[NAME] you don't say which model you're buying, but I'm assuming it's the XPS 8940 with the spec you've given.  I have an XPS 8940, albeit with a lower spec. My thoughts are: 1) Dell wouldn't sell a computer with a power supply incapable of supplying enough power (would they?),2) Not all parts of the computer will be running flat out at the same time,3) Potential to upgrade in the future may be limited. With regards to the potential to upgrade the power supply, Dell supply four different power supplies with the XPS 8940 computer range, these are: 360W Liteon (2VD0G)500W Delta (5K7J8)500W Liteon (99TPH)500W Chicony (Y7R0X) The specifications of the three 500W power supplies all appear to be identical.  Also, the latest Dell desktop power supplies are of a bespoke size and also at least one of the power connectors to the motherboard is non-standard.  Therefore, 500W is as big as they go from Dell and putting a non-Dell power supply into the case is not going to be straight forward, if at all possible. 
Is 500W enough for 10700k + 3060ti? I just ordered a desktop with this configuration, and then I realize that it only has a max of 500w power. Is that enough? I used a calculator saying it needs at least 550w?  What should I do now? Is this available to upgrade power by myself afterwards?    Processor 10th Gen Intel Core i7-10700K processor(8-Core, 16M Cache, 3.8GHz to 5.1GHz) Video Card NVIDIA GeForce RTX 3060 Ti 8GB GDDR6 Memory 32GB, 2x16GB, DDR4, 2933Mhz Hard Drive 512GB M.2 PCIe NVMe Solid State Drive + 2TB 7200RPM Hard Drive Chassis Options 500W Mineral White Bezel Chassis including optical drive Wireless Killer Wi-Fi 6 AX1650i (2x2) 802.11ax Wireless and Bluetooth 5.1
Is 500W enough for 10700k + 3060ti?