Off-grid Solar Cable Size Calculator

Cables are an essential component of any solar project. You could have the most efficient solar panels money can buy and the best inverter, but if you don’t connect the two together with the right cable, that’s money, and free energy, down the drain.

The Sunstore team are always on hand to offer guidance and advice on anything solar. However, we find it’s always useful to approach a challenge from a position of knowledge.

That’s why we created the Off-grid Solar Hub and this cable size guide.

To provide accurate information to everyone interested in generating free, renewable energy.

What follows is an outline of how to specify cable for off-grid solar power. It is not a recommendation or designed to replace expert advice. It’s designed to help

Solar cable primer

Solar cable is also referred to as ‘PV wire’ or ‘PV cable’. Cable is the correct technical term as wires are simpler connectors than what we typically use for solar.

Cable will typically run throughout your system, connecting solar panels to the inverter, charge controller, batteries and then to your home’s grid or the national grid.

A cable is made up of several components.

Cable core

You have the wire core, Class 5 or EIC 60228, made of copper or aluminium. This is arguably the most important part of the cable.

The core is made up of a single or multiple metal wires wrapped together to create a cable of the desired thickness. This is called a stranded core.

Some cables come with a solid core instead. Solid core cables are used in buildings where cables are out of the way and won’t be disturbed.

Stranded core cables are used where the cable will be exposed and subject to knocking or the elements.

The copper core is typically 4mm or 6mm thick. The higher the voltage of the solar system, the thicker the core.

Each end of the core should be crimped and set into an appropriate connector.

Insulation and sheath

You have the insulation layer surrounding the core to help insulate the cable from resistance and prevent loss.

Finally, you have the sheath, the plastic or rubberised outer layer that protects the cable from damage and from the elements.

These will be manufactured from PVC or other tough but flexible material.

Our Best Off-grid Solar Kits

200W, 12V, 1.2kWh Professional Off-Grid Solar Kit

£826.25 – £1,633.92Price range: £826.25 through £1,633.92 including v.a.t.

Select options

This product has multiple variants. The options may be chosen on the product page

475W, 12V, 1.6kWh Professional Off-Grid Solar Kit

£923.01 – £2,262.56Price range: £923.01 through £2,262.56 including v.a.t.

Select options

This product has multiple variants. The options may be chosen on the product page

950W, 12V, 2.3kWh Professional Off-Grid Solar Kit

£1,612.23 – £2,721.22Price range: £1,612.23 through £2,721.22 including v.a.t.

Select options

This product has multiple variants. The options may be chosen on the product page

Solar cable rating

The rating, or carrying capacity, of a solar cable is a measure of how many amps it can reliably supply without overheating.

Heat is a byproduct of energy transfer and the cable will produce heat as it transports energy. The cable rating will include a thermal limit, the amount of heat the cable can safely dissipate.

When deciding the rating, you should add a slight margin of error and account for voltage drop.

For example, if you were wiring a 50W low energy 12v appliance, you would use I = P/V.

That’s 50W/12v = 4.17A.

Your cable would need to be able to safely carry slightly more than 4.2 amps.

If in doubt, going up a cable size is usually safe as long as it’s economically viable.

Voltage drop

One thing to bear in mind when specifying solar cable is voltage drop. This is how much voltage you lose through the length of the cable.

The longer the cable, the more significant the drop.

As metal has natural resistance, voltage running through it will gradually drop over distance and cause heat as a byproduct.

You can minimise voltage drop by using short cables and using slightly higher voltage cables than we need.

The typical allowance is a 3-4% drop, or V = IR

The calculations are a little complex but can provide an accurate estimate on how much drop you may experience over a given distance.

It can also tell you whether a particular rating of cable would be sufficient for the task.

In the example, we’ll use 5m of 0.5mm2 cable with a resistance of 0.037 Ω/m, which is typical resistance for a cable of this type.

The calculation would be: voltage drop = IR = 4.17A x (5m x 0.0371Ω/m) = 0.774V or 6.45%

As you can see, using 0.5mm is not sufficient for the task because it exceeds the ideal 3-4% voltage drop (6.45%).

That means you would have to increase the cable size until you met that desired percentage.

Voltage drop is one of the most common causes for system faults in off-grid solar.

Use a cable that’s too small and the voltage drop means there will be insufficient voltage delivered for the purpose.

Cable that’s too large costs more and won’t provide the efficiency we want from a solar system.

Our Best Off-grid Solar Kits

Out of stock

1600W, 24V, 1.1kWh Deluxe Off-Grid Solar Kit

£2,402.82 – £3,648.66Price range: £2,402.82 through £3,648.66 including v.a.t.

Select options

This product has multiple variants. The options may be chosen on the product page

Out of stock

800W, 24V, 1.1kWh Deluxe Off-Grid Solar Kit

£1,561.43 – £2,807.27Price range: £1,561.43 through £2,807.27 including v.a.t.

Select options

This product has multiple variants. The options may be chosen on the product page

2375W, 24V, 7.0kWh Professional Off-Grid Solar System

£3,609.59 – £4,627.95Price range: £3,609.59 through £4,627.95 including v.a.t.

Select options

This product has multiple variants. The options may be chosen on the product page

Length of solar cable

As we now know, if we keep cables as short and as direct as possible, we can minimise voltage drop as well as installation cost.

That means running cable in as straight a line as possible from one device to the other. That can require some creative installation but is definitely achievable.

This isn’t something we can plan for you on this page, but we can definitely offer advice on running cable across a typical solar setup.

Choosing the right cable for solar power systems

There’s a lot to take in here but it’s all very straightforward once you get your head around it.

If in doubt, follow these three simple rules:

1. Make sure the cable has enough rating for the expected load plus a margin of safety
2. Ensure the cable length will not exceed the ideal 3-4% voltage drop
3. Always buy quality cable suitable for the task at hand (solid core vs. stranded, copper vs. aluminium)

If in doubt contact one of the team at Sunstore Solar and we would be happy to help.