Was there any other machine draining the power from the electric engines? And are they hooked up to energy conduits or conductive pipes?
I read somewhere they take from 10-20 mj/t so I set up 4 biogas engines to start with, and one portable tankful as a starter. Then I hooked up a fermenter with liquiducts to continually feed the engines. Power was never an issue and I quickly produced more biomass than needed, so I sent it to storage. I don't have the calculations on that. I'll try and figure out a way to test that in a bit. I'll also test the power consumption of strictly the apple oak and for funsies test the power consumption of the apple orchard and peat bog.
Gusmahler, you should probably hook up a squeezer to the system and not only does it product apple juice for more efficient biomass output, it produces mulch as a byproduct which will be more than enough to supply a single fermenter with fertilizer. That saves the fertilizer for the multi-farm. Trust me, you may think you have enough apatite for now, but you really don't. Especially if you expand into an orchard which eats up the fertilizer much faster. You'll have to devise a system where the fermenter uses both water and applejuice as the squeezer doesn't put out enough to keep up with sapling production and occasionally the fermenter stops producing. If you could gate it so that when it doesn't have enough applejuice then it switches to water, you'll be golden. I've tried that a little but can't quite figure it out.
Dirt has been an issue for me as well. I've been trading stuff for dirt with people as a supplement. I tried a peat bog but it runs too slow. It's decent, but not enough to keep up with dirt consumption. I tried scrapboxes, but turns out it's 1.37% not 10%, so that's no reliable. I am in the process of getting enough copper to make another multifarm for reeds to turn into plantballs to macerate into dirt. I think that will finally be my solution. I'm using xycorium soil so I don't need any water holes, so the entire thing is reeds. They also grow really fast.
I'll try and do a test on the wood/sand/sapling/apple production per hour. I know that after 6 days running, nearly continuous (maybe 15 hours it stopped) I have 35 or so barrels full of wood and 6 barrel fulls of sand, including using up a lot of that sand for fertilizer. Apples and saplings I have no idea as they are being consumed once produced, although I definitely produce way more saplings than are consumed by one fermenter. I now have three, and it's staying somewhat even, but my sapling overflow system is buggy. Poor gate design on my part.
As for refilling the multifarm with fertilizer and dirt, I suggest having the dirt in a chest two blocks away and connecting a wood pipe to the chest, a stone/cobblestone to that and the multifarm. Get 2 autoarchic gates. Set the one against the multifarm to be <96 dirt emit redstone signal. Set the one attached to the chest to be on redstone signal emit pulse. Then hook the two gates together with redstone tubing or jacketed wire. It's definitely failsafe. You can do the same thing with the fertilizer.