Trade Calculation and Estimation
Trade Calculation and Estimation
This domain tests quantity and cost estimation for PV jobs: bill of materials, labour, taxes, energy yield, savings and payback.
Core rules
- Estimate from a load list and a single-line diagram; include modules, inverter, mounting, DC/AC cables, isolators, protection, meter, earthing and labour.
- Energy (kWh) = power (kW) × time (h). For PV, annual kWh ≈ kW × PSH × 365 × PR.
- Simple payback (years) = net capital cost / annual savings.
- Include taxes, transport, scaffolding, commissioning and maintenance in the estimate; state assumptions clearly.
- Subsidy or capital incentive reduces the customer's net cost, not the technical system cost.
Worked example
A 3 kW rooftop system costs ₹50,000/kW = ₹150,000. A 30% subsidy is ₹45,000, so net cost = ₹105,000. At 4.5 PSH/day and PR 0.75, annual generation = 3 × 4.5 × 365 × 0.75 = 3697 kWh. At ₹7/kWh, annual savings = ₹25,879. Simple payback = 105,000/25,879 = 4.06 years. If generation is 4.0 PSH/day, annual output falls to 3285 kWh and payback rises.
Traps that cost marks
Applying subsidy to the annual saving instead of capital cost; forgetting labour and balance-of-system items; using peak kW as annual kWh; mixing GST-inclusive and exclusive prices; ignoring degradation and maintenance in long-term estimates.