Probably the easiest thing would be to charge them by however much *more* your electric bill is than it would normally be.
If you could monitor this over a period of several months and come up with a percentage, then that would keep things fair in the future when you had any electric rate increases.
Otherwise you would need to route all the wiring through some sort of electric meter and total this up monthly.
AND... There is nothing so complex as an electric bill! There are base amount rates which are lower, then the rate goes up after a certain amount of use, then an allowance for lower rates for an electric water heater in some cases - up to a certain amount, then transmission charges, taxes which could be a percentage or a set fee per customer, or the taxes could be tiered depending on how much electricity is used, etc.
So if you use a small amount of electricity, the rate per kWh will be cheaper than what you will be charged for beyond that basic amount.
Here is an example...