kay

Return On Investment can be stated in several ways. In the case of your solar heating, the idea is that you would save enough from the electricity that you no longer use to heat the water, to pay for the cost of the equipment and installation, in two and a half years. (If you didn’t heat your pool previously, then stating the ROI this way is useless.)
When dealing with investments like stocks, ROI is stated as a percentage: That stock has an 8.5% ROI would imply that the combined growth and dividend payments come out to 8.5% in a year. Or, if I bought the stock at 1000, sold it at 1080, I would say I had an 8% ROI (without regard to how long I held it).
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