The most important number describing a solar electric, or photovoltaic (PV), system is its “rated output” or “rated capacity”.
This is the power (in watts) that a given PV module or system can produce from a certain, standardised amount of sunshine (1,000W of sunlight per m2 surface area). It is sometimes referred to as the peak capacity (Wp) because the standardised condition is close to the maximum amount possible on earth. A standard test using this amount of sunshine is useful to get a standard power output figure for all panels (to help when comparing them).
When a panel is rated at, say, 180W then this means that the panel will produce 180 watts when the sun is shining on it at full strength (1000 W/m2) and at a perfect angle. Of course in the UK we don't really see a full strength sun very often! - plus there will often be some clouds, and the angle between sun and panel will not usually be perfect.
So in practice, the output is often less - when it is cloudy or the sun is lower in the sky. In average UK weather conditions, a PV system rated at 1000W (1kW) will produce between 700 and 900 units (kWh) of electricity per year.
To illustrate this: A PV roof consisting of 15 modules rated at 180W can produce a maximum of around 2,700 watts (2.7kW rated capacity). Over the course of a year, this roof will produce between around 1,800 and 2,400 kWh units of electricity. The output over time is the important figure for most applications.
PV produces much more energy in summer than in winter. A 1 kW array may produce more than 100 units in July but only 20 units in December.
The life expectancy of a PV panel is likely to be 30 years or longer though there will likely be some cosmetic physical decay and a decrease in energy output - see How long do photovoltaic panels last?