Also how do you figure out how many rigs I can put per PDU? I don't really understand the watts / volts / amps etc. - so to me when I see online that the T2T runs around 9-10 amps - that makes me think that I can only run 2 on the PDU since it's connected to a 20amp circuit.
[...]
Can I really run 3 x Innosilicon T2T 30th on each 30amp PDU? If so that means I could essentially run 45 of them. For some reason I don't think I can run that many.
The shortest explanation is a basic electrical calculation - Watts = Volts X Amps , quick switch can get you amps by going Amps = Watts/Volts. Then you look at the circuit and apply an 80% safety factor, so your 30 Amp circuit can hold 24 Amps. This will also mean that your 30 Amp PDU's can do the same but likely distributes 12 Amps on two different circuits internally - mine have 2 sets of sockets.
I don't know what PDU you have but if they are similar to mine, 12 amps per side you won't be able to run 3 T2T models, as you can't split the load of the 3rd across the 2 PDU circuits. Hope that makes sense, it's similar to when people used to use 2 different 110V 15amp circuits to run s9's on 2 power supplies.
Yes, I plan to go for the long haul, at least a few years mining with the next upgrade I make. I can probably only afford about 15k (including the profits from the sale of my S9s). So I am trying to figure out what rig I'll be able to buy to fill my slots for that much, as well as what makes sense for the power I have. And of course how many of that rig I should buy.
[...]
I guess I'm just looking for some advice on what rigs I can run on my PDU's safely, how many of them, and of course which ones are going to be the most affordable, but still increase my hashrate instead of just continuing to run my S9s.
With your budget it's a tough call where to go for a nice upgrade. You might want to do a blend of the T2T and maybe something like the T17 which is a better bargain due to tariffs if they come into play. With the way the number look now here is pretty much your best case scenario, and becuase it is only an upgrade on fairly efficient still profitable gear for you waiting for a further drop in prices makes sense. It's just not a guarantee what will be available for what price in the future.
When you factor in the US tariffs the T2T is 30 TH at 566$ so 18.86$ if you long play it and order 1 at a time at least a week apart; more like 707.5$ so per TH it's - 23.58$. It can be done though and might actually walk you into the option I think works best for you; blending. Do this steadily for the very cheap hash, but also order T17's, maybe a couple in stock with coupons you can buy from members here on the forum. T17 numbers are as follows - The T17 is 42TH @ 898 in stock so per TH it's 21.38$, Delivery in January is 20.14. Now with a coupon you can save an additional 10%, but you will have to buy at least the first one from a member so more like 6.5%. There are a few of these for sale for 250$.
If I had the budget and the mindset I would order 8 T17's, buy a 689$ coupon from a member here selling 2, your order would be 7184$-689$=6495$ plus shipping. I would double check that these avoid the tariffs being assembled outside China first, but assuming so that is a good first batch to swing into the farm and make space trying to move S9's. With maybe 3 T2T models ordered and delivered over the next month you will be at 416 TH of upgraded gear with 22 S9's cleared out or up for sale. This is a little over half of your upgrade budget to this point, being 8193$.
Now you can see if Bitmain gave you a coupon for this order. If so you can apply it to a less expensive future batch of T17's maybe, depending on if you keep ordering T2T's or re evaluate when you see what pricing looks like at the end of January. I don't know what that leaves you for your old hashrate but we are taking off less than half your S9's so you should still have around 400TH old gear and 416 TH of new gear. New total hashrate with 8443$(added in coupon cost) invested, 22 S9 sets available for sale = 816 TH an increase of 66TH.
Good luck with the upgrades.