you can put 10kW into a 2Kw inverter and nothing would happen! The inverter will only generate 2kW of power and the other 8kW would be a gross oversizing.
Technical Trainer, Solar Academy
SMA America, LLC
From SMA Australia: (They don't really seem to have been properly briefed by their peers in Germany and the Americas however they still understand that the inverter is a current limiting device as stated here).
Technically, it doesn’t do physical damage, the inverter is a current limit device which means it will not take anything more than what it’s rated for and it will only track the MPP up to the maximum of 5.3kW of DC power but it will put the inverter in full load for long period of them which will then increase the internal temperature of the inverter
Service Contact Manager
SMA Australia Pty. Ltd.
67 Epping Road, North Ryde NSW 2113