電 electricity

I flip the switch and DEN ('then') the electric light goes on.

Electricity comes from lightning in the rain and also from the lightning breath of dragons.

train ★★★★★
電 (electricity) + 車 (car) = 電車 (train)

telephone ★★★★★
電 (electricity) + 話 (conversation) = 電話 (telephone)

electricity ★★★★★
電 (electricity) + 気 (mood) = 電気 (electricity)

electricity. Also, instead of saying 'lamp,' most people will merely say, 'Turn on the denki, would you?' (BOOBOO)

battery ★★☆☆☆
電 (electricity) + 池 (pond) = 電池 (battery)