This idea was suggested by @meatlug82 in the comments in Chapter 15. It's a bit of that and a bit of theories I have myself, so, sorry if it's not what you were expecting.
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"Life in the Undergroud had been hard but not impossible. However, it was not an ideal place to live for multiple reasons. One of them being the obvious that we never see the sun, but the fact that we are also without a permanent source of electricity makes things harder. Monsters with high levels of magic are fine because they can simply form magical electricity. However, not all monsters are apt at magic, and the ones with low levels have been complaining. Asgore has decided that I should be the one to look into it. Our plans to escape the Underground have been useless so far. I have and idea, but it could be years before it's finished."
"Gaster, you know I usually find your work fascinating, but I already know half of this. Just get to the solution already." I smiled at my boyfriend as he handed me an order of fries. "Burger's gunna be a sec." I nodded, before continuing my excited rant.
"Geothermal Power."
"Excuse me?"
"Geothermal Power. That's the answer. If I could create a facility that converts geothermal power into magical electricity, we'd be golden. It'll take a lot of work, and a lot of sleepless nights but I think it's possible."
"Don't work to much, you've still got two boys to take care of."
"I know, nothing could come before my sons. Still, you've got to admit, it's rather exciting."
"For you perhaps, but I'm sure your average monster wouldn't understand half of it."
"That's about all I understand of it, so I'll ignore the fact that you implied I wasn't normal."
"Oh? The Great Royal Scientist W. D. Gaster doesn't understand something? It must be a very difficult subject."
"Yes and no. There are multiple different types of geothermal power, and I just have to find which one is best for conversion to magical electricity. All of the original types use a generator, so making one should be at the top of the list." Grillby placed the burger down on the bar and I passed him my Gold in response. Before I could continue my rant, I was tackled by a tiny ball of orange.
"Dad! Dad look! Undyne gave me this new scarf so Snowdin won't be as cold anymore!" Papyrus said, still hanging off my shoulders. Sans, Scorch, and Undyne were right behind him. Undyne was flushed bright red, and Sans was still snickering.
"Sans, no teasing your brother's friends." I scolded, making his mouth drop open. Undyne giggled, but flushed again when I winked at her.
"Gaster, no teasing you son's friends." Grillby said, walking out of the back room.
"Fine." I replied, pouting slightly. My smile returned when he kissed the top of my skull. I pulled him down by the shirt to kiss him where his mouth should be.
"Abort! Abort! Gay old men alert!" We both laughed as Papyrus and Undyne ran out of the restaurant.
"C'mon Sans, we better go catch them." Scorch said, smiling at her dad. Sans got up from the chair, but not before grabbing a ketchup bottle.
'You'll have to pay me one day kid!" Grillby yelled after him as the two rushed out of the diner.
"Kids are kids Grillby. I'll pay for him." I said, pulling out some more Gold.
"Don't. I've already got a pretty large tab on him. I wanna see how big it'll get before he pays." I chuckled as Grillby moved around the bar to sit in the stool next to me.
"I love you." I said, leaning into him.
"I love you too." He replied.
Things are finally changing for the better for all of the Underground.