Despite some slightly questionable practices, PayPal continues to be the best (or only) way for millions of online businesses to take your money. So, an outage, even just a few hours long, can cost millions.

PayPal’s services were down for about two hours on Thursday evening, caused by a problem in one of PayPal’s data centres. PayPal didn’t elaborate on the outage, just saying:

“The network experienced an intermittent interruption during a period of relatively low traffic. At 7pm PT, the majority of volume is in APAC [Asia and Pacific countries] not in EMEA [Europe and the Middle East] or USA. This interruption was caused by a data center power outage and we apologize for any inconvenience this may have caused.”

But some back-of-the-napkin math can help show the scale of the loss. Last year, PayPal processed $644 million per day, or $26 million an hour. So even at a period of ‘low volume’, two hours of PayPal outage translates to tens of millions of non-processed payments.

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For some merchants, that might not be such a big deal—PayPal’s just one of many different ways to hand over digital money. But for some big-name sites like eBay, PayPal’s really the only way to handle transactions.

[Fortune]