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Solar Voltage Drop Calculator (UK)

Free UK solar voltage drop calculator. Enter system voltage, current, cable length and mm² to see drop in volts and percentage. BS 7671 compliant.

Solar Voltage Drop Calculator

Voltage drop
0.3 V
1.3% of system voltage
Verdict
Excellent
NEC recommends < 3% on solar circuits

How to use this calculator

Enter four values:

  1. System voltage — typical UK domestic strings run at 250–600 V DC; off-grid leisure systems use 12 V, 24 V or 48 V
  2. Current — the maximum amps the circuit will carry (read the panel Imp on the data sheet, or the charge controller output rating)
  3. One-way length — distance in metres from the array to the inverter (the calculator doubles it for the return path)
  4. Cable cross-section — 1.5, 2.5, 4, 6, 10, 16 or 25 mm² copper

The calculator returns drop in volts and as a percentage, plus a verdict on whether you meet the BS 7671 5% combined limit and the MCS 3% DC recommendation.

Why voltage drop is the silent killer of UK solar systems

Every cable has resistance. When current flows through that resistance, some of the voltage is “dropped” and converted to heat instead of reaching your inverter or battery.

On a 230 V mains AC circuit, 3% drop is barely noticeable. On a 12 V leisure-battery off-grid setup, 3% drop means the inverter sees only 11.6 V instead of 12 V — enough to trigger low-voltage disconnect on a cloudy day. On a 48 V battery bank with a 100 A inverter draw, 3% drop equals 144 watts of waste heat in the cable under full load.

This is the most common reason DIY solar systems underperform their predicted MCS yield: under-sized cable creates a bottleneck that doesn’t show up on a multimeter at idle but eats power under real load.

The formula

Voltage drop on a DC circuit:

V_drop = 2 × Length(m) × Resistance(Ω/m) × Current(A)

The 2× accounts for the round trip (out through the positive conductor, back through the negative). Resistance values come from standard BS EN 60228 copper tables at 25°C.

Resistance per kilometre (Ω/km @ 25°C) for common UK cable sizes:

Cross-sectionΩ/km
1.5 mm²12.10
2.5 mm²7.41
4 mm²4.61
6 mm²3.08
10 mm²1.83
16 mm²1.15
25 mm²0.727

Each step up roughly drops resistance by 35–40%, which is why moving from 4 mm² to 6 mm² is usually enough to fix marginal voltage-drop issues on UK domestic strings.

When to size up

If your drop exceeds 3% on the DC side and you cannot shorten the run:

  • Move up one cable cross-section (4 → 6 mm², 6 → 10 mm²)
  • Run the array at higher string voltage — combining two 250 V strings into one 500 V string halves the current and quarters the drop
  • Add a parallel conductor (effectively halves resistance, but adds connector and labour cost)

For long ground-mount or barn-roof to house runs, increasing string voltage is almost always cheaper than larger copper. Solar PV cable costs scale steeply with cross-section above 6 mm².

BS 7671 and MCS code references

The 18th Edition Wiring Regulations (BS 7671:2018+A2:2022) Appendix 4 governs voltage drop in UK installations. The MCS Standard MIS 3002 for grid-connected PV and the MCS Installer Code of Practice require a calculation showing total system drop is within tolerance — DNOs increasingly request this with G98/G99 connection paperwork.

For installations beyond domestic scale, refer to BS EN 50618 (specific to PV DC cable) and the IET Code of Practice for Grid-Connected Solar PV Systems.

Real-world UK examples

  • 3.6 kWp roof array, 12 m run, single 600 V string, 6 A current — 4 mm² gives 0.22 V drop (under 0.1%). 2.5 mm² is also fine here.
  • 48 V off-grid cabin, 30 m to battery shed, 60 A peak — 16 mm² gives 4.1 V drop (8.5%) — way too high. Go to 25 mm² (5.2%) or, better, run the system at 96 V or 192 V with an MPPT charge controller.
  • Caravan 12 V, 5 m from panel to controller, 8 A — 2.5 mm² gives 0.6 V drop (5%) — borderline. 4 mm² brings it to 3% and is the standard upgrade most installers fit.

Verifying this calculator against UK design tools

Two free reference tools agree with this calculator to within rounding:

  • Schneider Electric Voltage Drop calculator (schneider-electric.co.uk free tool)
  • IET Wiring Matters online calculator (theiet.org)

Both use the same BS EN 60228 copper resistance values and the same 2× round-trip multiplier as this tool.

What it costs to get cable wrong

A 4 kWp UK domestic system installed under MCS in 2026 typically costs £6,500–£8,500 turnkey including VAT (Energy Saving Trust survey, MCS Installation Database). Annual generation is around 3,400–3,800 kWh. A persistent 4% voltage drop above the 3% target costs roughly 40 kWh/year — about £14/year at the 2026 Ofgem price cap of 27 p/kWh. Across a 25-year warranty that’s around £350 — small versus the cost of upsizing 50 m of 6 mm² to 10 mm² (about £80 in materials), so cable upgrades almost always pay back.

For installation, only an MCS-certified installer can register your system for the Smart Export Guarantee (SEG) and submit DNO G98/G99 paperwork. Always demand a written voltage-drop calculation as part of the system design pack.

Frequently asked questions

What voltage drop is allowed under BS 7671 for a UK solar PV circuit?
BS 7671:2018 Amendment 2 (the 18th Edition Wiring Regulations) Appendix 4 sets a 5% maximum total voltage drop from origin to load on standard installations and 3% on lighting circuits. For solar PV, the MCS Standard MIS 3002 and most DNO connection agreements recommend keeping DC drop below 3% to protect inverter MPPT efficiency, with AC drop also under 3%, giving roughly 5% combined. Below 2% is excellent and worth aiming for on long roof-to-inverter runs.
Why does voltage drop matter on UK domestic solar?
Voltage drop is energy lost as heat in the cable, never reaching the inverter or battery. A 4% drop on a 4 kWp string means roughly 160 W is being dissipated in the cable every hour the array is at full output. Across a 25-year MCS-warranted system that's hundreds of pounds of lost generation, plus higher cable temperatures that age the insulation faster. On 12 V or 24 V leisure-battery systems, drop also reduces the voltage the inverter sees and can trigger low-voltage cut-out under load.
Should I use one-way or round-trip cable length?
Use the one-way distance from the array (or string combiner) to the inverter or charge controller. The calculator automatically doubles it because current flows out through the positive conductor and back through the negative — both contribute to the total drop.
Does cable temperature affect the voltage drop calculation?
Yes. Copper resistance rises about 0.4% per °C above 25°C. Cables run on a hot south-facing roof in summer can sit at 60–70°C inside conduit, which means real-world drop is 14–18% higher than calculated. BS 7671 Appendix 4 includes correction factors — for cables in roof voids or sun-exposed trunking, either apply the temperature correction or simply size up one cross-section.

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