Wave Energy Converters (WEC) are usually designed to achieve maximum efficiency by impedance matching with the incoming waves. We first review the typical features of a heaving buoy. One feature is that the buoy must be large enough in order to resonate in the usual frequency range of the sea waves. Another is that the frequency range of high efficiency is band-limited, as in many other devices based on impedance matching. Inspired by the FO3 system being developed in Norway by Fred Olsen and ABB Associates, and the Manchester Bobber from UK, we examine theoretically power extraction by a compact array of small buoys. It is shown that such systems have certain advantages over a single large buoy.