What's the maximum wattage a PCI-E slot can handle on a dell Optiplex 790 SFF?

Industry standard is that 75w is upper limit of wattage that can be safely drawn from PCIex16 slot.  Just like most 6 pin PCIe connector with 2 x12V yellow wires is certified to safely handle up to 75w.  This comes down to about 3A for each 12V wire, which is max load current rating for 20 G wire.  If you use 18 gauge wires rated for 4.2 A, then a 6 pin connector can handle up to 100w.

75w is max spec power for PCIe slot in 790 MT.  790 SFF has 50w max in Dell tech guide probably because SFF has smaller psu than MT, and probably not because SFF had smaller electrical wire gauge for the slot itself.  SFF board is just a smaller version of MT board with some additional slots curtailed.  They most likely use same electrical wire gauge for PCIe slot etc.

When you wire a psu of unlimited power to 790 SFF motherboard, gpu draws 12V power from PCIe slot via motherboard 24 pin connector which has several 12V wires.  There is no built in safety mechanism in motherboard that controls upper limit of 12V power it can draw, i.e., no fuse.  The upper limit in practice is when current drawn exceeds safety limit, wire will melt and become a fire hazard.

it is good idea not to greatly exceed safety upper limit power.  So if you have a gpu that needs much more than 75w, it most likely has additional 6 pin connector for that need.  A 125-150w TDP gpu that has a 6 pin but no power connected to 6 pin will scream a loud beep to warn you when powered on in PCIe slot.

on the other hand, if you have a gpu with TDP of 75w, but you overclock it during use, it will likely draw a bit more than 75 w from PCIe slot as long as psu is powerful enough.  No built in safety mechanism to limit current drawn until wire melts.

Let's say I put this GPU in the Optiplex: https://pcpartpicker.com/product/3GrmP6/galax-geforce-gtx-1660-6-gb-ex-1-click-oc-video-card-60srh7ds03ex. If I put this in the PCI-E 16x slot and connected it using the 6 pin connector, what would happen? Would it work without problems? (I didn't actually do this.)

What if I installed a 750 watt PSU (Outside of case) and I put that GPU (Outside of case) in the PCI-E 16x slot via riser cables and connected it with the 6 pin PCI-E power connecter from the PSU? What would happen?

There is no 6 pin connector on 290W or smaller Dell power supply.

You cannot use a 225 to 300W card in a system that has LESS THAN 305W.

Smallest system that has 6 pin PCI-E power connector has 315W power supply.  Optiplex XE2.

https://www.ebay.com/itm/233917249072

4FCWX XE2 SFF 315W H315ES-00 D315E001L

And to all the people who say that Dell is ATX

Note Dell 875w combined power as well as the ATX design Guide from INTEL.

A large card requires riser and power supply ouside the DT or SFF case.

https://www.amazon.com/dp/B06Y1SX7ZN/

Bare minimum for 225w Gpu is EVGA 700br

https://www.bestbuy.com/site/evga-br-series-700w-atx12v-eps12v-80-plus-power-supply-dc-dc-technology-black/6346160.p?skuId=6346160

https://www.evga.com/products/product.aspx?pn=100-BR-0700-K1

EPS 12v not ATX 12vDT and SFF have much less power

Note over 150W for 3.3v/5v

6 Pin and 8 pin which is 2X 6 pin

Re: If I put this in the PCI-E 16x slot and connected it using the 6 pin connector, what would happen? Would it work without problems? (I didn't actually do this.)

I do not have video of exactly same pc and gpu, but here is a video of optiplex 380 SFF with GTX 960 using upgraded psu with 6 pin. No problem running.  The 380 SFF PCIex16 slot likely has low max watt (25-50w) in spec, although tech guide does not disclose that explicitly.  This video gives you idea that as far as gpu is concerned it does not care where power comes from.  If there is not enough from slot (much less than 75w), it will draw more from the 6 pin to satisfy its need.

Outside the case for PSU and Video card works.

However power supplies are not a single spec of WATTS.

Max combined power for 3.3v/5v rails is 90 to 130W max even for some vendors 1000W units which means they wont work due to insufficient power on the 3.3v/5v rails.

https://seasonic.com/pub/media/pdf/consumer/datasheet/PRIME-ULTRA-Gold-GD-GD2.pdf

This is why EVGA 700BR is as low as you can go.

https://www.evga.com/products/product.aspx?pn=100-BR-0700-K1

Dell marks the BLACK slots 25W on some models because they are wired X4 not X16 and that is the max power rating for an X4 slot.  Max power for X1 Slot is 10W.  This is also why a riser cable is needed for smaller cases.

https://www.amazon.com/dp/B06Y1SX7ZN

This is also why X1 to X16 riser cables have power connection because you cannot draw 75w from an X1 slot without damage.

https://www.amazon.com/dp/B08ZT3SKMX

This is from an Optiplex Desktop Model

This is a known safety standard for decades now.

https://pcisig.com/specifications/

PCI-SIG has reps from: Agilent, AMD, Dell, HP, Intel, Synopsys, NVIDIA, and Qualcomm.

Blue slot is wired X16 so it can handle 75w

Black slot is wired X4 so its 25W max regardless of the system size aka MT DT SFF. USFF has no slots so thats not an issue.  Nothing whatsoever to do with power supply size has to do with maximum current for the slot for UL94V fire safety regulations.  PCI Express for Dell has 4 slots with link widths of ×1, ×4, ×8, ×16. Larger models like the precision line have more slots.

NOTE BLUE SLOT VS BLACK SLOT

PCI SIG PER SLOT POWER SPEC