Have had a 'Solara M. Series' semi flexible panel, which claims to be 40Wp, on the roof for a few weeks now. It is about 2 ft. 6 in. by 1 ft. 6 in. and were amazed to find that its regulator indicated a charge from sunrise to sunset even when cloudy. Boat batteries are usually at their lowest in the morning and I was in the habit of running the engine for about 2 hours and a further hour in the evening before 8 o'clock. I now find that there is no need to run the engine in the morning as the voltage is generally higher than before. 2 hours in the evening seems to be enough for the lights and TV. In order to run the lights and TV in the evening you need enough battery capacity to store the power. You can generate and store the power with more panels during the day. Get it right and you wont need to run the engine! But then you wont have hot water!