<.< with all the Mk I's floating around find one that outputs the amount of energy you need (should match the tier of the adjacent storage block or less) and set it up like so. Viola! It runs til it fills up the storage block and then shuts itself off. Good for backup power when you've been overclocked macerating everything your miner just dug up or just want to refill that MSFU you just dumped into the Matter Fabricator. Click the button and it'll safely shut down within ~1 tick of the storage filling up, and will continue to run if you are pulling power out of it.
BatBox Tier Reactor 20eu/t 2EFF
Might be good for an off-base charging station.
MFE Tier Reactor 100eu/t 3.33EFF
Getting above that is more complicated, and prefer to keep the reactors chamberless for this idea.
MSFU Tier Reactor x5 = 497eu/t 3.33EFF
Any MK I should do for an MSFU, but 5 MFE 0 chamber reactors connected like screenshot 2 should work and output 497eu/t which is almost enough to saturate the output. You can actually setup multiples of the screenshot and have it fill to the MSFU faster than the it can deplete itself guaranteeing shutoff even with a load on the MSFU.
Those reactors are self designed and there might be better ones on the Reactors thread but I wanted to match the output to the tier's capacity. Any reactor can be setup like this as long as the detector cable is adjacent to it or one of the chambers but I was experimenting with the idea of one directly coupled to the storage block to pull power from the reactor as a back up charge and having a green power source fill it in times of low/no load.
Theoretically when storage redstone is working properly you'd be able to setup a self-filling storage block with "Emit when empty" as the trigger, and then the reactor will hold itself on until the storage is full. Drain it, rinse lather repeat.