Side-project AI (ComfyUI, LoRA, small fine-tunes) often starts with “should I just buy a 4090?” The better question is: how many effective GPU-hours per week will you actually burn?

A simple break-even sketch

Estimate capital for a used consumer flagship + PSU headroom.

Spread over 12 months → weekly ownership tax (ignore power/noise for a moment).

Divide by your cloud $/GPU-hour → hours/week where owning starts to win.

If your real work is bursty — a weekend LoRA, a deadline sprint — hourly rental usually wastes less money on idle silicon. Effective hours = GPU busy time, not an open SSH session.

Three billing pitfalls

Stopped compute ≠ stopped bill. Disks often keep charging after the GPU stops. Export weights, verify, delete unused volumes.

Data transfer eats wall-clock. Pushing large checkpoints can take longer than the job. Test with a tiny file first.

Bare images cost setup time.…