@Bc1609 - Re: And this covers......?
I think the charge as you have stated it would not stick on the grounds that, as long as the CPU and RAM were not 100% committed at the time, the malware was only transiently using spare capacity, hence the owner was deprived of neither.
However, it could be argued that the malware would have made the processor do more work and hence it would have consumed battery power. Therefore the battery would have expired earlier. The malware would therefore have deprived the owner of his/her phone completely for a length of time equal to how much earlier it had to be put aside for recharging than would otherwise have been the case.
I'm not sure if 'theft for a limited time' is a concept, but you could give it a go. Alternatively, the cost of recharging the consumed portion of the battery could be directly considered theft. A minor amount, but still - it is theft.