Balance of System
Your solar modules and mounting system make up the largest portion of your solar PV system. You still need a variety of components to finish your installation which could include an inverter, controller, batteries, combiner box, fuses, grounding, and more.
Grid-tie or Standalone Inverters
Do you want power when the grid goes off-line? Are you selling all power produced back to the grid?
Grid-tie inverters are "tied" to the grid. They only work while the inverter is connected to the electrical grid AND the electrical grid is UP. If the grid goes down, your inverter disconnects the power you are producing, from the grid... it does not send power down the wires, or into your house, while the grid is down. If the grid has gone down and your neighbors lights are off, your lights will also be off. Grid-tie inverters have fewer components than a standalone inverter and as a result, grid-tie inverters are cheaper to purchase than standalone inverters.
Standalone Inverters can provide power to the electrical grid, but will also operate when the electrical grid is not available. These inverters usually have a string of batteries in the circuit, to provide power when the solar panels are not producing power (night). If you are connected to the electrical grid, your system must be installed to ensure no power is transmitted onto the grid, when the grid is down. All installation will need to be inspected before receiving authorization to connect to the electrical grid. Some standalone inverters also include the charge controller functions. See below for more information on charge controllers.
With a central inverter, the solar panels are connected together in strings, which then connect (via a combiner box) to a large central inverter. There may be more than one central inverter in a system, depending on the overall size of the solar system, design, etc.
Central inverters have been in production for many years and have proven to be very reliable. They are also very competitively priced. On the down side, because the solar panels are connected in series, if one panel is producing less power, the whole string will be pulled down. A panel would produce less power if it became defective, if it was shaded (even partially), or if it was covered in dust or debris.
A micro inverter is connected to one solar panel (though two micro inverters may reside in one case). The biggest advantages of micro-inverters are in small installations, below 10KW. Small systems on rooftops often have very different orientations of PV panels, there is partial shading, there is snow, there is dust. With the traditional string inverter technology, all modules are connected in series, and the maximum power that can be delivered in the system is decided by the weakest module. For example, if one module is trying to produce 250W, but it is under partial shading and can produce only 50W, then all the panels will be governed by the lowest amount of power. With micro inverters, only the panel that has the shading on it, is impacted.
On the down side, micro inverters are relatively new, without a long track record. They tend to cost more than a central inverter. And their life span has not been proven as yet. Having said that, there is one new product coming out at the end of 2010, that will replace the junction box on the back of each solar panel, with a new micro inverter. This design has eliminated some of the traditional micro inverter components and will have a 25 year warranty - same as the warranty of the solar panel.
A charge controller manages the electrical charge of the batteries in your solar system. If you don't have batteries, you won't need a charge controller.
When a battery reaches full charge, it can no longer store incoming energy. If energy continues to be applied at the full rate, the battery voltage gets too high, water separates into hydrogen and oxygen and bubbles out rapidly. The battery will also degrade rapidly and may possibly overheat. Preventing overcharge is simply a matter of reducing the flow of energy to the battery when the battery reaches a specific voltage. Unlike wind turbines and hydro plants, a charge controller can simply disconnect the solar panels from the batteries, and the solar panels will not be affected. With wind turbines and hydro electric plants, a load must always be applied to the power generator.
Most charge controllers will also disconnect the inverter if the battery voltage drops below a set point. This again, protects the battery and prolongs it's life span.
Maximum Power Point Tracking (MPPT) charge controllers usually cost more than traditional solar charge controllers, but the gain in overall charging efficiency (up to 30%) makes them well worth it in most cases.
Solar Combiner Box
A combiner box, takes the DC output form a solar module or a string of solar modules and combines them in parallel, into one DC circuit that then feeds the inverter or charge controller. Each input is individually fused. This is "sort of" the reverse of what a typical home AC circuit breaker panel does, which takes 1 AC input and distributes it to several AC circuits.
Grounding, Disconnects, Inspections
Grounding, disconnects and electrical inspections are all part of the electrical code. Contact your local electrical safety inspector or a qualified electrician, for clarification of what is required for your specific system.
Solar Production Monitoring
Central inverters will usually include a limited display of production information, on the front of the inverter. This may include the kilowatt output at that moment in time, total production so far for that day, total production since the inverter was energized, error messages, line voltages, etc. A remote display is often available at a reasonable price. Micro inverters, by their nature, do not provide a "front of box" display, as the inverter is often located remotely, on a rooftop.
Most inverters can also be connected to a computer or the internet via additional hardware. These monitoring systems can store the data produced and provide historical information via text and charts. As micro inverters connect to just a single solar panel, production numbers are available by panel, which makes identifying a poor performing panel, quick and easy.
In addition, most inverter manufactures have a free internet site that can be used to track your production. One of the advantages of receiving production numbers from thousands of sites - you can compare daily outputs to other nearby installations. This is another way of pinpointing potential production issues.
Here's a link to a 48 panel installation - ENPHASE 48 PNEL MONITORING EXAMPLE
In this "real" system, you can look at the day's production, the month's production and see the lifetime numbers. You can also scroll through a complete day, week or month - with this real life example, you can see where a shadow crosses a number of panels - each panel in the bottom righthand road, turns darkm then brightens up as the shadow passes over it.