Physics is basically applied math, so it shouldn't be that hard. :-)

I often think of the units like unknown variables and pull them with me in the equations. If I get the incorrect units in the answer, I know I've done something wrong. Units can to some extent be converted - one Watt is the same as one Ampere multiplied with one Volt. One Joule is the same as one Watt multiplied with one second.

Things tend to get complicated as the battery capacity is often measured in Ampere-hours (Ampere multiplied with hours!) rather than Joule. To get the actual energy storage in the battery, it's needed to multiply it with the battery voltage (perhaps 12.3V is a good number). The result will be some number multiplied with Ampere multiplied with Volt multiplied with Hours. One Watt is one Ampere Volt, so you have the answer in Watt-Hours. If you want it in Joules, just multiply it with 3600 seconds / hour (that's equivalent to multiplying it with one!). Watt multiplied with Hours multiplied with seconds multiplied with hours divided by hours ... that collapses into Watt Seconds, which is the same as Joule!

Kilo Watt Hour is also a unit often used for energy. Remember that one kilo is basically just 1000 ...

(This may look complicated in writing - but try doing the maths with a pen and a paper! It's easy!)