Run this in an elevated Command Prompt:
powercfg /batteryreport /output C:\battery_report.html
Open the HTML file and check:
Design Capacity vs. Full Charge Capacity — if Full Charge is significantly lower than Design (e.g. 35Wh vs 45Wh original), the battery has degraded or is being misreported.
Recent usage — look for abnormal discharge rates at idle or sleep.
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Open Resource Monitor → CPU tab → sort by Average CPU
Also check the Services tab for anything unexpected running high
Run the full Surface diagnostics suite and note any battery, thermal, or firmware warnings.
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For a Surface Pro 9 purchased in December 2023, the battery should generally still be performing reasonably well unless there has been unusually heavy use or a hardware issue.