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Solar PV Electrical Calculator

DC & AC cable sizing, string configuration, and protection for solar PV installations

Reference Info & Formulas
Key Rules

DC cables: Rated for 1.25 × Isc

DC VD: Aim for ≤3%

AC VD: Max 2.5%

String Voc: Must not exceed inverter max

Panel Data

Find Vmp, Imp, Voc, Isc on the panel data sheet (STC values)

Apply temperature correction to Voc for cold site temps

Panel Data (from data sheet)
Enter STC values from the panel specification
Array Configuration
String layout and inverter
Cable Lengths
One-way cable run distances

Data: BS 7671:2018+A2:2022 Section 712, IET Code of Practice for Solar PV

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

Solar PV electrical design requires sizing both DC and AC circuits, with string voltage and current calculations governed by BS 7671 Section 712.

A PV array consists of panels wired in series (strings) and sometimes in parallel. Panels in series add their voltages — so a string of ten panels at 37V Vmp produces 370V DC at maximum power point. The calculator determines string voltage, verifies it falls within the inverter's operating window, and sizes cables for both the DC and AC sides of the installation.

V_string = N × Voc × (1 + (T_min - 25) × α)
N
= Number of panels in series
Voc
= Open-circuit voltage per panel at STC (25°C)
T_min
= Minimum expected cell temperature (°C)
α
= Temperature coefficient of Voc (typically -0.3%/°C)

BS 7671 Section 712, IEC 62548

DC cables must be rated for at least 1.25 times the short-circuit current (Isc) of the string. This safety factor accounts for irradiance levels that can momentarily exceed the STC rating of 1000 W/m², particularly with cloud-edge enhancement effects.

I_cable ≥ 1.25 × Isc
I_cable
= Minimum cable current rating required
Isc
= Short-circuit current of the string at STC

BS 7671 Regulation 712.433.1

Quick Reference — Common PV Panel Specifications

Typical electrical data for residential PV panels

Typical manufacturer datasheets — always use actual panel specs for design
Panel RatingVmp (V)Voc (V)Imp (A)Isc (A)
330W (60-cell)33.540.59.8510.44
400W (108 half-cell)37.044.810.8111.47
420W (108 half-cell)38.245.611.011.65
500W (132 half-cell)41.749.912.012.72

STC conditions: 1000 W/m² irradiance, 25°C cell temperature, AM 1.5 spectrum.

Quick Reference — DC Cable Sizing

H1Z2Z2-K solar cable current ratings

EN 50618, manufacturer datasheets
Cable Size (mm²)Current Rating (A)Typical Use
4mm²40ASingle string up to ~12A Isc
6mm²55ATwo parallel strings or long runs
10mm²75AMultiple strings in parallel or long DC runs

Ratings shown are for free air installation. Apply derating for bunched cables, conduit, or high ambient temperatures.

Practical Notes

Cold Weather Increases String Voltage

PV panels produce their highest voltage in cold weather. At -10°C, a panel with a Voc of 45V and a temperature coefficient of -0.3%/°C will produce approximately 49.7V. A string of twelve panels could reach 597V — dangerously close to or exceeding a 600V inverter limit. Always calculate the worst-case cold temperature Voc before finalising string lengths.

Use the Inverter MPPT Range for String Design

The inverter has an MPPT (maximum power point tracking) voltage window, typically 150-500V. Your string Vmp at the hottest expected temperature must remain above the lower MPPT limit, and the cold-temperature Voc must stay below the absolute maximum DC input voltage. Design the string length to keep Vmp within the MPPT range across all seasonal temperatures for optimal energy harvest.

AC Side Protection — Type B MCB

The AC connection from the inverter to the consumer unit typically requires a Type B MCB or RCBO. Where the inverter feeds back to the grid, the circuit is treated as a source of energy and the voltage drop limit is 2.5% rather than the 5% permitted for final circuits. A Type A (or Type B) RCD with 30mA sensitivity is required for the AC output circuit.

Frequently Asked Questions

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