Enter the site’s monthly usage and tariff details, then model solar output and financial items.
Use these sample values to test the calculator flow.
| Monthly kWh | Rate | Peak kW | Demand Rate | System kW | Yield | Self-Use | Export Rate | O&M | Financing |
|---|---|---|---|---|---|---|---|---|---|
| 45,000 | 0.18 | 120 | 12 | 80 | 120 | 85% | 0.10 | 150 | 950 |
| 22,500 | 0.22 | 60 | 15 | 40 | 110 | 75% | 0.08 | 90 | 520 |
| 75,000 | 0.16 | 180 | 10 | 120 | 130 | 90% | 0.12 | 220 | 1,450 |
Export(kWh) = max(SolarGen − SelfUse, 0)
DemandAfter = PeakkW × (1 − DemandReduction%) × DemandRate
BillAfter = (GridkWhAfter × Rate) + Fixed + DemandAfter − (Export × ExportRate)
NetSavings = BillBefore − BillAfter − O&M − Financing
Notes: This is a planning model. Real bills can differ due to time-of-use rates, minimum charges, taxes, and tariff rules.
- Enter monthly energy use and your utility energy rate.
- Add fixed and demand charges if your bill includes them.
- Provide system size and typical monthly yield for your site.
- Set performance ratio and the share of solar used on-site.
- Enter export credit rate, plus O&M and financing if relevant.
- Press calculate to view savings, credits, and exports.
- Download CSV or PDF for proposals and internal reviews.
Baseline Utility Cost Drivers
This calculator separates monthly cost into energy, fixed fees, and demand charges. Baseline bill equals monthly kWh times the energy rate, plus fixed monthly charges, plus peak kW times the demand rate. Capturing demand cost is essential for commercial sites where demand can represent 20–60% of the invoice. When peak demand is unknown, use interval data or the highest billed kW from recent statements to avoid underestimating savings.
Solar Energy Production Estimate
Monthly solar generation is modeled as system size (kW DC) multiplied by monthly yield (kWh per kW) and a performance ratio. Performance ratio reflects soiling, temperature, inverter losses, and downtime. For many rooftops, values from 0.75 to 0.90 are typical, depending on maintenance and climate. Use a conservative yield for monsoon months or shaded periods, and adjust as measured data becomes available.
Self-Use Versus Export Value
Self-use percent determines how much solar directly offsets on-site consumption at the full retail rate. Exported kWh receives the export credit rate, which may be lower than the import rate under net billing. Increasing daytime load alignment or adding storage can raise self-use and improve savings. For construction sites, shifting pumps, batching, or temporary offices to solar hours can increase the offset without resizing the array.
Demand Reduction and Operations
If solar or storage reduces peak demand, the calculator applies a demand reduction percentage to peak kW before pricing demand charges. Even a 5–15% reduction can materially improve monthly economics on high-demand tariffs. Add monthly O&M to represent cleaning, monitoring, and insurance. If you include financing, compare net savings to cash flow targets to decide between ownership, lease, or hybrid procurement.
Net Savings and Reporting Outputs
Net monthly savings equals baseline bill minus the post-solar bill, minus O&M, minus any financing payment. The results summary reports solar generation, self-used kWh, exported kWh, export credit, and estimated CO2 avoided using your selected factor. Use CSV and PDF exports to support bids and stakeholder review internally.
1) What monthly yield should I use?
Start with a conservative kWh per kW value from site irradiance tools, EPC estimates, or nearby systems. Use a lower number for cloudy seasons and validate later with measured production data.
2) Why can savings be negative?
Savings can drop below zero when financing payments and O&M exceed bill reductions, or when export credits are low and self-use is limited. Recheck yield, performance ratio, and tariff components.
3) How do demand charges affect results?
If your tariff includes demand charges, the peak kW portion may be a large monthly cost. Even modest demand reduction can improve savings significantly, especially on commercial or industrial schedules.
4) What does self-use percentage mean?
Self-use is the fraction of solar generation consumed on-site. Higher self-use offsets energy at the full import rate, while exported energy earns only the export credit rate, which is often lower.
5) Does this include taxes or time-of-use pricing?
This model focuses on core energy, fixed, and demand charges plus export credits. If your utility adds taxes, riders, or time-of-use periods, treat the output as a planning estimate and adjust inputs conservatively.
6) How should I present results in a proposal?
Run at least three scenarios: conservative, expected, and optimistic. Export the CSV and PDF, then summarize baseline bill, post-solar bill, net savings, and key assumptions like yield, self-use, and demand reduction.