Generac shares jumped as much as 45% after Amazon agreed to buy backup generators worth up to $8 billion for its data centres. Only $2.4 billion (or 30%) of that is actually contracted, the rest is maximum spend Amazon may incur. Besides, Amazon also got the right to buy almost 3% of the company. It is the second time in five months that a tech giant has taken a stake in its power supplier just before announcing the order that sends the stock up.