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 panels system will often produce more electricity than you consume. This excess electricity becomes a credit based on the energy generation rate and is applied to the current month electric bill. If the credit covers the electric bill and there is a remaining credit balance, then the amount due will be $0 and the remaining credit will be applied to the following month’s bill.

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?

There are “fixed charges” on your bill, which are charges that exist no matter how much energy you use. Since your solar panels 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.

What is Net Energy Metering Service?

AEP Ohio’s net energy metering service is a special billing tariff, available to customers who generate their own electricity using private generating equipment that is connected to our utility system. To qualify for net metering, your generating equipment must be fueled by a renewable energy resource (solar, wind, biomass, landfill gas, or hydropower) or use a micro-turbine or a fuel cell. Your generating equipment must also be located on your premises behind the meter and be designed to offset part or all of your electric service needs.

How is my electric bill reduced by having a solar panels system?

If AEP Ohio is your Generation Supplier, then our net energy metering service enables you to offset the cost of your total bill from the energy that your generating equipment produces. If you generate more energy than you need, a bill credit (based on the generation component only of your total rate) from the leftover energy is applied to your current month’s bill. If this results in a $0 bill with a portion of the ($) credit amount still remaining, then the remaining credit is applied to future months. You will not receive this credit as a cash payment. If you have a different Generation Supplier, you will need to contact them to learn how the excess electricity is addressed, since their rates and policies can vary.

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%

Environmental Savings

Environmental impact figures for CO2 are computed using EPA regional emission rates derived from the fuel mix used to generate electricity within the region. These savings are then displayed as equivalent measures(trees planted or miles not driven) using EPA approved conversion factors.

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 80% 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.

Tax Implications

WattPlan makes several assumptions about the tax impact of owning a solar panels system. WattPlan makes no guarantees about tax benefits, and users should check with their tax advisor with regard to any tax treatments.