How our calculators work
A full audit trail of every formula, tariff schedule and assumption. If a number on our site disagrees with your bill, this page tells you where to look.
1. Slab math (residential)
Every calculator applies NEPRA's inclining-block schedule as a step function: units in each slab are priced at that slab's rate, not the marginal rate of the highest slab reached. Protected-consumer status is checked at 200 units — if 6 consecutive months are ≤200, the protected schedule applies to all units.
energy = Σ (units_in_slab_i × rate_i)
2. FPA + QTA application
FPA is a flat Rs./unit adder for the current billing month, multiplied by total units. QTA is billed as a discrete line in months when NEPRA notifies it (typically once a quarter). Both are exempt for lifeline consumers (≤50 units, ≤3 months).
3. Tax stack (in order)
- Electricity duty: 1.5% of energy + FPA
- PTV fee: Rs. 35 (domestic only)
- GST: 18% of energy + FPA + electricity duty + PTV fee
- WHT: 7.5% if pre-tax total ≥ Rs. 25,000
4. Solar payback (advanced model)
Our Solar Payback Calculator models 25 years of cash flows with three drivers: (a) panel degradation at 0.5–0.8%/year, (b) tariff hike at 8–15%/year (historical Pakistan average), (c) net-billing export rate under SRO 251(I)/2026 rather than the old retail-tariff net-metering credit. Payback year is the first year where cumulative savings exceed system cost.
5. TOU calculator
Peak-hour units × peak rate + off-peak units × off-peak rate. Windows source from NEPRA's TOU notification; typical summer peak 6–10 pm, winter peak 5–9 pm. Weekend exemptions do NOT apply.
6. What we deliberately don't model
Municipal water surcharges (province-specific), TR fee variations on feeders undergoing loss recovery, and one-off arrears adjustments printed on bills. These typically move a bill by less than 2%.