Solar System Design Calculators

Technical tools for checking a solar design: inverter size, string length, charge controller rating, cable cross section and voltage drop.

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Solar Inverter Size CalculatorMatch inverter kW to your solar array, or size an off-grid inverter for load and surge.Voltage Drop CalculatorVolts and % drop for mm2 or AWG cables, 230 V, 400 V or DC, checked against BS 7671 limits.Solar String Size CalculatorMinimum and maximum panels per string from cold Voc, hot Vmp, inverter DC limit and MPPT window.

Design checks protect equipment and people. A string that is too long can exceed the inverter's maximum DC voltage on a cold morning, and an undersized cable wastes energy and can overheat. These calculators show each check against the limit so you can see how much margin you have.

They are planning aids, not a substitute for a competent designer. Fixed installations must be designed and installed by qualified people and, in the UK, meet BS 7671 and the relevant connection rules.

All solar system design calculators

Solar Inverter Size CalculatorMatch inverter kW to your solar array, or size an off-grid inverter for load and surge.Solar Cable Size CalculatorFind the minimum DC cable size in mm2 and AWG for a voltage drop limit on solar circuits.Voltage Drop CalculatorVolts and % drop for mm2 or AWG cables, 230 V, 400 V or DC, checked against BS 7671 limits.Solar String Size CalculatorMinimum and maximum panels per string from cold Voc, hot Vmp, inverter DC limit and MPPT window.MPPT Charge Controller CalculatorCharge controller amps for 12, 24 or 48 V with cold Voc and start-up voltage checks.

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Guides on this topic

Frequently Asked Questions

How do I size a solar inverter?

For a grid-tied system, many designs use a DC to AC ratio between about 1.1 and 1.3, so a 5 kWp array often pairs with a 4 to 4.6 kW inverter. Off-grid inverters are sized for the largest continuous load plus motor start-up surges.

What voltage drop is acceptable?

UK guidance based on BS 7671 commonly uses 3% for lighting and 5% for other uses on a public supply, measured from the origin of the installation. Many solar designers aim for lower drops on DC cables to reduce losses.

Why does cold weather matter for string size?

Panel open-circuit voltage rises as temperature falls. On a cold, bright morning a long string can exceed the inverter or controller's maximum input voltage, so the maximum string length must be checked at the coldest expected temperature.

Checked October 2026 by the Solaxyra Editorial Team. Read how we build and check our tools in our editorial policy.