Sri Lanka Solar Payback Calculator — Net Metering, Net Accounting & Net Plus

The calculator runs three deterministic steps for each year of a 20-year horizon, then chooses how to settle the energy flow based on the scheme you pick. Every formula and table reflects the PUCSL Net Plus Tariff Order (01 January 2024) and the PUCSL March 2024 Domestic D-1 tariff schedule.

1. Annual generation

Generation is modelled as gen = systemKW × insolation × 365 × orientation × (1 − degradation)^(year−1). District insolation is read from the Sri Lanka Sustainable Energy Authority solar atlas — values range from 3.9 kWh/kW/day in Nuwara Eliya to 5.2 in Jaffna. The orientation multiplier is 100% for south-facing, 92% for east/west, 80% for north. Annual panel degradation defaults to 0.5%/year, the mid-range warranty figure across major Tier-1 module manufacturers.

2. Allocating self-consumption, exports, and imports

Under Net Metering, the calculator caps offset at the smaller of generation and consumption. Any surplus is treated as banked kWh credit with no cash value — that matches CEB's SOP for net-metered customers. Bill-after is computed by walking the same PUCSL block schedule our Sri Lanka electricity bill calculator uses, at the reduced monthly import, never falling below the minimum monthly fixed charge. Because the block tariff is steeply progressive, shaving the top blocks first is what makes solar pay back so fast — the units you stop buying were the Rs 45 and Rs 70 ones, not the Rs 13 ones.

Under Net Accounting, self-consumption is assumed at 35% of generation (SEA reference figure for a residential daytime load profile) and capped at actual consumption. Exports are credited at Rs 22.00/kWh per the PUCSL gazette. The remaining import is billed at the block schedule.

Under Net Plus, all generation is exported. Earnings are paid at the PUCSL feed-in tariff for the tier matching your system size (≤10 kW, 10-100 kW, 100-500 kW) — different rates for years 1-7 vs years 8-20. The customer continues to pay their full electricity bill since Net Plus is a pure-export contract.

3. 20-year cash flow

Each year's net cash impact is net = billBefore − billAfter + earnings. Avoided-bill components grow at the tariff-inflation rate you set (default 8%/yr, the CEB rolling average since 2020 — you can pressure- test that assumption against the Sri Lanka inflation calculator). Net Plus feed-in tariffs are PUCSL-fixed for 20 years and therefore not inflated — this is the single biggest reason a Net Plus contract that looks generous in year 1 quietly loses ground to Net Metering by year 10, once grid tariffs have doubled but the FIT has not. The simple payback year is the first year cumulative net cash flow crosses zero (linear-interpolated within the year).

4. Carbon offset

CO₂ saved is generation × 0.53 kg/kWh — the SEA Sri Lanka Energy Balance 2023 grid emission factor. Reported per year and across the 20-year horizon.

5. Financing and the loan-vs-cash decision

The 20-year cash flow above assumes you pay the system cost up front. Many households instead take a solar loan — the SLSEA / Bank of Ceylon Surya Bala Sangramaya facility historically ran at concessionary rates. If you borrow, the honest comparison is monthly loan instalment versus monthly bill saving: as long as the year-1 net cash from this calculator exceeds twelve loan instalments, the system is cash-flow positive from month one. Run your quoted price and rate through the Sri Lanka loan EMI calculator and set that annual instalment against the “Year-1 net cash” figure here. A loan lengthens the true break-even by the interest you pay, but it does not change which scheme wins — that ranking is driven by tariffs and system size, not by how you fund the panels.

6. Edge cases the model handles

Three inputs are deliberately clamped so the numbers never mislead. A system size of 0 kW returns zero generation and a nullpayback (“Never”) rather than a divide-by-zero. An implausibly large monthly consumption is capped at 100,000 kWh so the block walk stays finite. And a heavily oversized system on Net Metering — where generation far exceeds consumption — correctly banks the surplus as zero-value credit rather than inventing a cash payout, because CEB net-metering never pays for surplus units. That last case is exactly where Net Plus or Net Accounting overtakes Net Metering, and the scheme-comparison strip makes the crossover visible without you having to re-run the tool three times.

Cross-check

A second helper computes the layperson's simple payback as systemCost ÷ year-1 net cash and surfaces it next to the modelled payback. The two should agree to within ±0.5 years when degradation and inflation are modest. A larger gap is a hint to sense-check your inputs.

• PUCSL — Net Plus Tariff Order (01 January 2024)
• PUCSL — Electricity Tariff Determination (Domestic D-1 schedule)
• Ceylon Electricity Board — Solar Programme rules
• Sri Lanka Sustainable Energy Authority — Solar resource atlas
• CEB — Tariff Schedule (2024 revision PDF)

PUCSL tariffs, SEA insolation factors, and the grid emission factor were last cross-checked on 2026-05-16. The page is reviewed when any gazette is amended. Calculations cited on this page reconcile to PUCSL/CEB sources within ±1%.