For those of you who want to know more details about how inverters really work, including the nerdgasm inducing concept of “Maximum Power Point Tracking” (which is all about maximising the power from your panels), then this blog post is for you!
Inverters have 2 main roles in life:
- Converting constant, Direct Current (DC) electricity from the panels into oscillating, Alternating Current (AC) electricity for your power points.
- Transforming the low(ish) voltage from your panels to the high(ish) voltage needed by your appliances.
But they also have to:
- Keep you safe – i.e., the grid connected inverter should shut down if there is a power blackout or a fault with your Solar Power system.
- Export electricity to the grid (and get paid for it) when your solar panels are producing more power than you are using.
- and (ideally) do some funky electronic stuff to maximise the power from your panels if, for example,one panel is giving more output than its neighbour
Let’s go through how that box of electronics bolted to the wall manages to do all these things:
a) Converting DC electricity to AC electricity
Solar panels produce direct current (DC) electricity. This is the type of electricity used in car batteries. However, the electricity we use in our homes for lighting and power is 240 volt Alternating Current (AC) electricity.
The transformation of DC electricity to AC electricity is achieved by use of very efficient electronic switches to alternate the flow of the DC electricity produced from solar panels.
That is, switch one opens and switch 2 is closes and the current flows one way across the circuit. Then switch 1 closes and switch 2 opens and the current runs the opposite way across a circuit. Thus the DC electricity is converted to AC electricity
b) Transforming the voltage of the electricity produced by solar panels
The transformation of the voltage from the solar panels (23 to 38 volts) to match the voltage of the electricity grid (240 volts) is achieved by using a transformer.
Put simply, transformers have 2 sets of coiled wire on each side of a circuit. The current of electricity flowing through the first coil causes a current to flow through the second coil.
The voltage across each coil is related to the number of coils in each set of coils. The higher the number of coils the higher the voltage. Thus, in a grid connected inverter, there are more coils in the second set of coils than in the first set of coils and this increases the voltage of electricity produced. The number of coils in the second set is adjusted so the output voltage matches the voltage of the electricity grid (240 volts). See diagram below.
c) Maximising the electricity you get from your solar panels
In order to get the most electricity out of your solar panels the grid connected inverter uses a maximum power point tracker (MMPT).
Each cell in a solar panel has a maximum power point (MPP). This is the point at which the maximum power (i.e. electricity) can be extracted from a cell. The MPP is determined by the voltage and current of the cell, the cell temperature and the amount of sunshine hitting the cell. The MPP for a given cell can vary by as much as 25%.
Thus, to get the maximum power out of a cell we can use a MPP tracker (MMPT) to vary the current and voltage of the cell (using software logic or circuitry controls) so the cell is operating at its MPP.
Most current grid connected inverters have one MPPT that will seek to get the MPP of a system as a whole. Since individual cells will each have a different MPP (due to differences in manufacturing, differences in shading, etc.) then some cells will not be operating at their MPP. Therefore, there is a loss of efficiency.
Some inverters do have more than one MMPT. These are called Multi String Inverters. This can be useful if you have a system where you might have strings of panels in different situations (e.g. a string facing north and a string facing west). As each panel will have a different MPP depending on where they are facing, each MMPT can find the MPP for each string of panels. This will increase the overall efficiency of the system.
Multi string inverters may also be useful if you want to connect different panels types provided your inverter can handle the increase power from the solar panels (e.g. if you want to upgrade but your original panels have been superseded).
d) Safety of your grid connected inverter
All grid connected inverters are required to have certain safety features to protect you, the grid technicians, your electrical appliances and your inverter. These are:
- A DC isolation switch isolates the inverter from the solar panels. This prevents damage to the inverter or household appliances if there is a fault with the solar panels.
- An AC isolation switch isolates the inverter from the grid to protect the inverter in case of a fault in the grid.
- A safety switch that will turn off the inverter and isolate it from the grid in case of a blackout. If this did not happen your Solar System could send electricity into the grid. This, in turn would be a danger to any mains grid technicians that come to check the grid.
e) Exporting Electricity to the grid (and measuring the power output from your panels)
If you want to get paid for the electricity coming out of your panels then you need to do two things: measure the electricity, and export it to the grid (if you don’t use it all in your home). If you are on a Gross Feed In Tariff then you need a “Gross” Meter that measures all the electricity coming out of those panels whether you export it or not. If you are on a Net Feed In Tariff then you need a meter that measures the difference between what you generate and what you use, because you are only gonna get paid for the exported electricity.
Warning: Make sure that any quote you get includes a new meter and connection of the meter to the grid. It amazes me that anyone can quote a system as “fully installed” without doing this, but a lot of companies are doing just that. Kind of useless to get a solar power system for your home that isn’t connected to the grid. The worst case scenario is that they connect it to your existing meter which doesn’t know the difference between what you have generated and what you have used, then you will actually be charged for the solar power you generate (not credited) ! Ouch.