Tool 01 and 02
Run the numbers before anyone visits
Two models on one page. The first turns kilowatt hours into panels. The second turns panels into a payback period, with every assumption exposed as an input you can change.
- Inputs exposed10
- Model horizon25 years
- Degradation applied0.4 %/yr
- Escalation applied3 %/yr
Model 01
Kilowatt hours to panels
Consumption divided by specific yield gives kilowatts. Kilowatts divided by module wattage gives panels. Panels multiplied by module area gives the roof you need. Nothing here is proprietary, it is just arithmetic most quotes hide.
- 01Required array9.24 kW DCannual kWh × offset ÷ specific yield, rounded up to whole modules
- 02Module count21 panelsarray watts ÷ module watts, rounded up
- 03Panel area452 sq ftmodule count × area per module
- 04Roof area needed533 sq ftpanel area plus 18 percent for fire setbacks and walkways
Model 02
Cost, credit, cashflow and the year it crosses zero
Payback is net cost divided by savings, except that savings move every year. Production falls with degradation and the value of a kilowatt hour rises with utility escalation. Both are applied year by year here.
- Array required9.1kW DC21 modules at 440 W
- Net cost after credit$17,836$25,480 gross, less $7,644
- Year one saving$1,58512,067 kWh at a blended $0.125
- Payback9.8yearscumulative position crosses zero
- 25 year net$30,412after recovering the net cost
- Cost per kWh produced$0.061net cost ÷ 25 year production
| Year | Production | Value per kWh | Saving | Cumulative |
|---|
Show your working
Every assumption, named
If you are comparing quotes, these are the inputs to ask other installers for. A payback figure without them cannot be checked.
| Assumption | Value used | Why, and where it can be wrong |
|---|---|---|
| Specific yield | 1,340 kWh per kW | Sample average from our monitored Triangle fleet. Your survey figure replaces it and can be 20 percent lower on a shaded lot. |
| Year one degradation | 1.0 percent | Light induced degradation, applied once. Module warranties are written this way. |
| Annual degradation | 0.40 percent | Typical for N-type TOPCon. PERC modules run closer to 0.55 percent, back contact closer to 0.25. |
| Utility escalation | 3.0 percent per year | The most abused input in the industry. Anything above 4 percent should be challenged, and 6 percent roughly doubles a 25 year saving on paper. |
| Export value | $0.035 per kWh | Sample avoided cost figure for kilowatt hours not consumed on site or credited at retail. Your rider decides this and riders change. |
| Installed cost | $2.80 per watt | Sample mid point for a straightforward Triangle residential job. Complex roofs, microinverters and panel upgrades push it higher. |
| Tax credit | 30 percent, illustrative | A reduction in federal tax owed, not a rebate. Worth nothing in a year with no liability. Confirm your own position with a tax professional. |
| Maintenance | Not deducted | We leave it out rather than guess. Budget for an inverter replacement somewhere between year 12 and year 18 on a string system. |
| Financing cost | Not included | This model is a cash purchase. A loan adds interest, which lengthens payback. See the financing page for that comparison. |
Every figure on this page is a sample used to demonstrate the method. It is not a quote, an offer or a guarantee of savings. Production we will guarantee against our own model. Savings depend on future utility rates and on how you use energy, and nobody can guarantee those.
Questions
About the money side
Net cost after credits, divided by first year savings, with escalation and degradation applied year by year. Net cost is the contract price less any credit you can actually use. First year savings is production multiplied by what each kilowatt hour is worth to you, which is not the same as your headline rate once export credit is treated properly.
For a purchased residential system in this region our models typically land between nine and thirteen years, as a sample range, depending on consumption, shading and whether storage is included. Storage lengthens payback and buys resilience instead.
We guarantee production against the model, within 10 percent, because production is what we control. We cannot guarantee savings, because savings depend on future utility rates and on how you use energy, and anyone promising otherwise is guessing.
It reduces federal income tax owed for the year the system is placed in service. It is not a rebate and not a discount on the invoice. If you owe less tax than the credit, the remainder generally carries forward. Confirm your own position with a tax professional.
Lower up front, higher over the life, and the tax credit goes to the system owner rather than to you. Leases suit households without the tax appetite or the capital. We will tell you plainly which case you are in.
Ready for the real version
A site check replaces every sample figure above with a measured one: your specific yield, your shade fraction, your panel condition and a fixed price.
No pressure appointment. The site check is free and the design is yours to keep.