To generate the most power from your Solar PV units, you should place the panels on a South facing roof as this will get the most exposure during the day.
Roofs that face South-East or South-West can also produce reliable amounts of energy. If you only have a roof that faces East or West you can still put solar panels on them, but they’ll typically lose between 10%-15% of their efficiency.
If your roof faces North, then Solar PV units may not be a good investment due to fall in efficiency.