If you've got a tier 1 generator but make it generate more than 32 EU/t, it will fill. There's no way to increase the tier/actual maximum output of the generators purely because they're not meant to generate that much in the first place.
If they wanted to make it so it could devs could add and upgrade slot to generators. The transformer upgrade would work great for increasing the output. Would especially work well for Geothermal Generators.