Calculator Inputs
Example Data Table
| Controller | System (V) | Modules (S×P) | Module (W) | Voc (V) | Isc (A) | Tmin (°C) | Design Voc (V) | Design Current (A) | Suggested Rating (A) |
|---|---|---|---|---|---|---|---|---|---|
| MPPT | 48 | 2×3 | 550 | 49.8 | 13.9 | -5 | ~113.3 | ~43.3 | 50 |
| PWM | 24 | 1×4 | 300 | 40.0 | 9.8 | 0 | ~42.0 | ~53.9 | 60 |
| MPPT | 12 | 1×2 | 200 | 22.5 | 11.2 | -10 | ~26.1 | ~46.7 | 50 |
Formula Used
- P_array = W_module × N_series × N_parallel
- Voc_string(25°C) = Voc_module × N_series
- Voc_string(Tmin) = Voc_string(25°C) × [1 + (TCvoc/100) × (Tmin − 25)]
- Voc_design = Voc_string(Tmin) × VoltageMargin
- MPPT output current estimate: I_out ≈ (P_array × η_controller) / V_system
- PWM conservative current estimate: I_out ≈ Isc_module × N_parallel
- I_design = I_out × CurrentSafety × (1 + Growth%)
How to Use This Calculator
- Enter controller type and system voltage from your battery bank.
- Add module datasheet values for Voc, Isc, and temperature coefficient.
- Set your series and parallel counts to match the array layout.
- Choose a realistic minimum temperature for cold-voltage rise.
- Set safety and margin factors to reflect your design standard.
- Click Calculate and review voltage and current checks.
- If results are acceptable, export CSV or PDF for records.
Cold-Weather Voltage Headroom
Controller PV input rating is driven by string open-circuit voltage in the cold. This calculator applies the module Voc temperature coefficient to Tmin, then adds a voltage margin. Example: a 2S string at 49.8 V Voc becomes about 108 V at −5°C. If the resulting design Voc is near the controller limit (100, 150, or 250 V), reduce modules in series or select a higher model.
Output Current From Array Power
For MPPT controllers, battery-side current is estimated from array STC power and controller efficiency: Iout ≈ (Parray × η) / Vsystem. A 3.3 kW array at 48 V and 98% efficiency yields roughly 67 A before safety factors. This links electrical sizing to financial risk: clipping lowers kWh delivery, reduces savings, and stretches ROI assumptions.
PWM Current Rule Of Thumb
PWM controllers behave like a switch between PV and battery, so current is bounded by array current rather than array power. The calculator uses Isc × parallel strings as a conservative sizing basis, then applies a continuous-duty safety factor and optional growth allowance for later expansion. If Isc is 13.9 A and there are 3 parallel strings, PV-side current is about 41.7 A, which can justify a 60 A controller after multipliers.
Safety Factors And Expansion Planning
The current safety factor (often 1.25) helps account for continuous operation, irradiance spikes, and wiring temperature. The growth allowance adds budget-friendly headroom when a future string is likely. Combining these produces a design current that maps cleanly to common controller ratings (30, 40, 50, 60, 80, 100 A). Verify battery charging limits and cable ampacity also support the selected rating.
Documentation For Proposals And Audits
CSV output supports quick integration into spreadsheets for bill-of-materials pricing, while the PDF report provides a simple attachment for quotations and compliance files. Recording Tmin, margins, and limits alongside the recommended rating makes review faster and reduces rework during procurement. Exported voltage and current checks keep the cost-versus-headroom tradeoff transparent for teams.
FAQs
1) Should I size from PV current or PV power?
Use PV power for MPPT because the controller converts voltage to battery current. Use PV current for PWM because it largely passes array current to the battery near battery voltage.
2) What minimum temperature should I enter?
Use the coldest expected ambient at the site, not the annual average. Colder values increase Voc and protect against controller overvoltage during clear, cold mornings.
3) Why is there a voltage margin factor?
Margins cover manufacturing tolerance, measurement uncertainty, and transient conditions. A small margin helps prevent nuisance faults when string voltage sits close to the controller’s PV limit.
4) Does controller efficiency affect voltage sizing?
No. Efficiency changes the estimated output current for MPPT sizing. PV input voltage checks are based on module Voc, temperature coefficient, series count, and your voltage margin.
5) What if the recommended amps falls between sizes?
Choose the next standard rating above the design current. Also confirm the controller’s maximum PV power and battery charge limits so the higher amp rating is usable and safe.
6) Is Isc always the right current for PWM sizing?
It is a conservative ceiling for array current. Real operating current is usually lower, but sizing with Isc and a safety factor provides headroom for high irradiance and component tolerances.