Solar Inverter Sizing

Solar Inverter Sizing

One question we’re often asked by our customers relates to inverter sizing. As we normally specify inverters with a nominal power rating that appears lower than the peak power rating of the array, the implied concern is that we’re compromising the system. In fact the opposite is true – we’re optimising the system and improving the overall value for money. This technical article explains in more detail…..

The electricity generated by a solar PV array is in the form of direct current (DC), similar to the electricity produced by a battery. However, the electricity used by the National Grid, and by the electrical system of a house, is in the form of alternating current (AC). The DC power generated by the PV array must be converted to AC power for use in domestic applications or for export to the electricity network (Grid). This conversion is achieved by the inverter. This conversion is done with a series of transistors which carry the electricity, and some sophisticated electronic control systems to allow it to efficiently and safely convert DC electrical energy to AC electrical energy.

Most inverters used by the PV industry typically achieve a conversion efficiency of between 90-98%. However, this efficiency is achievable only if the inverter is properly selected to match the PV array that it is connected to. The efficiency of an inverter depends on several factors, but one of the most important is the size of the inverter in relation to the size of the PV system to which it is connected. Inverters operate at their most efficient when they are operating under a reasonable load.

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The graph left shows the efficiency of a SMA Sunnyboy SB3000, which is a typical inverter used in domestic PV systems, rated at 3000 Watts maximum AC power. The efficiency stays relatively constant (with a small drop off) once the inverter is operating above about 600 Watts, or approximately 20% of its peak capacity (This boundary depends on inverter type and design, and can be obtained from the inverter manufacturer). However, below this level of loading, the efficiency of the inverter drops off dramatically, leading to power generated by the PV array being lost as heat rather being converted to AC power.

The rated power of a solar PV array (given in kWp) refers to the power it is capable of producing under what are known as Standard Test Conditions (STC), this is given as 1000W/m2 at 25ÂșC. It is very rare that normal weather conditions anywhere in the world would provide these conditions, so PV system will always vary in their output depending on location and weather. PV Module performance will also degrade over time to about 80% of its initial value after 25 years.

The graphs below show the amount of power available to a 1.5kWp solar PV array on sunny, mixed and cloudy days. It is only on the sunny days that the array is actually producing 1.5kWp at any point, and then only on a couple of occasions during the day. Energy is still captured on cloudy and mixed days, but at a slower rate than on sunny days.

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Because an inverter will be less efficient under a light load, it is common practice to under size inverters by 10-30% (or occasionally more) in countries such as the UK as the environment tends to be less favourable than more southerly countries. Inverters will tend to be more heavily undersized the further North the installation is, and the more heavily the array will be shaded.

An inverter that is undersized in this way will spend more of its life operating at a more optimal load. The advantage of this approach is that during the large part of the year, and the course of a single day, when the PV array is producing less than about 20% of its peak output, the inverter will be working at a higher efficiently and more energy will be captured than with a larger inverter. This does mean that a small amount of energy will be lost on the rare occasions that the PV array is operating at peak power (Usually only around mid-day in mid-Summer) but this small penalty is worth the gains at every other time.

For the owner of a properly designed PV system in the UK, a higher yield over the course of the year will be achieved, maximising financial return, and the initial expense incurred by installing an unnecessarily large inverter is avoided.

One final thing: as the likely array output is different for each building (because of orientation, roof angle and local shading), the output curve above will be different and therefore so may the optimum inverter – this is why we don’t buy kits like many of our competitors – we design for each system individually. We can do this because we have the expertise many less experienced companies can’t afford.

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