Types of Solar Inverter Efficiency

by Solar Selections on October 17, 2011

Types of Inverter Efficiency

Solar panels make up only one part of a grid-connected solar power system. In order to run your appliances and receive any solar feed-in tariff revenues, the direct current (DC) electricity produced by your solar panels must be converted into alternating current (AC) electricity through a device called an inverter. You’ll find a number of different brands of solar inverters, each with its own specifications and reputation. This article will guide you through one of the most important figures you will consider in selecting the right solar inverter; its efficiency.

One of the first figures (besides the price!) considered when buying a solar panel is ‘How efficient is it?’ This question is arguably of even greater importance when asked of the inverter for a solar photovoltaic system. The efficiency of your inverter needs careful consideration because it is a key point in the balance-of-system (BOS).

Balance of System (BOS)

The BoS is essentially the collective presence of every component in the solar photovoltaic (PV) system besides the panels themselves. The system’s BOS chain is the link of these other components and is also where the power produced by your system is lost through transmission and storage. Small losses are inevitable, but with a good inverter, you ensure that a greater amount of the electricity flowing from your solar panel array ends up where you want it to; powering your home or fed into the electrical grid.

Types of inverter efficiency

It’s important to note that inverter efficiency is not a fixed number. Inverter’s have what is called an ‘efficiency curve’, usually displayed in a chart that shows how efficiency fluctuates with the input power or voltage fed into it. Each brand and model of inverter has its own efficiency curve. (An example is shown below; the green arrow indicates the optimal operating range.)

You may encounter the following terms referring to efficiency when shopping around for an inverter: peak efficiency, Euro (η) efficiency, and CEC efficiency. The difference between these is explained below.

Solar Inverter Efficiency Curve

An example of a solar inverter efficiency curve. The left side indicates the efficiency, while the bottom is the power in watts (W) being fed into the inverter. The green arrow indicates the peak operating range.

Peak Efficiency

The nominal rating for an inverter is usually its ‘peak efficiency’. For example, a 3kW inverter’s peak efficiency is reached when it is receiving 3kVA (essentially 3kW in DC electricity parlance) from the solar panel array. Throughout the day your solar array output does not remain constant; it’s lower in the morning and evening, and higher in the afternoon when the sun is shining.

Simply put, peak efficiency is calculated as DC input to AC output when the inverter is operating at (usually) its rated capacity. Peak efficiency for some of the best inverters can get up to 99%. That sounds pretty impressive, but remember that although this is a noteworthy number, it is not the final word on inverter efficiency. Your inverter may only operate in its peak efficiency range for a very small part of the day or not at all. This is why the CEC and Euro weighted efficiencies have been developed. They recognise that inverters don’t always operate in optimal conditions, and instead these measurements offer an indication of how an inverter might perform throughout the day.

Solar Inverter Peak Efficiency

Solar inverter peak efficiency is a measure of your inverter's efficiency at a specific level of input power (watts). In this efficiency curve, peak efficiency is about 250W.

Euro and CEC Inverter Efficiency

Euro efficiency and CEC efficiency are ‘weighted’ efficiencies. In calculating them, the efficiency of an inverter at different spots within the operating range are taken into consideration and ‘balanced’ against each other depending on importance. These measues are generally more useful than peak efficiency because they measure inverter performance across the range of the inverter’s capacity. This gives you a better idea about the inverter’s operating profile over the course of the day.

In calculating CEC and Euro efficiency, instead of looking only at how efficiently the inverter functions at its power input ‘sweet spot’, calculating weighted efficiency requires first selecting a few DC input levels relative to the inverter’s rated capacity. For Euro and CEC (California Energy Commission) efficiency, there are 6 efficiency points in the calculation (see the formulas below).

CEC and Euro Efficiency measures for solar inverters

CEC vs Euro weighted inverter efficiency formulas. The difference between the two is in how each input level is weighted.

The formulas below may look quite technical, but it’s not important if you don’t understand the details. Helpful hints for reading them: ‘η‘ can be read as ‘efficiency’, and the subscript ‘XX%’ can be read as ‘when operating at XX% of rated capacity’. The numbers in front of the η‘s are weighting coefficients (they add up to ’1′ for ’100%’).

The coefficients ‘weigh’ the importance of inverter output efficiency at different efficiency levels, based on assumptions (based on observations and monitoring) about how often your inverter will function at each level. The difference between CEC and Euro efficiencies can be found in which levels are included and how the weights (%) at each level are determined.

You can see, for example, that both CEC and Euro weight an inverter’s performance at 30% capacity with a coefficient of about 0.10. This means that the inverter is assumed to operate at  30% of its rated efficiency 10% of the time. You’ll also notice a difference in the fact that the Euro formula includes a meausure at 50% but not 75%, and CEC vice versa. One is not necessarily better than the other, but you should be aware that these differences may result in different efficiency numbers for the same inverter.

Checking your Inverter’s Efficiency Profile

Although both these efficiency ratings are great ‘at-a-glance’ perspective on an inverter’s efficiency, the best thing you can do for your own solar system is to look at your inverter’s efficiency profile. This will tell you in detail how an inverter will perform in your area’s climatic conditions with your solar array at different output levels. In combination with the knowledge of how well your solar panel array performs throughout the course of the day, will enable you to more accurately understand how your inverter performs. Ask a solar broker for help with any and all of this by contacting us today.

Written by James Martin

Analyst for Solar Selections

© 2011 Solar Choice Pty Ltd

Sources and links:

Weighted inverter efficiency formula image from: http://www.scribd.com/doc/41948202/Weighted-Inverter-Efficiency

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{ 2 comments… read them below or add one }

paulbeartil April 27, 2012 at 10:32 am

hello please pm me, i lost your details
bearty

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admin April 27, 2012 at 2:22 pm

Hello Bearty, if you email us at sales@solarselections.co.uk, we can then help assist you to the best of our ability.

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