A solar project that looks attractive on a simple payback basis may look very different when modelled on NPV or IRR. Understanding all three metrics — and knowing when each one matters most to your decision-maker — is essential for building a compelling business case that gets approved and funded.
The three financial metrics and when to use each
Simple payback period
Payback period = initial investment ÷ annual savings. For a $350,000 system saving $80,000/year in electricity costs: payback = 4.4 years. Simple, intuitive, and easy to compare across projects. The main weakness: it ignores what happens after payback — two projects with the same payback period can have dramatically different 25-year returns if their annual savings differ after year 4–5 (e.g., one system degrades faster, one site has higher electricity price escalation).
Use simple payback for initial screening and for communicating with non-financial stakeholders who want a single, clear number.
Net Present Value (NPV)
NPV discounts all future cash flows back to their present value, then subtracts the initial investment. It answers: "How much value (in today's money) does this project create above the cost of capital?"
NPV formula: NPV = -C₀ + Σ [Cₜ / (1+r)ᵗ] where C₀ is initial investment, Cₜ is cash flow in year t, r is the discount rate, and t runs from year 1 to year n (typically 25).
A positive NPV means the project creates value above the cost of capital. A higher NPV is better. NPV is the most rigorous single metric for evaluating absolute value creation — it properly accounts for the time value of money, the full life of the project, and degradation in later years.
Internal Rate of Return (IRR)
IRR is the discount rate at which NPV = 0. It represents the annualised return on the investment over its full life. IRR is easily comparable to other investment options: a project with 15% IRR earns the equivalent of 15% per year on the invested capital. It is the metric most commonly used by financial decision-makers and equity investors, because it can be directly compared to the hurdle rate (minimum acceptable return on capital).
Weakness of IRR: it assumes interim cash flows are reinvested at the same IRR rate, which is often unrealistic. For project selection between mutually exclusive options (e.g., 200 kWp vs 400 kWp), NPV is a more reliable ranking metric — always maximise NPV, not IRR, when capital is not constrained.
Worked example: 500 kWp commercial rooftop
| Parameter | Value |
|---|---|
| System size | 500 kWp |
| Installed cost | $350,000 ($0.70/Wp) |
| Annual generation (Year 1) | 700,000 kWh (1,400 kWh/kWp specific yield) |
| Self-consumption rate | 80% (560,000 kWh avoided grid import) |
| Export | 20% (140,000 kWh at $0.06/kWh FiT) |
| Retail electricity tariff (avoided) | $0.12/kWh |
| Annual O&M cost | $3,500 ($7/kWp/year) |
| Module degradation | 0.5%/year |
| Electricity price escalation | 3%/year |
| Discount rate | 8% |
Year 1 revenue calculation:
Avoided electricity cost: 560,000 kWh × $0.12 = $67,200
FiT revenue: 140,000 kWh × $0.06 = $8,400
Less O&M: -$3,500
Year 1 net cash flow: $72,100
| Metric | Result |
|---|---|
| Simple payback | 4.9 years |
| NPV (25 years, 8% discount rate) | $412,000 |
| IRR (25-year project) | ~18% |
| Lifetime savings (undiscounted) | ~$1,900,000 |
Key sensitivity variables
The solar financial model is most sensitive to:
- Electricity tariff level: Every $0.01/kWh increase in the retail rate adds ~$5,600/year in Year 1 savings. Moving from $0.10 to $0.15/kWh roughly halves the payback period.
- Electricity price escalation: 0% vs 3% escalation changes the 25-year NPV by approximately 20–30% in typical models.
- Self-consumption rate: Higher self-consumption means more savings are at full retail rate vs lower FiT export rate. Increasing self-consumption from 70% to 90% typically adds 8–12% to NPV in markets with low FiT rates.
- System cost: At $0.70/Wp installed, the base case. At $0.60/Wp (competitive tender), payback drops to ~4.2 years. At $0.85/Wp (premium roof or small system), payback rises to ~5.8 years.
- Discount rate: Increasing from 8% to 12% roughly halves the NPV for a project with a 5-year payback.
Building a sensitivity table for stakeholders
Present sensitivity as a matrix — electricity tariff on one axis, system cost on the other — showing payback period in each cell. This immediately communicates the robustness of the investment case across scenarios and is far more persuasive than a single-point estimate. Include a "base case" cell highlighted in green and a "downside case" (higher cost, lower tariff) to show that the project remains viable even under conservative assumptions.