PV panel power

Why PV panel power in this website is expressed with W not Wp?

The capacity of a solar installation is expressed in watt peak i.e. Wp. Solar panel output is expressed in units of watts (W) and it represents the panel’s theoretical power production under ideal sunlight and temperature conditions.

So if someone tells me they are offering a 10kW (output) system, what capacity in kWpeak would I need to install?
Given that the ideal sunlight and temperature is at its best.

They are one of the same. When referring to a system size, kW is kW peak.