Eureka!
The energy answer is simple:- Merely provide each grunt with a chewing gum generator, as they chew the energy used in masticating a special piezoelectric gum can be diverted to lithium ion batteries which are light and have a good watts/kilo ratio. When in action the adrenalin produced will make them chew more rapidly and with greater force so producing valuable needed energy at just the right time.
In the event of an extreme emergency needing rapid recharge they can just snort amphetamines which will provide the energy boost and increase aggression at the same time.
I would prefer to be paid in gold at the moment please.