Energy quotes often use different sizes, efficiency ratings, financing terms and utility assumptions. First normalize the proposal, then calculate a scenario rather than treating vendor savings as guaranteed.
Normalize scope and capacity
Record equipment size, included labour, permits, warranty, incentives and exclusions. For solar, compare price per installed watt before incentives as well as the final cash price.
Use measured consumption where possible
Start with recent bills or interval data, not a generic household. Match electricity and fuel units, distinguish fixed charges from usage charges and note time-of-use pricing.
Test payback boundaries
Run lower savings, degradation, maintenance and tariff-change cases. Simple payback ignores financing, opportunity cost and equipment replacement, so show it beside total cost rather than as the only decision rule.
Worked example
Scroll the table horizontally on small screens to see every column.
| Installed price | System size | Price per watt | Annual saving | Simple payback |
|---|---|---|---|---|
| 18,000 | 6,000 W | 3.00 | 1,500 | 12 years |
The simple payback excludes financing, degradation and future electricity-price changes; test those separately.
Sources and checks
- https://www.energy.gov/energysaver/energy-saver ↗
- https://www.energy.gov/eere/solar/homeowners-guide-going-solar ↗
Examples are illustrative and can be checked with the linked tools. Read each tool’s assumptions before applying its result. How we verify methods.