Big GPUs need big, good-quality power supplies.
How many watts?
Add up the GPU’s board power and your CPU’s power, then leave plenty of headroom. For an RTX 5090, NVIDIA recommends at least a 1000 W unit.
Look for ATX 3.1 and 12V-2×6
Modern NVIDIA cards use the 12V-2×6 power connector. An ATX 3.1 power supply with a native cable is the cleanest option and avoids bulky adapters.
Plug it in properly
Push the connector in fully until it clicks, and avoid sharp bends right next to the plug. A loose connection is the main cause of melted connectors.
Quality matters
Choose a reputable brand with an 80 Plus Gold rating or better and a long warranty. A cheap PSU is the worst place to save money in a high-end PC.