If you have enough of them it may be good enough to power the electric fence that you use to keep the zombies at bay.
Using said zombie power to store energy in some other form (like compressed air or recharging batteries) your energy need to power a war tank would only depend on whether you have the resources to build charging stations and charge holders and how many zombies you could obtain. IE a billion zombies could provide a lot of power but not if you don't have the charging stations. A billion zombies trundling a billion charging stations are wasted if you only have one battery to charge.