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Rooftop solar planning estimate

Sri Lanka Solar Calculator – Size, Savings & Payback

Estimate a suitable rooftop solar capacity, first-year electricity generation, scheme-based financial value and simple payback using assumptions you can verify and edit.

120 kWhplanning yield per 1 kWp each month

Calculator

Model a rooftop solar quotation

Use twelve months of bills where possible. Replace the example cost and unit values with figures from your electricity bill, utility agreement and written installer quotation.

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kWp

Use the complete cash price including panels, inverter, structures, protection, installation and applicable connection costs.

Planning assumptions
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%

The 1,440 kWh/kWp/year starting yield follows Sri Lankan sustainable-residence guidance. The Rs. 27.06 export value is the PUCSL-published flat-tariff reference effective from 1 July 2024 for systems up to 500 kW—not a promise of the rate available to you. Replace it with the figure in your PPA.

First-year estimate

Rooftop solar planning results

Suggested capacity to match annual use—Rounded up to the next 0.5 kWp
Estimated annual generation—Proposed capacity × entered yield
Estimated first-year value—Using the selected scheme and unit values
Simple payback—Installed cost ÷ first-year value
Average monthly generation
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Generation used or credited against consumption
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Estimated exported generation
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Annual consumption covered by generation
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Approximate clear roof area
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Transparent methodology

How the solar estimate is calculated

Suggested capacity divides annual consumption by the entered annual yield and rounds up to the next 0.5 kWp. Generation uses the proposed system capacity, not the suggested capacity. Net Metering values generation credited against annual consumption; Net Accounting separates directly used and exported energy; Net Plus values all generation at the entered export tariff.

The roof-area indication uses 6.5 square metres per kWp as a screening allowance. Actual module efficiency, spacing, roof shape, access paths and fire-safety requirements determine the installable capacity.

Example: estimate solar size from monthly electricity use

A household using an average of 500 units a month uses about 6,000 units a year. With the default annual yield of 1,440 units per kWp, the screening calculation suggests about 4.17 kWp and rounds that up to 4.5 kWp. The quotation should still be checked against roof space, daytime use, shade and the selected connection scheme.

Rooftop solar calculator questions

Should solar capacity match my highest monthly bill?

Not usually. Twelve months of electricity use provide a better starting point because consumption can change with weather, holidays and appliance use. Your connection scheme and export terms also affect the most useful size.

Why is the installer’s generation estimate different?

A site-specific estimate can account for roof orientation, shading, equipment, system losses and local solar conditions. Replace the default annual yield with the documented figure from the quotation when comparing proposals.

A screening estimate is not a solar design

Shade, orientation, tilt, temperature, dust, downtime, grid curtailment, inverter sizing, equipment degradation, financing, maintenance, taxes and tariff terms can materially change the result. Obtain a site survey and written generation estimate from an SLSEA-registered provider, and confirm the connection scheme and payment rate with the utility.

Authoritative planning sources