Solar Savings Calculator: The Inputs an Honest Rooftop Estimate Needs

How it is usually done

A customer asks what solar would save them. A salesperson looks at one electricity bill, applies a rule of thumb about system size, multiplies by a generation figure from a supplier presentation, and produces a monthly saving on the spot. The number is memorable, and the customer repeats it back at commissioning.

A calculator does not make the sale more cautious. It makes the assumptions explicit, so the same site quoted by two salespeople produces the same answer, and so the estimate can be defended when generation is measured against it a year later.

Grouped by where the information comes from, which is also the order it arrives.

• Monthly units for twelve months — from bills, because seasonal variation drives sizing.
• Tariff category and applicable slab — domestic, commercial or industrial, with the rate that will be displaced.
• Sanctioned load — often a constraint on permitted system size.
• Shade-free roof area and orientation — measured at survey, not estimated from a photograph.
• Roof type and structure notes — RCC, metal sheet or tile, since mounting cost follows from it.
• Proposed system size — in kilowatts, with module wattage and count.
• Generation assumption — units per kilowatt per year, sourced from your own monitored sites.
• Self-consumption share — how much generation is used on site rather than exported.
• Export treatment — the assumed value of surplus units, with the date of the policy assumed.
• System price and subsidy status — the quoted price, and whether an application is being made.
• Financing — loan amount, tenure and instalment, where the customer intends to borrow.
• Annual maintenance provision and escalation — the two costs most estimates quietly omit.

Illustrative example only. Every figure below is a placeholder used to show the structure of the calculation, not a benchmark for any location or system.

The right-hand column is the important one. An estimate whose inputs cannot be traced to a source is a guess with a decimal point.

One salesperson with a good sheet can run twenty live enquiries. A team of six cannot, because the sheet fragments into six versions with six different yield assumptions, and nobody can see which enquiries have gone quiet.

In a CRM the survey is a record with photographs, the quote versions sit on the deal, the assumptions come from a shared template, and follow-up runs on a sequence suited to a decision that takes weeks. When the deal is won, the same record carries the survey and design into installation.

Sizing from one bill. A single summer bill oversizes the system and a single winter bill undersizes it.

Hiding the yield assumption. If it is not on the quote, the customer cannot check it and neither can you.

Treating export like self-consumption. They are usually worth different amounts, and the difference is large.

Omitting maintenance. Cleaning and servicing are real annual costs and belong in the payback.

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