A homeowner receives three quotations with payback periods of six, nine and eleven years for similar systems.
The differences are almost entirely assumptions. One assumes a high proportion of generation is used in the house, one assumes rising electricity prices, and one uses a conservative generation estimate.
None of the three explains its assumptions. She is comparing three arithmetic opinions presented as facts, and the honest one looks like the worst offer.
What the benefit is actually made of
Two components. Electricity generated and used in the house, which avoids buying it at the full rate, and electricity exported, which earns a considerably lower rate.
Component 1: self consumption
The valuable part. Every unit used as it is generated avoids the full purchase price, and the proportion achieved depends entirely on when the household uses electricity.
Component 2: export
The smaller part. Exported units earn a rate well below the purchase price, which is why a house that is empty all day gets considerably less from the same system.
Component 3: the installed cost
Panels, inverter, fitting, scaffolding and any electrical work. This is the figure everybody compares and it is only one side of the calculation.
Component 4: the ongoing costs
Inverter replacement at some point in the system's life, occasional maintenance and any monitoring. Frequently omitted entirely from a payback figure.
A worked example
A system costing six thousand pounds generating three thousand five hundred units a year. With forty per cent used in the house at thirty pence, that is four hundred and twenty pounds.
Adding the export
The remaining two thousand one hundred units exported at around six pence gives a hundred and twenty six pounds. The total annual benefit is five hundred and forty six.
The payback that produces
Six thousand divided by five hundred and forty six is around eleven years. The same system in a household using seventy per cent directly returns nearly eight hundred a year, and pays back in under eight.
What that comparison actually shows
That the system is the same and the household is different. A quotation that does not ask about consumption patterns is producing a number about somebody else's house.
Ask about the daytime occupancy
Somebody home during the day, working from home, or out entirely. This single fact moves the payback period by years and it is the first thing that should be established.
Ask about a battery separately
It raises self consumption and adds cost, and it should be evaluated on its own arithmetic rather than bundled into a single payback figure.
Ask what electricity price was assumed
A calculation using an optimistic future price is a forecast rather than a figure. Using the current price is conservative and defensible.
Ask what generation estimate was used
Orientation, pitch, shading and location all affect it, and an optimistic generation figure is the easiest way to produce an attractive payback.
Publish the assumptions with every figure
An installer that shows the arithmetic wins against one that shows a number, which is the same openness as solar enquiries from homeowners still doing the maths and commercial solar enquiries from business owners.
Do the calculation with the customer at the enquiry
An assistant that asks about occupancy, current electricity use, roof orientation and shading, then shows the arithmetic with its assumptions, gives a homeowner a figure she can compare properly. The same payback framing appears in website assistant payback period and physio insurance and self pay questions. See pricing, run the numbers on the chatbot ROI calculator, and the sector view sits under solar installers.