No vessel is 'required' to use their radar. The COLREGS do require you to use all available means to ascertain the threat of collision, and boards of inquiry look dimly upon navigational gear not used. Most merchant ships leave their radar on continuously at sea, even in good weather, as glancing in it occasionally may show something far off, not yet in sight, or something small, not seen visually in the prevailing seas. The latter might be a sub. periscope or a small sailboat, both of which it is nice to keep away from. Of course the merchant ship does not have power consumption issues. Run your radar when you need it.