Since it's not a battery storage setup, the energy being sent into your home circuit alleviates demand by a small amount. Where did they come up with 10 to 25% savings? Factors such as an optimal view of the sun for as much as possible, south-facing or biased East or West, would be the max. payoff. Night would be a zero net gain. At a savings of $7 a month, the panel would pay for itself in maybe 10 years not factoring in government subsidies. You need to keep it clean as well for it to maintain its potential output.