Our calculation methodology

1. Solar irradiance data — PVGIS v5.2

All irradiance figures come from the PVGIS v5.2 dataset, maintained by the Joint Research Centre (JRC) of the European Commission. PVGIS uses the CM SAF SARAH-2 satellite dataset for Africa, which provides monthly irradiation data at a spatial resolution of approximately 5 km × 5 km.

We query PVGIS using each city's latitude/longitude coordinates with the following fixed parameters:

• System type: Grid-connected (fixed mounting)
• Tilt angle: Latitude-optimised (typically 25–35° in SA)
• Azimuth: North-facing (0° = North in Southern Hemisphere)
• Output: Annual specific energy (kWh/kWp/year)

We report annual kWh per kWp installed (kWh/kWp/year). Peak sun hours (PSH) per day is derived as: kWh/kWp/year ÷ 365.

Source: PVGIS Interactive Tool — EU JRC. Data was last extracted in January 2025.

2. Electricity tariff data

We use the residential block tariff approved by NERSA for each municipality for the 2025/26 financial year. Tariff data is sourced from:

• Municipal tariff schedules published on each municipality's official website
• Eskom's published residential tariff for areas on direct Eskom supply
• NERSA's Electricity Tariff Approval database

For cities without an exact tariff on record, we use the provincial average derived from the closest known municipality. All tariffs are reviewed annually after the NERSA approval in June. The Eskom tariff page and City of Cape Town tariff page contain the most recent approved figures.

3. System energy yield calculation

Annual energy yield is calculated as:

We use a loss factor of 0.80 (80%), which accounts for the following real-world deductions:

This is consistent with PVGIS's own default loss setting of 14% plus additional practical losses. Industry installers typically estimate 18–22% total losses for a well-designed South African installation; we use 20% as a conservative midpoint.

4. Self-consumption assumptions

The fraction of solar energy actually used by the household (vs. exported to the grid or wasted) depends on battery backup and usage patterns:

• Without battery (grid-tied only): 60% self-consumption — used for the lower bound of savings ranges.
• With battery (hybrid system): 85% self-consumption — used for the upper bound of savings ranges.

These figures are consistent with SolarEdge and Victron Energy's published South African self-consumption studies and validated against installer reported data.

5. Monthly saving formula

We round to the nearest R50 for readability. All savings figures represent avoided electricity cost — the money you would otherwise pay the municipality or Eskom for the same units.

6. Payback period

This is a simple nominal payback calculation — it does not account for annual tariff increases. Including the typical 10% annual tariff increase would shorten actual payback by 0.5–1.5 years. We use nominal payback as it is the most conservative and verifiable method.

System cost benchmarks used for calculations (mid-range, fully installed with inverter and panels, no battery):

• 3 kW: R57,500 (mid-range 2026 market)
• 5 kW: R87,500 (mid-range 2026 market)
• 8 kW: R130,000 (mid-range 2026 market)
• 10 kW: R172,500 (mid-range 2026 market)
• 12 kW: R220,000 (mid-range 2026 market)
• 15 kW: R290,000 (mid-range 2026 market)

7. Data freshness policy

• PVGIS irradiance data: Updated when PVGIS releases a new dataset version (typically every 2–3 years). Current: PVGIS v5.2.
• Municipal tariffs: Reviewed and updated every June/July following NERSA approval of the new financial year tariffs.
• System costs: Reviewed quarterly. Costs are mid-market estimates and vary ±20% depending on installer, equipment brand, and site complexity.
• Page revalidation: All programmatic pages use Next.js ISR with a 24-hour revalidation window.

8. Limitations and disclaimers

• Savings estimates are indicative only. Actual savings depend on your specific usage profile, roof orientation, shading, and the equipment installed.
• We do not account for Time-of-Use (TOU) tariff optimisation or feed-in tariff revenue (Eskom SSEG / municipal buy-back), which can improve ROI further.
• System costs exclude battery backup unless otherwise stated.
• SARS Section 12B rebate savings are not included in the payback calculation — factoring these in would reduce payback by 0.5–2 years depending on your tax bracket.
• FlowLeads is a lead generation platform, not a solar installer or financial advisor. All financial decisions should be verified with your installer and a qualified financial planner.

9. Commercial disclosure

FlowLeads is a free service for homeowners. We are paid by SAPVIA-registered installers on a referral basis when one of them wins your business after we route your quote — and in some cases by accredited comparison-engine partners we redirect homeowners to (clearly labelled at the point of redirect).

This referral fee comes out of the installer's margin and does not affect the price you pay. It also does not influence which installer we match you with: routing is driven by suitability (system size, location, roof type), response time, and homeowner ratings. We have removed installers from our network for slow response times, inflated quotes, and POPIA breaches — and we will continue to do so.

Any product reviews, brand comparisons, or installer rankings on this site are editorially produced. We do not accept payment for inclusion in those.

10. Questions or corrections

If you believe a tariff figure or irradiance value is incorrect, please email hello@flowleads.co.za with a link to the official source. We aim to update verified corrections within 5 business days.