Calculator Inputs
Example Data Table
| Monthly consumption (kWh) | Solar production (kWh) | Retail / export rate | Fixed charge | Baseline bill | Bill with solar | Monthly savings |
|---|---|---|---|---|---|---|
| 1,200.0 | 950.0 | $0.18 / $0.18 | $25.00 | $241.00 | $70.00 | $171.00 |
Formula Used
Exported kWh = max(0, Solar − Consumption)
Self-used kWh = min(Consumption, Solar)
Energy charge = (Import×Retail) − (Export×Credit)
Solar bill = max(0, Energy charge − Starting credit) + Fixed + Demand
Savings = Baseline − Solar bill
Credit carryover: if energy charge becomes negative, it is stored as bill credit for future months. This simplified model keeps fixed and demand charges payable each month.
How to Use This Calculator
- Enter your site’s monthly consumption and your retail rate.
- Pick a credit method: full retail credit or a custom export rate.
- Choose solar input: estimate from system parameters or enter measured kWh.
- Add fixed and demand charges to match your utility bill.
- Press Calculate to see bills, savings, and payback.
Load and generation alignment in project planning
Net metering value rises when solar output overlaps active site loads. In construction, daytime welding, lifting, and temporary HVAC often create predictable peaks. Enter a representative monthly consumption, then estimate production from kW, sun‑hours, and performance ratio. Compare self‑used kWh to exported kWh. If exports dominate, savings depend heavily on credit rates and may require resizing or load shifting for improved utilization and steadier monthly savings.
Credit valuation and tariff structure sensitivity
Tariff structure changes results more than panel brand. Use the retail rate for imported energy and select either full retail credit or a custom export rate. A 0.18 to 0.10 $/kWh credit drop can cut savings when production exceeds consumption. Always include fixed charges and demand charges so the baseline bill matches invoices. The calculator separates demand costs to keep assumptions explicit during exports.
Construction budgeting with carryover credits
Carryover credits help during commissioning phases and seasonal swings. If the energy charge becomes negative, the model stores it as a dollar credit and applies it to later months. Record the starting credit from the most recent statement to avoid overstating cashflow. When carryover is off, the energy charge is floored at zero, which is useful for quick worst‑case reviews and conservative budgeting across analysis months.
System sizing and performance ratio assumptions
Production estimates must reflect losses, not nameplate ratings. Performance ratio captures soiling, temperature, inverter limits, wiring, and downtime. Many commercial projects plan between 0.75 and 0.90, but harsh sites may be lower. Test multiple system sizes and billing days. Watch how imported kWh falls and exported kWh rises. The best size often maximizes self‑use before exports grow. Use notes to justify chosen ratio values.
Reporting and stakeholder communication
Stakeholders need clear, repeatable reporting. Present the baseline bill, bill with solar, monthly savings, and any ending credit. Use the CSV export to audit assumptions and build a 12‑month true‑up model if required by your utility. The PDF summary helps field teams and finance review quickly. For investment decisions, pair simple payback with incentives, escalation, and degradation analyses. Document units rounding rules and billing period length.
FAQs
1) What is “imported” versus “exported” energy?
Imported kWh is energy you draw from the grid when solar is not enough. Exported kWh is surplus solar sent to the grid when production exceeds on-site consumption during the billing period.
2) When should I use a custom export credit rate?
Use a custom export credit when your program pays less than retail for exported energy, such as avoided-cost or time-limited credits. Keep the retail rate for imported energy so savings remain realistic.
3) How does credit carryover affect the bill?
With carryover on, negative energy charges become a stored credit and can reduce future energy charges. Without carryover, the model floors the energy portion at zero for the month.
4) Does the calculator account for annual true-up periods?
This version models a single billing period and optional monthly carryover. For annual true-up, run representative months and track the end credit across a 12‑month sequence in a spreadsheet.
5) Are demand charges reduced by solar here?
Demand charges are included but not netted by default, because demand depends on peak kW timing. If your solar meaningfully reduces peak demand, adjust the demand kW input to reflect that scenario.
6) What performance ratio should I use for estimating production?
Many planning models use 0.75–0.90 depending on system quality and site conditions. Use a conservative value if access is difficult, soiling is high, or maintenance is uncertain.