Inverter
PV modules and energy storage systems function with direct current (DC), yet due to the advantages of alternating current (AC) the majority of the appliances produced in the world are built to function with an AC input source. This means that it is common to incorporate an inverter, which can convert from DC to AC, into any any system that is intended to function with more than just basic appliances like lighting, cell phones and radios. There term inverter covers many different products with different functionality and cost, thus it is important to understand the different factors that go into choosing an inverter to determine the right one for your application.
Inverters are the most electronically complex component and thus are a likely failure point, meaning that investing in a quality inverter is a wise choice. If the inverter in a system that is built around AC fails, then the system will stop functioning completely. For this reason, many system designers of small offgrid systems choose to incorporate DC lighting or a DC-based refrigerator into a system to offer the a more robust system that will continue to provide these basic functions even in the event of an inverter failure.
An inverter will have to be chosen for every project based upon the overall system design (DC for some loads, all AC) and the intended power demand for the system. How to properly select an inverter based upon the intended power demand is covered in more detail in inverter selection. The characteristics common to all inverters that go beyond power demand are covered below.
Waveforms
The most important characteristic of an inverter - that helps to define its functionality and quality - is the waveform of its alternating current output. The AC that the grid supplies comes in a pure sine wave, which is what all AC appliances are designed to use as their input. A smooth variation between directions of current flow that is operating is necessary for the proper functioning of various complex appliances, but for other simpler appliances it doesn't matter. The three types of output wave forms that are available in the market are the following:
- Pure sine wave (PSW): An inverter that outputs AC in a sine wave that is indistinguishable from that supplied by the electricity grid. The creation of a pure sine wave requires a more complex inverter design that costs more, but the additional cost of a pure sine wave inverter often bring additional efficiency and quality. It is recommended that any system that relies on AC continously to supply loads incorporate a PSW inverter.
- Modified sine wave (MSW): An inverter that outputs AC in a waveform that is more rough than a pure sine wave, but that is indistinguishable for most appliances. MSW are a more economical option for small systems that require AC, but that are only going to supply simple loads (cell phones, radios, lights etc.) Should not be considered if a system is intended to be used with certain types of loads - motors, laser printers, battery chargers, washing machines, high-end music equipment - as it cause them to work improperly or damage them. Motors will consume roughly 25% more energy with a MSW inverter compared to a PSW inverter and the life of the motor will be shortened as that extra energy will be converted into heat. If a system doesn't rely on AC continously, but only periodically for smaller loads, then it is an option that should be considered.
- Square wave: The simplest and cheapest form of inverter. Current direction switches very rapidly and can damage certain appliances. Will work fine with simple loads like cell phones and lighting, but not recommended for use in a PV system. Frequently poorly designed and manufactured. A modified sine wave or pure sine wave inverter will not cost very much more.
Idle consumption
An inverter requires energy even if it is not currently supplying loads. Larger offgrid inverters may require more than 30W when not supplying loads, smaller inverters tend to require around 4-8W. Inverter idle consumption can greatly affect the design of smaller PV systems as a constantly operating inverter may be the most energy intensive load that the system supplies. It is common practice with smaller systems that use DC for lighting and cell phone charging to incorporate an inverter that is only used as needed to reduce the size of the PV source.
Example 1: A small offgrid PV system incorporates an 800W inverter that consumes 7W of power as it sits idle. How much energy will it consume if left on continously?
- Daily idle consumption = Watts × hours
- Daily idle consumption = 7W × 24hr
- Daily idle consumption = 168W
This is more energy than two efficient 3W LED lightbulbs - a common size in offgrid applications - would consume if left on continuously.
Efficiency
Inverters vary in terms of how efficiently the transform DC into AC. Many inverter manufacturers offer a maximum or peak efficiency number for their products, but this number is not likely to be achieved in practice. Inverters will only achieve these efficiency numbers when under a sufficient load, but the number drops rapidly with less loading. An offgrid inverter in practice will likely be somewhere in the 85-90% efficiency range. The efficiency of an inverter can have a significant impact on system design and performance.
Example 1: Two inverters are being considered for an offgrid PV system. Both inverters have an 800W power rating, but one is 90% efficient and the other is 85% efficient. It is anticipated that the inverter will have to supply 2400Wh of energy each day. How much extra energy will each inverter require to meet this demand?
- Total energy demand = energy demand of loads ÷ inverter efficiency
- Total energy demand inverter 1 = 2400Wh ÷ .85 = 2824Wh
- Total energy demand inverter 2 = 2400Wh ÷ .90 = 2667Wh
The less efficient inverter will require 157Wh more of energy each day to supply the loads. This could mean that the PV source needs to be larger.
