Community Solar Subscriber Allocation Balancer
Determine how many households you can enroll, how large their bill credits will be, and whether anchor tenants or carveouts are consuming too much energy from your community solar array.
Why equitable allocation matters
Community solar thrives when developers can fill every block of capacity with satisfied subscribers whose bill credits match their expectations. Misalignments between project production, anchor tenant contracts, and low-income carveouts can leave capacity stranded or, worse, lead to under-delivery that erodes trust. This calculator helps you find a stable mix of households by modeling the energy available for allocation, the size of each subscription, and the cash flow impact on participants. If you are still evaluating whether community solar beats rooftop ownership, review the community solar vs rooftop solar cost calculator for a portfolio-level comparison. For developers layering in smart controls, the residential demand response ROI calculator can quantify complementary value streams.
Inputs capture the physical and contractual features of your project. Project size and capacity factor define how much energy the array will generate each year. Anchor tenants, such as a municipal building or school district, often reserve a portion of output at a negotiated rate. The low-income carveout reflects policy requirements to dedicate a share of energy to income-qualified subscribers. Combining these factors tells you how much energy is left to distribute across general subscribers who may have caps on how much of their usage can be offset.
The calculator first transforms project size and capacity factor into expected monthly generation. It subtracts anchor tenant energy, sets aside the carved-out portion, and then divides the remaining energy by the per-household subscription size. Because many states limit subscriptions to a percentage of a customer’s historic usage, you specify the coverage cap so the tool can compute a realistic energy block for each household. It outputs the number of low-income and standard participants you can support, the leftover energy cushion, and the average bill credits after accounting for program administration costs.
Passing through savings is a delicate balancing act: administrators must retain enough margin to run the program while still delivering meaningful bill reductions. The pass-through percentage captures how much of the raw credit value is shared with subscribers. The calculator subtracts per-subscriber administrative costs to show the net benefit. You can vary the pass-through value to see how trade-offs impact participant savings and the ability to fund customer support, outreach, and reserve funds.
Formula foundations
Solar production is estimated by multiplying the array size in kilowatts by annual sun hours expressed through the capacity factor. Dividing by 12 yields an average monthly output. Anchor reservations are removed, and the low-income carveout is applied to the remaining energy. The remaining energy for general subscribers is what you can allocate across households in fixed subscription blocks tied to their electricity usage.
The monthly production calculation can be summarized as:
where P is the project size in kW and f is the capacity factor expressed as a decimal. Anchor tenant energy is Eanchor = Panchor × 8760 × f / 12. Low-income carveouts reserve a portion of the remainder: Elow = (E − Eanchor) × c, where c is the carveout share. Dividing each bucket by the per-subscriber block, B = usage × coverage cap, reveals the number of subscribers you can accommodate.
Financial outcomes are derived from the value of bill credits minus administrative costs. The bill credit per subscriber is simply B × credit rate. Net savings equals that value multiplied by the pass-through share minus administrative cost. Degradation reduces production over time, so the calculator applies the annual factor exponentially to project output in the tenth year, allowing you to check long-term subscription adequacy.
Worked example
Consider a 1.5 MWdc community solar garden in a Midwestern cooperative territory. The developer expects an 18% capacity factor, reserves 300 kWdc for a school district anchor tenant, and must deliver at least 25% of remaining energy to income-qualified households. Typical households in the territory use 600 kWh per month, and regulations cap subscriptions at 90% of usage. Bill credits are worth $0.11/kWh, of which 85% is passed through to participants. Administrative costs run $3.50 per subscriber each month, and new enrollees pay a $50 one-time fee to cover credit checks. The array is expected to degrade by 0.5% annually.
Monthly production equals 1,500 × 8,760 × 0.18 / 12 ≈ 197,100 kWh. The anchor tenant consumes 300 × 8,760 × 0.18 / 12 ≈ 39,420 kWh per month. That leaves 157,680 kWh. The low-income carveout at 25% claims 39,420 kWh, leaving 118,260 kWh for standard subscribers. Each household subscription is limited to 600 × 0.9 = 540 kWh per month. The carveout therefore supports 73 low-income households (39,420 / 540), while the remaining pool supports 219 standard subscribers. About 420 kWh remain unallocated, serving as a cushion for cloudy months.
Each subscriber earns credits worth 540 × $0.11 = $59.40 per month. With an 85% pass-through, households receive $50.49 before administrative costs. Subtracting $3.50 leaves $46.99 in monthly net savings. At that rate, the $50 enrollment fee is recovered in just over one month. Annual savings per household reach $563.88, and aggregate annual savings for all 292 participants reach $165,656. The tenth year of production, after 0.5% annual degradation compounded, still delivers around 187,000 kWh monthly, easily supporting the subscription levels identified.
Scenario comparison
Use the table below to see how different pass-through percentages affect participant savings while holding other inputs constant in the example scenario.
| Pass-through share | Net monthly savings | Payback period | Annual program margin per subscriber |
|---|---|---|---|
| 75% | $39.05 | 1.28 months | $144.60 |
| 85% | $46.99 | 1.06 months | $86.52 |
| 95% | $54.92 | 0.91 months | $28.44 |
Higher pass-through shares dramatically improve household economics but shrink the program margin that covers staffing, marketing, and reserves. Developers must weigh financial resilience against community impact. This calculator makes it easy to iterate those trade-offs before entering into binding agreements.
Limitations and practical notes
Real-world community solar programs must account for weather variability, subscriber churn, and regulatory compliance. Production can swing by 10% or more month to month, so maintaining an energy buffer or securing standby subscribers is wise. The calculator treats anchor tenants as consuming a fixed share, but some contracts allow anchors to absorb overproduction, which could reduce the cushion available for households. Consider modeling best-case and worst-case anchor behavior when negotiating agreements.
Subscriber energy usage also fluctuates seasonally. The average usage input should represent a rolling 12-month value to avoid undersubscription in winter or summer. Many programs use historical data to size subscriptions and conduct annual true-ups. You can revisit this calculator whenever usage patterns change to rebalance allocations. Incorporating a waitlist policy ensures that attrition does not leave energy unassigned for long periods.
Administrative costs are unique to each program. Some developers outsource billing and customer support, converting costs into a per-kWh fee. Others rely on grant funding to subsidize low-income outreach. The per-subscriber value used here should reflect your fully loaded expenses, including software, accounting, and regulatory filings. If costs vary between low-income and market-rate subscribers, you may want to run separate calculations to avoid cross-subsidies.
Finally, degradation is only one long-term risk. Policy shifts, virtual net metering rules, and retail rate changes can alter bill credit values. Build conservative assumptions and stress test your program annually. Pairing this tool with a cash flow model or the electric school bus depot charging scheduler can reveal complementary infrastructure that shares administrative overhead or demand response payments.
Frequently asked questions
How should I treat community solar projects with multiple anchor tenants? Combine their reserved capacity into the anchor input so the calculator removes their energy before sizing household subscriptions. If their contracts have different escalation clauses, perform separate analyses for each anchor scenario.
Can I model time-varying credit rates? This tool uses a single blended credit value. To approximate tiered rates, run multiple scenarios using weighted averages for each billing period and compare the results. For complex rate structures, integrate outputs from a dedicated tariff model.
What happens if I over-enroll? The results section shows leftover energy. If the number becomes negative, it means subscriptions exceed expected production, signaling a need to trim enrollments or secure supplemental credits.