My observatory is at my club's dark site. Six years ago (or so) the club I belong to decided to open twelve small plots at the dark site to members for construction of private observatories. The club provides 10A of 120VAC for each observatory site. I ran a buried conduit from the "drop" provided by the club to my building. I ran the conduit through the slab so it enters the wall with no exterior fittings.
Like others, I installed a small panel on the exterior. If I remember correctly my panel has four slots, I use three. The external panel is intended to allow a club member to turn power off if something goes wrong. Luckily it's only been needed once, luckily again, not for a fire but just because my interior lights came on when I wasn't there. They were able to cut power and turn the lights off.
One circuit provides 120VAC to my observatory. I have a thermostatically controlled exhaust fan to keep my building vented. I also have power outlets on the west, south and east walls inside. I also have power on the west and north walls outside. Finally, there is an outlet at the base of my pier. I keep a clock and use a hair dryer (dew control) there.
One circuit provides power for a 30A(?) 12VDC power supply which goes to my pier and a set of 12V connections on the outside of my west wall. Currently only my mount and telescope fans draw power from that power supply. I have an 8'x8' slab outside my west wall and I've had guests connect to the outside power sometimes.
Finally, the third circuit provides for a second 30A(?) 12VDC power supply for everything else. I have 12V LED lighting and eventually will install an old car stereo head unit and speakers. That last is a project I've been meaning to get to since I installed my scope five years ago...
As to the question, "how much do you need?": if you think about it, not much. The only big load you're likely to have is a small air conditioner. Just check at Home Depot to see what the current draw is for what you're planning to install, if you're planning to install one. After that, your astronomy gear will draw hardly anything, at least when measured on the 120V side of the circuit. Even if you count up all the amps your mount, fans, camera, filter wheel, focuser, guider, etc will draw at 12V, you get to divide that by 10 when figuring out how much 120V power you need to supply to your power supply. OK, for your Paramount you only get to divide by 2.5...
I think your current plan (in your original post) is way more than you'll need. Perhaps you'll need a dedicated circuit for your heat pump (again, find out what the current draw is for the unit you want to install). Having a separate circuit for it may be a good idea anyway, it may generate a bunch of electrical noise you'd rather not have on your mount and camera circuit. After that, I'd bet a single 15A or 20A circuit will provide enough 48VDC and 12VDC power for everything else.
As to the question about running directly off your house's existing panel or installing a subpanel in your house or at your observatory, that's up to you. The most cost effective thing is to install a couple new breakers in your house's panel and then run wire directly to your observatory site. I don't see how having a subpanel in your house gains you anything. A subpanel in your observatory may get you a small amount of convenience since you wouldn't have to walk from your observatory to your house to cut power. I have never used the breaker panel I installed in my observatory (but I needed it because I share my circuit with another observatory and had to protect my wiring).
Edited by macdonjh, 05 July 2020 - 09:14 AM.