ROTTERDAM, Netherlands—Fire up a joint in the Netherlands? No big deal. Grow marijuana? That's a crime. Getting smoked: the nation's power companies.

Volt-hungry pot farms have been stealing hundreds of millions of dollars of electricity a year. The problem has gotten so bad that one firm has blown a fuse. Stedin Netbeheer BV, a grid operator with 1.8 million customers, is now sending employees on raids with armed police officers, using sophisticated grid analysis to unearth pot plantations.

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