In simple terms, you have costs and you have income generated.

Costs will be:

- Initial capital costs
- Interest payments
- Operating costs

Income generated will be from:

- Power Generated
- Tourists
- etc.

(OK, so the only likely one will be (1))

You keep your capital costs to one side.

Then you look at your monthly income and subtract the monthly expenses to get a net income (you may or may not have to subtract taxes from this).

What is left is expressed as a proportion of the capital cost.

So, if your capital was $100,000, interest is $750 per month, and operating costs are $1000 per month, and your gross income is $3500 per month then your net income is $1750, or about 1/57.

This tells you that the payback period (in this case before tax) is 57 months.

In real life the calculations will be more complex as the amount of power generates will vary and the tariffs received will not be constant. If you need to pay tax, then you may also need to look at the cost of depreciation because that can be offset against your tax. Whether you include that in your payback figure depends on what you're trying to prove.