Looking for an estimate for a solar installation

Our neighbor's average monthly electricity bill here in Hamilton MT was $185.00 9 years ago. I designed a highly energy-efficient house and had a solar system installed. It paid off in seven years and our monthly bill is $4.20; the hook-up charge to the system. Now that we're adding an addition, the old P/V system needs to be enlarged and some parts replaced, since they're too small. Curious to see how long it'll take to pay off. We also have 4 solar panels that make hot water for under-floor heating and domestic use.