Path-Q Solar

How should I compare a solar proposal that promises maximum savings with one that uses conservative assumptions?

Answer

Compare the assumptions and the range of plausible outcomes, not the headline savings. A conservative proposal may be more credible if it clearly accounts for shading, degradation, downtime, export limits, energy-use changes, and uncertain future prices.

Key points

  • Put both proposals on the same basis: system size, financing or cash price, analysis period, electricity-use forecast, production estimate, and treatment of excess generation.
  • Ask for the underlying production model and a sensitivity analysis showing lower, expected, and higher outcomes.
  • Separate guaranteed terms from estimates, and compare total cost, payback, cash flow, and expected bill reduction.

What it depends on

  • Actual interval or annual electricity use
  • Shading, orientation, equipment, and system losses
  • Future electricity prices, export treatment, financing terms, and battery operation

Risks and trade-offs

  • Optimistic assumptions can make a project appear to pay back sooner than it does.
  • Overly conservative assumptions can understate a sound project or obscure useful design differences.
  • Savings estimates are not guarantees unless specific contractual performance terms apply.

Information you may need

  • Production model and assumptions from each installer
  • Itemized price, financing terms, degradation assumptions, and estimated operating costs
  • Treatment of exported energy and any battery dispatch assumptions
  • Your expected changes in occupancy, appliances, heating, or transportation

Your next step

What to do next

Create a side-by-side comparison using identical assumptions, then ask each installer to explain the largest differences and provide sensitivity cases.

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