What anime features robot armor and reference to "Dell Power"
I haven't watched the anime personally, however I came across this anime before and the synopsis fits your discription.
I believe the anime you are looking for is: 
 Delpower X Bakuhatsu Miracle Genki!

Synopsis


  Manami is your typical schoolgirl, despite the odd machines her grandfather is always building. One day, though, an odd evil robot comes to attack Minami's school, and commits the ultimate offense of threatening her friends... and ruining her lunch. Now, she will stop at nothing to return order to the world in her grandpa's newest creation: the mecha Del Power X.


ANN also describes the loss of her bento:


  Hanegi Manami is a junior high school girl with unusual relatives, to say the least. Her grandfather's passion is building unusual things and has just had his latest concoction delivered to their doorstep. The mecha known as Del Power X. With a driver seat specially designed for Manami, its powers are rivaled by no other. Unfortunately, her grandfather's rival Von Getsueru and a plethora of other people are determined to prove that fact wrong, regardless of who is in the pilot`s seat, Manami must fight for love, pride, and above all else, the loss of her precious bento. 


Both describe the "old man" as the grandfather who builds stuff, and also made the mecha in question and the girl being upset about her lost lunch. The obvious reference is, of course, "Delpower" probably because it is homophonic to "Dell power" which would be an understandable association.