India, July 6 -- When extreme heat hits, people turn to AC, but what if the grid fails? From France to the US, heat waves are pushing power systems to the limit, raising blackout risks and energy prices when electricity is needed most.

In late June, a heat wave in France left nearly 70,000 households without power after a transformer failed under high temperatures. For about 24 hours, as the mercury hovered near 40 degrees Celsius (104 Fahrenheit), residents in parts of Brittany sweltered in deadly heat without fans or air conditioning.

Then as the United States baked in extreme heat in the run up to its 250th birthday celebrations, federal authorities issued blackout warnings, allowing grid operators to order large energy users like da...