The problem before was that due to seasons on Mars, the winter had reduced
usage of the rovers because of the sun angle and dust problems.
The thing to bear in mind here is that the original rovers had a 90 day
life expectancy, nobody expected them to last as long as they did.. ie one
is still working now. as the life of this one is projected to be 2 years,
they needed to find a way to allow it to be independent of the seasons and
thus it brought its power with it from Earth.
Most long duration deep space missions have used RTG systems, a kind of
thermocouple generator using the heat from decaying plutonium.
Brian

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From the sofa of Brian Gaff -

Blind user, so no pictures please!
"The Natural Philosopher" wrote in message
...
"Curiosity is about twice as long and five times as heavy as NASA's two
previous Mars rovers, Spirit and Opportunity, and its plutonium-powered
nuclear battery will provide the mobile laboratory with about 2.7 kWh per
day, far in excess of the 1 kWh per day maximum of the earlier
solar-powered rovers. The nuclear battery is also much longer-lasting than
the solar power systems used by the previous missions, and is expected to
last through the rover's planned operations and beyond, providing the
potential for the mission to be extended."

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To people who know nothing, anything is possible.
To people who know too much, it is a sad fact
that they know how little is really possible -
and how hard it is to achieve it.