Every home as an annual kWh usage. If you simply add up your monthly electric bill statements or log into your online account which will have a usage tap you can see or easily figure out what your total kWh are. The same is true for solar panel production. It is a mathematical equation based on system size, quality of panels, quality of inverter system and of course weather. Weather will create a 10% variance from year to year the rest can be very accurately equated through mathematics. What this means is every system designed has a reasonable expected annual kWh production. So offering a guarantee simply gives you the piece of mind of knowing you are getting what you paid for.