Path-Q Solar
What evidence should support an installer’s promised bill savings?
Answer
A credible savings promise should be supported by a transparent production model, your actual or representative electricity use, documented rate and export assumptions, system losses, degradation, and a stated treatment of fixed charges and future rate changes. Ask for the underlying calculations rather than relying on a headline percentage.
Key points
- The estimate should show month-by-month or annual production and compare it with historical consumption.
- It should identify shading, orientation, tilt, weather assumptions, system losses, and expected degradation.
- It should distinguish avoided usage charges from export credits and include charges that solar cannot offset.
- Battery savings should separately state charging, discharging, efficiency, and backup assumptions.
What it depends on
- Roof conditions and system design
- Historical electricity consumption
- Utility tariff, export compensation, and bill structure
- Future usage, rate changes, and system performance
Risks and trade-offs
- Savings can be overstated by using optimistic production, consumption, rate escalation, or export assumptions.
- A production guarantee is not necessarily a bill-savings guarantee.
Information you may need
- At least several months of utility bills or interval data
- Production simulation and assumptions
- Current tariff and export treatment
- Written definition of any performance or savings guarantee
Your next step
What to do next
Request the complete savings model in writing and test it against your bills using conservative production, usage, and export assumptions.
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Some details may depend on your location, installer, utility, incentives, regulations, or specific property.
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