Before solar panels for your home were viable most solar panels were lower voltage and designed to work with off-grid systems where no grid power was available. These solar panels were usually lower wattage and lower voltage panels and were used to run, 12, 24 and 48 volt stand-alone power systems.
Now residential solar panels are mostly used for grid connect systems. Most solar panels used for homes are now in the range of 250-310 watts. They are designed to work in residential solar power systems either with a string inverter or with micro-inverters.
Usually, these panels individually have an open circuit voltage of around 40 volts. When they are installed for a string inverter they will be installed in strings usually between 7-10 panels and strung in series such that the total string of panels would have a total voltage within the acceptable voltage window of the string inverter (usually 280-400 volts).
When used with micro inverters each solar panel would have its own inverter that would convert the DC power output of each panel to 240 volt AC power. There are pros and cons of string inverters v's micro-inverters in terms of the way you choose to set up your solar panels for your home.
The solar panel size of 250-310 watts has become common because it is a manageable size for installation. If the panels were higher wattage they would become difficult for installers to handle, increasing both the risk that the solar panels would be damaged in installation and also the risk to the workers installing the panels.
In terms of cell technology most solar panels used for homes have crystalline silicon cells inside them. Most panels will have between 60-72 cells inside each panel to create its total power output. Within each panel the cells are strung in series.
There are two types of crystalline solar panels, monocrystalline and polycrystalline. A monocrystalline panel, as the name suggests contains cells cut from a wafer containing a single crystal of silicon. A polycrystalline cell is cut from a wafer containing a number of cells of silicon.
Theoretically, it is possible to make a monocrystalline cell that is more efficient than a polycrystalline cell because the faults between crystals can cause impedance to the flow of electrons across the cell. However, in practice mass produced polycrystalline cells are usually similar in efficiency to monocrystalline cells. Some argue that polycrystalline cells have their own advantages in that they may work better in lower light conditions but this is most likely also a marginal difference at best.
The real issue is that polycrystalline cell lines have become most common in recent times and as such polycrystalline cells are usually slightly cheaper than monocrystalline cells and so most often than not polycrystalline solar panels are most common for homes.
Efficiency isn't the big issue that many consumers think because when a panel is rated at a specific wattage it means that it produces that amount of power in standard test conditions. This means a more and less efficient 250 watt panel will still produce the same amount of power it is just that the more efficient panel may be very slightly smaller. However, this is only a real advantage where space on a roof is the biggest constraint and the differences are usually only slight.