Frequently Asked Questions
| Assumptions

Why does the Energy chart show “Excess from Solar” providing more electricity than I use?

During sunny summer months, your solar system will often produce more electricity than you consume. This excess electricity is typically carried over as a credit on your account to make up the difference in future months when days are shorter and your solar system is producing less.

What is included in the upfront cost of my solar system?

The upfront system cost represents what you can expect to pay initially to get your system up and running. Depending on how you finance your system, the upfront cost can be a portion or all of the net system cost. The net system cost is the complete cost of your system after incentives. Solar panels, balance of system components (wiring, mounting hardware, inverter, etc.), and labor for installation are all taken into account in the upfront cost.

Why don’t my energy savings match my bill savings?

Most utilities include “fixed charges” on your bill, which are charges that are the same no matter how much energy you use. Since your solar system can only be used to offset energy related charges (charges on a per kilowatt-hour basis), your bill savings will not exactly match your energy savings. Other times your solar system may offset energy from higher tiers that is expensive. This energy may represent a small portion of your total electricity consumption, but a large portion of your electric bill. In this case your electric bill savings will be greater than your electricity savings.

Bill Savings Calculation

To calculate utility bill savings, WattPlan calculates a before-solar and an after-solar bill, and compares the bill details. The after-solar bill is calculated by modifying the before solar load profile based on the estimated output of the proposed solar system.

Customer Energy Usage

When consumption data is not available, WattPlan generates an hourly consumption profile from the user provided average monthly bill. WattPlan uses statistical average profiles published by utilities to synthesize an hourly consumption profile for the user based on their electric rate plane and average monthly bill amount. This assumption will be reasonably accurate for the majority of users, but those with unique profiles, such as users with time - of - use rates, may experience notable differences.

Derate Factors

The solar system derate factors used in WattPlan are as follows:

ComponentDefault Derate Factor
Standard Test Conditions rating to PVUSA Test Conditions rating10%
Wiring losses, module mismatch and module soiling10%
Annual system degradation0.5%

Operation and Maintenance

In the course of this analysis, WattPlan assumes an annual maintenance cost of $10/kWdc, and a one-time inverter replacement cost of $400/kWdc, halfway through the life of the system. These costs can be seen in the cash flow details and lifetime cost breakdown.

PV Module Temperature

The solar production simulation process uses a module’s rating under PVUSA Testing Conditions(PTC). Because ambient temperature, and not cell temperature, is specified by PTC, a cell reference temperature of 45°C is assumed in the calculation of temperature derate.

Solar Data Source

To calculate solar system production, WattPlan uses SolarAnywhere® Typical GHI Year satellite data where GHI stands for Global Horizontal Irradiance. More info can be found at

System Size

The system size is calculated dynamically to offset 95% of the user's annual electricity use. Depending on the user’s electric rate plan, the annual bill savings may be greater than, or less than, the percentage of electricity that is offset.

Financial Assumptions

The lifetime savings or (costs) are not adjusted to incorporate the time value of money. The financial analysis only considers the nominal value of cash inflows and outflows. Where loan financing is selected, the analysis assumes that all applicable incentives are used as an additional lump sum principal paydown in year 1. The loan is then re-amortized in year 2, reducing the monthly repayments while maintaining the original loan schedule.

Tax Implications

WattPlan makes several assumptions about the tax impact of owning a solar system. Residential customers can receive income tax credits. Commercial customers are classified as taxable or tax exempt entities. WattPlan assumes tax exempt customers have no tax benefits, as they do not pay any taxes. Taxable customers can have tax benefits or costs related to investment tax credits, depreciation and change in expenses, etc. WattPlan makes no guarantees about tax benefits, and users should check with their tax advisor with regard to any tax treatments.