Inputs
All values are editableExample data table
| Monthly bill ($) | Fixed charge ($) | Rate ($/kWh) | Usage (kWh) | System (kW) | Sun hours/day | PR | Self-use (%) | Export rate ($/kWh) | Solar gen (kWh) | Bill offset (%) | New bill ($) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 200 | 20 | 0.16 | 1125 | 5 | 5 | 0.8 | 60 | 0.1 | 600 | 40.8 | 118.4 |
Formula used
- Usage (kWh) = Provided, or (Bill − Fixed) ÷ Rate
- Bill without solar = Fixed + Usage × Rate
- Solar (kWh) = kW × SunHours × Days × PR
- Self-use = min(Usage, Solar × Self% )
- Export = max(0, Solar − Self-use)
- Export credit = Export × ExportRate
- Energy after solar = max(0, (Usage − Self-use) × Rate − ExportCredit) (unless credits are allowed to exceed charges)
- Bill with solar = Fixed + EnergyAfter
- Offset value = BillWithout − BillWith
- Offset percent = (OffsetValue ÷ BillWithout) × 100
How to use this calculator
- Enter your monthly bill, fixed charges, and energy rate.
- Add your monthly kWh if you know it for accuracy.
- Set system size, peak sun hours, and performance ratio.
- Choose self-consumption and export credit assumptions.
- Press Calculate to view results above the form.
- Use CSV or PDF downloads to share the estimate.
Bill baseline and usage mapping
Start with the monthly bill, fixed charges, and an average energy rate. If monthly kWh is unknown, usage is estimated as (bill minus fixed) divided by rate. This keeps the baseline consistent for site cabins, offices, and mixed loads. For quick checks, set days to 30 and confirm the derived kWh matches the utility statement within 5%.
Solar generation estimate for the month
Monthly generation is modeled as system size (kW) × peak sun hours per day × days in the month × performance ratio. For planning, many projects use a performance ratio between 0.75 and 0.85 to reflect temperature, inverter, wiring, and soiling losses. Example: 10 kW, 5.5 sun hours, 30 days, and PR 0.80 yields about 1,320 kWh.
Self-consumption and export behavior
Self-consumption represents the share of solar generation used on-site. Higher self-consumption usually improves offset because it displaces retail-priced energy. Exported energy earns a credit at the export rate, which may be lower than the retail rate, especially where net metering is limited. Load shifting, daytime pumping, and scheduled batching can raise self-consumption from 40% to 80% without batteries.
Interpreting bill offset for construction budgets
Offset is reported as both currency and percentage. A 60% offset means the modeled bill drops by 60% versus the no-solar baseline, after fixed charges. Use the target offset input to test whether the planned array meets budget expectations before procurement. If you enable time-of-use pricing, the calculator blends peak and off-peak rates using a peak-share percentage for a single comparable rate.
Sensitivity checks and documentation
Small input changes can shift results. Re-check peak sun hours by season, confirm tariff rates, and run a low and high performance ratio scenario. Use the annual degradation input, often 0.5% to 1.0%, to preview multi-year savings. Use the CSV and PDF exports to securely archive assumptions, support approvals, and compare vendors during bidding.
FAQs
What if I do not know my monthly kWh usage?
Leave the kWh field blank. The calculator estimates usage from (bill minus fixed charges) divided by the selected rate. If your statement shows kWh, enter it for a tighter baseline.
What performance ratio should I start with?
Use 0.80 for a practical starting point. Adjust down for heat, shading, dust, or older equipment. Adjust up only if you have strong evidence from monitoring or a detailed design.
How do I estimate self-consumption percentage?
Use 50% to 70% for typical daytime activity without batteries. Increase it when loads run in solar hours, such as pumping, batching, or welding shifts. Decrease it for mostly evening demand.
Why is the export credit rate separate from the retail rate?
Many tariffs credit exported energy at a different value than purchased energy. Enter the export credit shown in your policy or bill. This affects how much exported kWh reduces the energy charge.
What does the time-of-use option change?
It creates a blended rate using your peak and off-peak prices and a peak-share percentage. This is a simplified view that helps compare scenarios, especially when a large share of usage occurs in peak hours.
Can this calculator match my utility bill exactly?
It is an estimate model. Taxes, tiered rates, demand charges, minimum bills, and seasonal rules can change the final invoice. Use it for sizing and budgeting, then verify with the full tariff for approvals.