The Sun-Pak is an experimental solar battery created by Sparkplug. The Sun-Pak is able to absorb enough energy from one hour of sunlight to power a transformer for 24 hours!
The Sun-Pak Prototype is made of metal and fiberglass, and secured externally to the Transformer it's powering by means of an electronic lock. As a prototype, the Sun-Pak is not a sealed unit; it can easily be rewired for adjustments (such as increased power output) but this also means that the prototype is not waterproof, shockproof, bombproof, or acid-proof.
Find Your Fate Junior
Sparkplug's invention of the Sun-Pak created a power-struggle among the Decepticons to possess its secret.
Sparkplug suffered brain damage when injected with a cerebro-shell and was unable to re-create the Sun-Pak after the prototype was destroyed. The Decepticons found Sparkplug's follow-up invention of the Portable laser-operated toothbrush much less interesting, and let the stupid human go.
The above is one of several endings.