When we started, we billed by the hour because that is how everyone did it. Then a customer ran a job that took 47 minutes, paid for a full hour, and emailed us annoyed. He was right to be annoyed. The GPU had been idle for 13 minutes and he paid for it anyway.
Per-hour billing makes idle time feel free. You rent an instance at 14:00, the job finishes at 14:35, and the cheapest thing to do is leave it running because you already paid for the hour. So people left instances on overnight "just in case." That is money we collect and a customer paying for a machine doing nothing, and neither of us is happy about it.
We moved to per-second metering in October 2024. The metering itself was the hard part, not the billing. An instance has a start timestamp, an end timestamp, and a price per second, which is the hourly price divided by 3600. Billing is just summing that. The hard part was deciding what counts as "running." Booting counts. Shutting down counts. An instance that crashed still bills until you delete it, which sounds harsh until you remember that the GPU is physically reserved for you the whole time.
Three things changed in the first month:
Per-second billing makes our invoices look fiddly. A 3 day instance shows up as a large number of seconds and people do sometimes ask what it means. We considered billing to the minute instead, and some of that survives: we round up to the minute on the invoice so nobody has to read 14-digit numbers. The pricing page says per-second, the invoice says per-minute. It is the same money.
The other tradeoff is ours, not the customer's. A customer who used to rent an instance for a week and forget about it now deletes it after two days. We make less money per customer and we are fine with that. The people who stay are the ones who keep coming back, because the invoice is always right.
Per-second billing is not a feature. It is a way of saying that you only pay for what you use, and meaning it. It cost us a couple of weeks of work and a small amount of revenue, and it made the whole thing feel fair. If you are running a job that takes 47 minutes, you should pay for 47 minutes.