What are inverters for?
Inverters are an essential part of a solar installation. They convert DC to AC electricity, and synchronise the electricity you generate with the mains electricity. They also control exported electricity, as well as allowing your system to import electricity, when required. Synchronous inverters have dual input (useful if you have more than one system installed), they are transformer-less and operate at a conversion efficiency of 97%.
Alternating Current (AC) electricity is the standard domestic supply. Unlike Direct Current (DC) electricity, AC rapidly alternates direction at a certain frequency per second. This ‘oscillation’ is measured in Hertz (Hz). The UK National Grid delivers 240v AC electricity at a frequency of 50Hz. The constant changing direction is a smooth, wave-like motion (called a sine wave) much like that of the smooth transition of a clock pendulum flowing from left to right.
Synchronous inverters (grid-tie inverters)
Grid-tie inverters synchronise your home’s production with the national supply by stepping up the voltage and regulating the current to match consumer electricity (240v) for home use or for export. Inverters avoid the need to re-wire your home. The alternatively named off-grid inverters are for standalone systems not connected to the National Grid.
Pure Sine Wave inverters (PSW)
A synchronised inverter must exactly match the sine wave of the National Grid supply if it is to export electricity, and must do so to within 1º. Most domestic inverters are therefore pure sine wave inverters using oscillators to create a smooth wave. They have on-board computers to constantly monitor the mains supply and match the two.
Modified Sine Wave inverters (MSW)
Cheaper MSW inverters produce a very jerky, modified ‘square’ wave. If you constantly click a switch from left to right at 50 times a second, the result would produce a jerky, square line wave rather than a smooth, pendulum-type transition. Modified Sine Waves create hums and crackles and are less efficient for most appliances that require AC.
A transformer adjusts the voltage of electricity. Electricity passes through a primary coil of copper wire placed next to another, secondary coil. AC electricity in the primary coil creates a magnetic field. In reverse, this magnetic field creates a current in the secondary coil. The voltage of the secondary current is the same as the primary current if both coils have the same number of turns. If they differ, then the ratio of turns of the two coils reflects the output. For example, if you have 5v going in and want 50v coming out then the secondary coil would need 10 times as many coils.
Coils require a lot of copper and make it very heavy and expensive. Modern inverters are transformer-less and use transistors to generate the AC current. This keeps the cost and weight down and also means the inverter can manage more than one input. If you have 2 different solar arrays, and one is less sunny than the other, the inverter will treat them separately to produce the same fixed frequency and voltage of electricity to supply your home or the domestic grid.
Efficiency of inverters
They are never 100% because all inverters use a small amount of input power to operate, usually about 10-25W. The efficiency of the inverter is also dependant on the power output it is operating at and, ideally, whether it’s at the inverter’s optimum. This optimum is better described as the inverter’s peak efficiency, which is usually when the inverter is working at around 20-30% of its maximum power rating.
Inverter power rating
The max amount of power the inverter can handle gives its power rating but the two important ratings to consider are its continuous rating and the surge rating.
Continuous rating for inverters
A 400W inverter tells you the continuous rating and is the amount of power the inverter can supply continuously. An inverter should have a continuous rating about 25% higher than the maximum power you will need to deliver to your household loads.
Surge rating for inverters
Large household items, such as refrigerators, require a short-term boost in power (a surge) to get started. This can be 2 to 3 times the normal operating power. The surge rating of the inverter is the ability to handle that amount of power for a few seconds and is given in terms of so many Watts for a period of time. Generally a 3-15 second surge rating is enough for nearly all appliances.
Solar power is a clean efficient means for houses to generate up to a third of the home’s electricity using renewable energy, whilst also benefiting from a generous annual Government incentive, interest linked and paid to you for 25yrs.