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 off-grid 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.
The inverter for an off-grid system must will be sized and selected based upon the load evaluation for a particular site - see Inverter sizing and selection for more information.
Contents
Inverter/charger
Many larger off-grid inverters - called inverter/chargers - are able to accept AC input from a generator and convert it to DC to be able to charge the battery bank if weather conditions are poor. This can be a wise investment for larger systems as the size of the PV source and energy storage system do not need to be as large to be able to meet demand during infrequent periods of bad weather. Additionally, an inverter can play an important role as a system with a bypass switch will permit the generator to directly power AC loads if the PV system is not functioning properly or needs to be disconnected for maintenance. An inverter/charger will have to be chosen in accordance with the size/type of generator that will be used.
Watt/VA output
All inverters are rated based upon the amount of power or volts/amps that they can continuously supply. In addition to this continuous rating an inverter will be able to supply higher amounts of power for brief periods of time in order to supply loads that require momentary surges of more current when starting. These additional ratings may come in 30 minute, 5 minute, 1 minute, 30 second, 10 second or 1 second ratings. All loads and their energy requirements will need to be evaluated to select the appropriate inverter - see inverter sizing and selection for more information.
DC input voltage
Inverters are typically available in 12 V, 24 V, or 48 V. Inverters with a smaller power rating will typically be available in 12/24 V configurations and larger inverters will be available in 24/48 V configurations.
AC output voltage
Inverters are manufactured for use in specific geographic markets given the voltage of the local grid, but is necessary to confirm that the specifications of the inverter - especially if importing the product - conform to the local voltage. Common output voltages for off-grid inverters are 120 V, 220 V, 240 V.
Frequency
The frequency of the grid varies globally between 50 Hz-60 Hz. An off-grid inverter that matches the local grid specifications should be chosen.
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 continuously to supply loads incorporate a PSW inverter.
- Modified sine wave (MSW): An inverter that outputs AC in a waveform that is rougher 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 can 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 continuously, 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 off-grid inverters may require more than 30W when not supplying loads, smaller inverters tend to require around 4-8 W. Some inverters will include a low consumption standby mode to reduce idle consumption, but standby modes often do not function well with small off-grid systems as the inverter only activates when a load of a sufficient size is connected to the system. A cell phone charger will likely not wake the inverter from standby mode.
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.
| Daily idle consumption | = idle watts × hours of operation per day |
|---|
Example 1: A small off-grid PV system incorporates an 800W inverter that consumes 7W of power as it sits idle. How much energy will it consume if left on continuously?
- Daily idle consumption = 7 W × 24 hr
- Daily idle consumption = 168 W
This is more energy than two efficient 3W LED lightbulbs - a common size in off-grid 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 off-grid 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.
| Total energy demand | = energy demand of loads ÷ inverter efficiency |
|---|
Example 1: Two inverters are being considered for an off-grid 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 inverter 1 = 2400 Wh ÷ .85 = 2824 Wh
- Total energy demand inverter 2 = 2400 Wh ÷ .90 = 2667 Wh
The less efficient inverter will require 157 Wh more of energy each day to supply the loads. This could mean that the PV source needs to be larger.
Additional inverter features
There are many other additional features that inverters offer that may be of value on a specific project.
User interface
The user interface is important as it can conveys vital information about the loads and the state of the inverter, which users need to revise regularly in order to be able to adjust their usage properly and protect the battery bank. Additionally, a user interface should be assessed for how much programming it allows the user to perform and if it allows the revision of historical system data.
Programmability
The larger the power rating of an off-grid inverter, typically the more user programming is permitted to enable customization according to the end user needs. There are basic functions, like the set point for the low voltage disconnect, and other more complicated functions related to its output, standby modes to save on idle consumption, generator input and monitoring. See inverter programming for more information.
Data logging/monitoring
A data logging/monitoring system can enable an inverter to share or record data about the performance of the system. The level of detail and amount of time for which an inverter can store data varies. Information about maximum power, usage and system voltage can be very useful in assessing how the system is performing in that location, if the user is treating the system properly and resolving any technical issues that may arise. Some systems may also offer the capability of remote monitoring through cell phone signals or the internet, which can be very useful in remote off-grid applications if possible.
Projected life
There is no specific projected life for an inverter as it varies significantly based upon its quality and how it is used. A low-quality inverter may only last for six months of heavy use before failing, whereas a high-quality charge controller used lightly could last decades. With inverters you generally get what you pay for.
Maintenance
The user manual for an inverter should always be consulted, but most inverters do not require much maintenance if they are used under proper conditions. They should be kept free of dust, insects, and water. Connections should be periodically revised - at least once a year - to make sure that they are still tightened properly and not creating unnecessary resistance.
Recyclability
Inverters contain a variety of different materials and chemicals that can be hazardous if not disposed of properly. They should be treated as electronic waste. Contacting the manufacturer is recommended.
