Community Solar Savings Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Community solar: A shared solar project where subscribers can receive electric bill credits without putting panels on their own roof [2].

Average monthly electric bill: Your usual electric bill in dollars per month before the estimated community solar savings.

Average electric price: The average dollars paid per kilowatt-hour, used here to estimate monthly energy use from your bill.

Bill covered by solar credits: The percent of your bill assumed to be eligible for community solar bill credits.

Community solar discount: The percent difference between the value of the solar bill credits and the amount you pay for those credits.

Extra monthly subscription fee: A fixed monthly plan fee added on top of the project charge.

Savings range to test: The number of percentage points added to and subtracted from the discount to make the lower and higher first-year estimates.

Mistake: Entering only the supply part of the bill for Average monthly electric bill when the offer is based on the full eligible bill.
Fix: Use your usual full electric bill, then lower Bill covered by solar credits if fixed charges or delivery charges are not credited.

Mistake: Leaving Average electric price at the default when your bill shows a very different $ per kWh.
Fix: Replace Average electric price with your bill's average price so Estimated monthly energy use is closer to your actual use.

Mistake: Treating Community solar discount as the percent of your whole bill that disappears.
Fix: Enter the discount on the solar credits only; for example, paying 90 cents for each $1 of credit means Community solar discount is 10.

Mistake: Forgetting an Extra monthly subscription fee because the advertised discount sounds simple.
Fix: Add any fixed fee that is not already built into the project charge so Estimated first-year savings includes it.

Mistake: Setting Savings range to test so wide that the low discount would be below 0 percent or the high discount would be above 100 percent.
Fix: Use a smaller Savings range to test or change Community solar discount so the tested range stays from 0 to 100 percent.

• The calculator does not model a specific utility tariff, state program, net crediting rule, tax treatment, or enrollment rule.
• Bill covered by solar credits is a user assumption. Some plans may not credit fixed charges, delivery charges, minimum bills, taxes, or other line items.
• Average electric price is used only to estimate Estimated monthly energy use. Replace the default with your own bill's average price when possible.
• Yearly electric bill increase applies only to the solar credit value. Extra monthly subscription fee stays flat in the calculation.
• Years to estimate is treated as a whole number of years, not a month-by-month contract schedule.
• Negative savings are allowed. A negative Estimated first-year savings means the fixed fee is larger than the estimated discount value for that scenario.
• The lower and higher first-year estimates test only the discount range. They do not test changes in usage, credit coverage, future rates, or fees.

How the estimate is calculated

The calculator starts with your Average monthly electric bill and the plan assumptions you enter. The default Average electric price is an editable starting value based on recent US residential electricity price context from EIA data, but your own bill is usually better for your home [1].

estimated_monthly_kwh = avg_monthly_bill / avg_price_per_kwh

monthly_credit_value = avg_monthly_bill * (bill_credit_coverage_percent / 100)

monthly_savings_base = monthly_credit_value * (community_solar_discount_percent / 100) - monthly_subscription_fee

first_year_savings_base = 12 * monthly_savings_base

For the savings band, the calculator subtracts and adds Savings range to test from Community solar discount. The tested discount must stay from 0 to 100 percent.

first_year_savings_low = 12 * (monthly_credit_value * ((community_solar_discount_percent - sensitivity_points) / 100) - monthly_subscription_fee)

first_year_savings_high = 12 * (monthly_credit_value * ((community_solar_discount_percent + sensitivity_points) / 100) - monthly_subscription_fee)

The bill-after-solar estimate subtracts the base monthly savings from the starting bill. The project charge plus fee shows the estimated amount paid for the credited solar value plus the fixed fee.

monthly_bill_after_solar = avg_monthly_bill - monthly_savings_base

project_charge_plus_fee = monthly_credit_value * (1 - community_solar_discount_percent / 100) + monthly_subscription_fee

Selected-year estimate

For Estimated savings over selected years, the calculator grows the monthly credit value once per year by Yearly electric bill increase, applies the discount, subtracts the fixed monthly fee, and adds each year together.

term_savings_base = sum for each year of 12 * (monthly_credit_value * (1 + yearly_bill_increase_percent / 100)^year_index * community_solar_discount_percent / 100 - monthly_subscription_fee)

Mini-example

If Average monthly electric bill is $150, Bill covered by solar credits is 90 percent, Community solar discount is 10 percent, and Extra monthly subscription fee is $0, the Solar bill credits counted each month are $135. Base monthly savings are $13.50, so Estimated first-year savings are $162. With a 2 percentage point Savings range to test, the lower estimate is $129.60 and the higher estimate is $194.40.