Solar-Powered Boat Carries 8.5 Tons of Lithium-Ion Batteries

bshell writes "The Verge has a great photo-essay about Tûranor PlanetSolar, the first boat to circle the globe with solar power. 'The 89,000 kg (nearly 100 ton) ship needs a massive solar array to capture enough energy to push itself through the ocean. An impressive 512 square meters (roughly 5,500 square feet) of photovoltaic cells, to be exact, charge the 8.5 tons of lithium-ion batteries that are stored in the ship's two hulls.' The boat is currently in NYC. Among other remarkable facts, the captain (Gérard d'Aboville) is one of those rare individuals who solo-rowed across both the Atlantic and Pacific Oceans, journeys that took 71 and 134 days, respectively. The piece has a lot of detail about control systems and design."

20 HP average?

[k2backhoe]

This has 512 m^2 solar array, incoming sun at directly overhead is roughly 1 kW / m^2, assume solar panel efficiency of 15%. This is a total power of about 76 kW or about 100 HP when the sun is directly overhead. Averaged over a 24 hour day, this is maybe 20-25 HP. 89,000 kg of lithium battery at 200 Wh / kg is 17.8 MWh. This would take 234 hours to charge with the sun directly overhead. That is about 40 days of clear sky charging, assuming you are not running the propeller at the same time. Something is fishy here. Sounds like he charges in port, then runs to the next port on solar plus battery (otherwise there is no need for this large battery / solar cell ratio). Then he repeats. Is my math wrong, or is this story a bit strange?