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 PUCSL block schedule at the reduced monthly import, never falling below the minimum monthly fixed charge.

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). Net Plus feed-in tariffs are PUCSL-fixed for 20 years and therefore not inflated. 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.

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%.