All About Inverters and Inverter Sizing
Inverters make up a critical component in solar-based or battery storage energy systems. Inverters are responsible for the conversion of DC Power from solar cell and batteries to AC power, for use in grids and households for many home or commercial appliances.
This article is motivated by the critical role played by Inverters since at some early stage of building your DIY Powerwall or solar system you will most likely encounter this in the design phase. It is \imperative to get an understanding of its functions and the process of sizing inverters for your next DIY build.
The DC-AC conversion process is achieved through a switching mechanism in which transistors are switched on and off to create pulses giving a representation of AC currents. The output voltage and frequency of AC from the inverter is dependent on the switching circuitry and control scheme adopted.
There exists different kinds of inverters, they're classified according to their power sources, load type it serves, switching techniques applied and output characteristics of AC it gives. The scope of this article only covers the latter class, the output characteristics of AC it gives. This article will mainly explain why prices of different inverters have wide and varying price ranges. This is partly due to inverter type and quality of AC power produced, which has a significant effect on the price.
According to the output characteristics, inverters are classified as:
- Square Wave Inverters – Outputs a snaggy square shaped AC waveform. This kind of inverters are cheaper and tend to produce more noise.
- Sine Wave Inverters - Outputs a smooth AC approximate sine waveform -such as the one given by the utility and is more expensive and efficient than square wave inverters. Costs more than square wave inverters and modified sine wave inverters.
- Modified Sine Wave Inverters – Its functionality lies between the two types of inverters mentioned above. It doesn’t produce an exact sine wave, but mimics it – giving a modified stepped sine wave output that gives a representation of a pure sine wave. Ideal for use in less sensitive electronic devices.
Knowledge of this is helpful in determining what kind of loads you have and the ideal inverter needed.
STRING INVERTER vs MICRO-INVERTER
Depending on inverter mounting location in solar-based energy systems, inverters are broadly classified into string inverters and microinverters.
String Inverters – also referred to as central inverters. Mounted on a central location. Series connection of solar panels lead to a large central inverter that carries out the dc - ac conversion. The main disadvantage of string inverters is that the lowest performing panel determines /dictates the production of the entire system.
Microinverters – This is a module level power electronic (MLPE) device mounted directly at the back of each solar panel. Each solar panel operated independently in parallel and thus converters its own DC power to AC Power that is aggregated to power homes or fed to the grid. Microinverters are more expensive than string inverters but they are superior and more efficient. Some come with features such as solar rapid shutdown systems to protect against fires. The National Electrical Code (NEC 690.12) provides recommendations for rooftop solar systems to have rapid shutdown systems, having these MLPES can help in having your projects meet this NEC standard. This will be discussed in greater detail in subsequent articles.
OPTIMIZERS IN INVERTERS
Power Optimizers work to maximize energy conversion rate and hence increase the efficiency of solar panels. It takes in DC energy from the panels, maximize the output and delivers this optimum energy to a central inverter. Optimizers are module level devices placed at each individual module of a solar panel. They are meant to maximize power extracted from each solar panel through tracking the panel’s maximum power output and optimize it’s performance by constantly adjusting the voltage and current output of the solar panel to ensure high output performance from the panels at all the time. They are meant for rooftop solar panels that are impacted by shading conditions. These power optimizers provide the ability to control and monitor the performance of each of the solar panels at modular level.
Of utmost importance in any DIY solar project is inverter sizing in order to best utilize these components. Wrong sizing of equipment may lead to projects that don’t meet your needs and can also lead to higher project costs. Sizing of inverters for solar projects is detailed here:
- Determine your total daily power consumption needs. This is the total energy in Watts needed to power devices in your household. A great place to find an approximate figure is through your current electricity utility bills. More details on how to calculate your energy usage for going solar is available in this article > here.
- Factoring in of the inverter efficiency and safety factor. Inverters incur losses in the DC to AC conversion process through switching losses related to frequency of operation and resistance the current flow encounters inside the inverter. These losses account for 20% to 30% depending on inverter type (some inverters have higher efficiency). This percentage can also be used as a safety factor, to account for the abnormal conditions that might occur, such as power surges, etc.
- Inverters should be 120% of your total load. Example: You have determined your total load is 500W, its best practice to select an inverter of 600W.
This is in looking at it from the load side.
More often than not, you will come across terms like "inverter oversizing" when in the market for solar. This is often used when sizing the inverter while taking considerations of the Solar PV capacity available (looking at it from the generation side).
"Inverter oversizing" is a term used to denote the concept of installing more solar capacity than what the inverter is rated for in an attempt to increase daily energy yield. Solar panels can be "oversized" because, in reality, there are many factors affecting the power output from the panel, ranging from sunlight strength to placement of solar panels. Because of this the inverter may output less than the rated capacity. The rated capacity is derived from standard testing conditions (theoretical), which often isn't how it works in real life. It therefore makes sense to select inverters with capacities below the installed DC solar power. Most literature and industry best practices recommend 33% less inverter capacity.
Hope this info is useful for your current and future solar builds. Also, browse through some of the JAG35 listed inverters > here. Keep on building!
Article contributed to JAG35, written by Japhet K, Electrical Engineer at http://charged-engineering.com