An inverter is an electronic device does one basic thing: converts direct current (DC) into alternating current (AC).
Why are you going to need it together with your solar panel, or wind microturbine? The main reason is that most of the microgeneration happens in direct current, but then if you want to use that power, you have to make it available to regular appliances, that run on alternating current.
Even in the few cases in which power generation is already in AC, it has to be converted into DC and then back into AC for the following reasons:
- because in this way the produced energy can be stored into batteries, which definitely require DC charging current
- because the AC current coming from example from a wind generator does not have the correct amplitude and frequency required by appliances.
In the first case, their job is just to convert DC current into regular AC current, with a nominal voltage amplitude and frequency. You can connect almost any appliance to these inverters. In some cases the quality of the AC output of the inverter is not enough for some appliances. It depends on the wave shape of the AC output, that can be square, quasi-square, or a sine (best).
Some metering device must be present in this kind of inverters, so that you know how much you are selling back to the grid and therefore you get paid for the correct amount.
Why choosing a stand-alone inverters and why choosing a grid-tie inverter?
Here are some guidelines:
- if you are producing a large amount of energy, then you probably prefer to sell it back to the grid, because otherwise you waste it in the case in which you don't have any running appliance (for example solar panels at midday, when you are at work)
- if you want to use your distributed generation system to have electricity even when the main network is not working (blackout), then you must have a stand alone inverter, because usually grid tie inverters stop working together with the distribution network (unless some smart solution is implemented and they can disconnect from the network and run independently)
- if your electrical energy supplier is paying renewable energy at a higher price, as a form of support for distributed generation, then you should sell it back to the grid with a grid tie inverter, instead of keeping it for you.