Inputs
Example Data Table
These sample scenarios show how limits change with site load and utility caps.
| PV (kW) | Inverter (kW) | Min Load (kW) | Utility Cap (kW) | Breaker (A) | Phase | Recommended Limit (kW) |
|---|---|---|---|---|---|---|
| 10 | 8 | 1.2 | 5 | 63 | 3φ | 5.00 |
| 15 | 12 | 2.5 | 10 | 80 | 3φ | 9.50 |
| 6 | 6 | 0.8 | 3 | 40 | 1φ | 3.00 |
| 20 | 15 | 4.0 | 12 | 100 | 3φ | 11.40 |
Formula Used
- PV peak (AC estimate): PV_AC = PV_DC × PR × (1−Soiling) × (1−Temp) × PeakFactor
- Peak generation (capped): Gen_Peak = min(Inverter_AC, PV_AC)
- Worst-case net export: Export = max(0, Gen_Peak − MinLoad − BatteryAbsorb)
- Breaker-based export capability: kW = (V × A × PF)/1000 (single-phase), kW = (√3 × V × A × PF)/1000 (three-phase)
- Allowed export limit: Allowed = min(UtilityCap, Breaker_kW × (1−FeederDerate) × (1−SafetyMargin))
- Recommended limit: If Export > Allowed, set limit to Allowed; otherwise, limiter may be optional.
How to Use This Calculator
- Enter PV and inverter ratings using nameplate values.
- Set the utility export cap from your interconnection agreement.
- Estimate the minimum site load during peak generation hours.
- If storage is available, include realistic absorption capacity.
- Enter POI voltage, breaker rating, and power factor assumptions.
- Submit to view recommended export limit and curtailment risk.
- Download CSV or PDF for design review and commissioning notes.
Why export limiting matters
Export limits protect the grid and keep projects compliant with interconnection agreements. When generation exceeds on‑site demand, net power flows outward at the point of interconnection (POI). Utilities often cap this flow to reduce voltage rise, thermal loading, and reverse power concerns. A sized limit reduces commissioning failures and avoids extra site visits.
Key drivers inside the sizing model
The calculator estimates peak AC generation by applying a performance ratio and loss factors to PV DC capacity, then caps the result at inverter AC rating. It subtracts minimum site load and optional battery absorption to estimate worst‑case net export. On the electrical side, it converts breaker rating and voltage into an export capability using power factor, then applies feeder derate and a safety margin.
Reading the results for decisions
Compare “Worst‑case net export” against “Allowed export limit.” If worst‑case export is higher, an export limiter is required and the recommended setting matches the lower allowed value. If it is lower, limiting may be optional, but many owners still set a conservative cap for operational certainty. The curtailment indicator shows how much surplus could be clipped during peak conditions.
Financial implications you can quantify
Curtailment reduces bill credits or market export revenue. For example, if peak surplus is 7.0 kW but the allowed limit is 5.0 kW, about 2.0 kW may be curtailed at the highest‑generation moments. Over a year, the cost depends on how often those peaks occur and the value per exported kWh. Documenting assumptions helps compare the cost of load shifting or storage against expected curtailment.
Practical steps before commissioning
Validate the utility cap in your signed agreement, confirm meter and limiter wiring diagrams, and verify CT polarity at the POI. Use measured minimum load rather than averages, especially for facilities with weekend shutdowns. If batteries are used for absorption, confirm charge power limits and control priorities. Finally, run an export test and record the observed limit, voltage, and breaker temperature.
FAQs
What is an export limit?
An export limit is a control setting that caps net power flowing from your site to the grid at the POI. It is typically enforced by inverter controls or an external limiter using CT measurements.
Why can the recommended limit be below the utility cap?
The allowed limit is the lower of the utility cap and the POI hardware capability after safety margin and derates. Breaker rating, voltage, power factor, and feeder constraints can reduce the practical export ceiling.
How should I estimate minimum site load?
Use the lowest expected load during peak generation hours, not an average. Review interval meter data, weekend schedules, and shutdown periods. If uncertain, choose a conservative low value to avoid accidental over‑export.
Do batteries always reduce export?
Only if the battery can reliably absorb surplus at the same time PV peaks. Use realistic charge power limits and ensure the control strategy prioritizes charging before exporting. Otherwise, treat battery absorption as zero for sizing.
Will export limiting increase curtailment?
If your surplus generation exceeds the allowed export limit, the excess must be curtailed or absorbed. Curtailment occurs mainly during high‑irradiance periods with low load. The calculator’s curtailment estimate is a peak‑scenario indicator.
What should I verify during commissioning?
Confirm the utility cap, CT placement and polarity, limiter wiring, and inverter control mode. Run a staged export test, observe measured export at the POI, and document voltages, temperatures, and any protective trips.