Solar glossary
NPV (net present value): what it means and why it matters
The value of all future savings in today's money, minus what the system costs to build.
Why it matters in Nigeria
A positive NPV means the rooftop is worth doing at today's cost of money. It captures the whole life of the asset, not just the first few years.
How PureWatts measures it
Total NPV (Merit-order sized, merit order)
sum_sc3_npv_meritorder_ngn
Sum over the buildings in the group. Net present value over 20 years at a 15% discount rate, 5% energy escalation. 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.
Sum over the buildings in the group.
Total NPV (Solar + battery, vs grid + generator)
sum_sc2_npv_mixed_ngn
Sum over the buildings in the group. Net present value over 20 years at a 15% discount rate, 5% energy escalation. 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.
Sum over the buildings in the group.
See these numbers for your area
Every state and local government area page reports npv (net present value) 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.