Community Solar Farm Calculator
Revenue from subscriber count and credit allocation.
About this calculator
A community solar farm doesn't sell power to one owner — it sells shares of its output to a roster of subscribers who never see the panels, crediting each one's utility bill in exchange for a discount off what they'd otherwise pay at retail. This calculator starts from the physics: multiply the farm's DC capacity by its daily peak sun hours, then by 365 to annualize, then by a system derate factor (this model defaults to 0.78, covering inverter losses, wiring, soiling, and temperature effects) to get total annual production, then divides that output evenly across the subscriber count to get each person's kWh allocation. The revenue side works backward from retail: subscribers are offered a discount (commonly 10-20%) off their utility's retail rate, and the developer collects the difference — retail rate minus the subscriber's cut — as the effective credit rate applied to every kWh produced. Two adjustments matter for realism.
First, churn: subscribers leave community solar programs over time (job changes, moves, dissatisfaction), so this model discounts gross revenue by an average subscription rate assuming half the annual churn rate is in effect on average across the year, not the full churn rate. Second, the 25-year lifetime revenue projection is deliberately simple — it holds net annual revenue flat with no degradation in panel output and no future changes to retail electricity rates, which in reality tend to rise over decades and would improve real returns. Because of that flat assumption, treat the reported payback period and lifetime ROI as a conservative floor, not a forecast, and rerun the numbers periodically as churn and utility rates become clearer.
Inputs
Results
Net annual revenue ($)
$306,729.00
How to Use This Calculator
- Enter the farm total DC capacity in kW and peak sun hours.
- Set the number of subscribers and the retail electricity rate they currently pay.
- Enter the subscriber discount percentage and installed cost per watt.
- Set the annual subscriber churn rate.
- Review net annual revenue, subscriber savings per year, and simple payback to evaluate project viability.
How the result changes with Farm capacity (kW DC)
| Farm capacity (kW DC) | Net annual revenue ($) |
|---|---|
| 1,000 | $153,364.00 |
| 1,500 | $230,046.00 |
| 3,000 | $460,093.00 |
| 5,000 | $766,822.00 |
What each input means
- Farm capacity (kW DC)
- Total DC capacity of the community solar farm.
- Peak sun hours (PSH)
- Average daily peak sun hours for site location.
- System derate factor
- Overall system efficiency including all losses.
- Number of subscribers
- Total subscribers allocated shares in the farm.
- Retail electricity rate ($/kWh)
- Local utility retail rate subscribers would otherwise pay.
- Subscriber discount (%)
- Discount off retail rate offered to subscribers. Industry standard: 10–20%.
- Installed cost ($/W DC)
- Total installed cost per watt before incentives.
- Annual subscriber churn (%)
- Percentage of subscribers who leave per year.
What each result means
- Net annual revenue ($)
- Developer revenue after subscriber discount and churn.
- Annual production (kWh)
- Total farm electricity output per year.
- kWh per subscriber
- Annual energy allocation per subscriber.
- Gross annual revenue ($)
- Revenue at full subscription before churn.
- Subscriber savings ($/yr)
- Annual bill savings per subscriber.
- Total project cost ($)
- Pre-incentive construction cost.
- Simple payback (years)
- Years to recoup investment from net revenue.
- 25-year ROI (%)
- Return on investment over 25-year project life.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersFarm capacity (kW DC) = 2000, Peak sun hours (PSH) = 5, System derate factor = 0.78, Number of subscribers = 200 = 8 input(s) provided
- Calculate Net annual revenueNet annual revenue = grossAnnualRevenue * avgSubscriptionRate306729 = $306,729
- Calculate Annual productionAnnual production = capacityKW * peakSunHours * 365 * systemDerate2847000 = 2847000
- Calculate kWh per subscriberkWh per subscriber = annualProductionKWh / subscriberCount14235 = 14235
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does adding more subscribers reduce each person's kWh allocation but not the developer's revenue?
Total annual production is fixed by the farm's capacity, peak sun hours, and derate factor — subscriber count only determines how that same output is divided up, so kWh per subscriber falls as subscriberCount rises. Developer revenue depends on the credit rate applied to total production, not on how many people share it, so adding subscribers changes each person's allocation and savings without changing net annual revenue at all.
How does the churn rate actually reduce revenue in this model?
The calculator doesn't apply the full annual churn percentage to gross revenue — it assumes subscribers leave gradually over the year, so on average only about half the stated churn rate is 'missing' from the subscriber base at any given moment. That's why avgSubscriptionRate is computed as 1 minus churn divided by 200, not by 100, and it's applied to gross revenue to get the net annual figure used everywhere else in the results.
Why is my subscriber discount percentage subtracted from the credit rate instead of added?
The credit rate is what the developer effectively collects per kWh, and it's defined as retail rate times one minus the discount fraction — so a 15% subscriber discount means the developer collects 85% of retail value per kWh produced. Raising the discount percentage to make subscriptions more attractive directly lowers the developer's credit rate and therefore both gross and net annual revenue.
Does the 25-year ROI account for panels degrading or utility rates changing over time?
No — this calculator multiplies net annual revenue by 25 flat, with no reduction for panel output degradation and no increase for rising retail electricity rates over the project's life. Both real-world effects tend to push in opposite directions (degradation lowers output, rate escalation raises revenue per kWh), so the reported lifetime ROI and payback period should be read as a simplified baseline rather than a precise long-term forecast.
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