Solar glossary
Payback period: what it means and why it matters
The number of years of savings needed to recover the capital cost of the system.
Why it matters in Nigeria
Payback is the number most Nigerian buyers decide on. Read it with the baseline: a system pays back far faster against generator fuel than against subsidised grid energy.
How PureWatts measures it
Median Payback (Merit-order sized, merit order)
med_sc3_payback_meritorder_yr
Median over the buildings in the group. Medians DO NOT COMBINE — never average these across groups; re-derive from the histograms instead. Payback ignoring the time value of money. Baseline: meritorder.
Solar + battery, merit-order sized: Same engine as sc2, but the target is what the building CONSUMES today x 1.5 (MO_SIZING_UPLIFT) rather than full latent demand.
In Osun, sizing sc2 to latent demand while crediting it only with displacing current spend produced a 6x-oversized system and a negative IRR that said nothing about solar. sc3 is the fix, not an alternative opinion.
Counterfactual: solar displaces the most expensive kWh first — generator before grid. savings = min(solar, genset_kWh) x genset_cost + max(0, solar - genset_kWh) x tariff. Approach B; the only baseline that is coherent for Bands B-E, where grid energy is rationed rather than purchasable.
Median over the buildings in the group. Medians DO NOT COMBINE — never average these across groups; re-derive from the histograms instead.
Median Payback (Solar + battery, vs grid + generator)
med_sc2_payback_mixed_yr
Median over the buildings in the group. Medians DO NOT COMBINE — never average these across groups; re-derive from the histograms instead. Payback ignoring the time value of money. Baseline: mixed.
Solar + battery: PV sized to cover FULL demand including round-trip losses: kwp = min(demand x (daylight + (1-daylight)/0.90) x 1.10 / net_yield, pv_max_kwp); battery = night demand / (0.80 DoD x sqrt(0.90)), capped at 2.0x PV kWp
Sized to LATENT demand — the energy the building would use if it could. Against a cash baseline that only reflects today's spend this looks oversized; that is what sc3 exists to fix.
Counterfactual: grid when available, generator otherwise, weighted by the NTL-derived grid availability. The realistic case, and what the headline numbers use. Always falls between grid and genset by construction.
Median over the buildings in the group. Medians DO NOT COMBINE — never average these across groups; re-derive from the histograms instead.
See these numbers for your area
Every state and local government area page reports payback period for its own rooftops. Browse the Nigeria rooftop solar data.
Related terms
How these numbers are produced
Approach A — energy value (upper bracket)
Sizes the system to the building's full latent demand and values that energy at the mixed grid/generator tariff. It answers what the energy is worth — not what anyone currently pays.
Approach B — cash saved (lower bracket)
Sizes the system to demand that is actually served today, values it by merit order (solar displaces generator kWh first) and caps it at current spend. It answers what a building stops paying.
Cohort: Viable rooftops (≥0.6 kWp potential).
- Demand is modelled latent demand, not metered consumption.
- Fuel is priced at ₦1,800/litre (operator-set). NBS reported ₦3,277/litre for May 2026 — on that basis every naira figure here is roughly 1.8× understated.
- Generator ownership is a model output, not a survey result: it follows from the grid-availability scale and the 25% demand-suppression assumption.
- Approach A prices energy nobody currently buys. Its savings must never be read as cash saved.
- Demand is MODELLED LATENT demand, not metered consumption.
- Generator ownership is model output, not survey.
- Medians do not combine across groups — re-derive from the histograms.
- Per-building files are float32 with 4 significant figures; cast to float64 before summing, because polars keeps Float32 sums in Float32.
- Null scenario fields mean the roof cannot host 0.6 kWp, not zero savings.
- `genset` was previously named `diesel` and now uses the segment's real fuel.
- Ordering holds by construction: grid savings < mixed < genset.