My Shed Is 75+ Metres Away, Can I Put Solar Panels On It?

Image: StarryAI “Big Shed With Solar Panels”

Top tips for installing solar on your man-cave. Or lady-lair.

So there’s a shed at your place that seems ideal for solar panels, but it’s well down the back of the block. Can you install solar panels on it, and if so, how much?

The short answer is: it depends on how thick you are prepared to go with the cables from the shed to the switchboard. The cable thickness required varies wildly depending on how much power you want to move over what distance. It’s Ohm’s law.

To start with, your DNSP¹ will have an SIR² book with rules about the SPD, MPD, meter isolator³, service size and meter location. But perhaps we should back up a little …

The Water/Electricity Analogy.

Voltage in Volts = pressure trying to burst out of the pipes.

Current in Amperes (amps) = the volume of water flow.

Resistance in Ohms = the drag or impedance that causes low flow and high pressure.

5% Voltage Drop Allowed In Your Home

Electrical rules (AS3000) say that from:

• the “point of supply” on your premises (which might be the fuse on the eave or the retail meter box)

to

• the farthest power point or light,

there can’t be more than 5% resistive loss at full load⁴.

For solar power systems, the rules are more stringent. Australian Standard AS4777.1 stipulates a maximum 2% voltage drop, (or voltage rise depending your perspective) from the solar inverter to the ‘point-of-supply’ (where your house connects to the grid). Some houses need the same fat cable that connects you to the street, just to connect the inverter. It’s a bit like having a fire hose to make perfectly sure your garden sprinkler covers the lawn properly.

The quality of grid electricity is the reason. By law, the pressure supplied to you isn’t 240 volts, it’s actually a nominal 230 volts, and it’s allowed to drop 6% or rise 10%. I’ll be honest here; I’ve seldom measured it below 240 anyway.

In the same vein, you’re not allowed to supply electricity back to the grid under 216  or over 253 volts. But the catch is, if you’re going to export energy, your inverter must push the solar electricity out at a higher voltage than the grid. You need to generate the pressure that pumps energy back into the street.

Here begins the conundrum. As more solar capacity is added to the network, all those inverters push up the voltage in the street, and the DNSPs have to do more things to keep the voltage down. Sometimes we have to call the power company and lodge a complaint if they’re not keeping it under control.

At the bleeding edge, SA Power Networks have taken to turning down (or turning off) solar power systems remotely if things begin to go pear-shaped using ‘world-first’ tools: substation-level voltage management and dynamic export limits.  It’s pretty interesting – grid stability is now partly dependent on a lot of dodgy customer-owned WiFi connections, but it seems to be working.

If you have solar panels and their inverter on a distant shed, the problem is compounded for you because in order to make the energy flow from your inverter back to your house, the voltage needs to rise even more. You can end up with 253 volts at the inverter, but because of drag, (think of a kinked garden hose), even at 253 volts there’s still not enough pressure to reach the house and efficiently use your own energy, let alone export to the grid.

Ohm’s law has been updated for the internet age but remains fundamentally the same concept.

Don’t worry, you won’t risk blowing things up with ever-increasing voltages. The inverter will derate itself by 5% for every volt above 253, and it’ll cut out completely at 258 volts after ten minutes.

These problems get worse still on rural SWER connections. Under the old standards, solar would rise to 263 volts and still be running at full output. This excess voltage creates waste heat in your appliances and I can assure you, having lived with it, 263 volts will incinerate your toast.

Image credit: http://www.pmichaud.com/toast/

What Wire Size/Thickness Will We Need?

It depends. Naughty electricians won’t bother with the proper calculations. Rules of thumb and assumptions abound, so it’s worth quizzing them for both your sake and theirs. It saves the embarrassment of your solar inverter tripping off because its increased electrical pressure climbs too high to overcome the drag on the way to the main switchboard

As a guide, the typical garden shed might have a 2.5mm^2, 16amp feed shared from the house somewhere. For a workshop, the rule of thumb is a 6mm² cable and 32 amp breaker in the house… the one that Mum switched on and off to get you to come up for tea, back when mobile phones weren’t a thing.

Coincidentally, that nominal 32 amp capacity is just what an average 7.5 kW EV charger calls for. That means when you get an EV, if you put your Level 2 EV charger in the shed, you won’t have enough flow to do more work there – not even to run the beer fridge.

Big Loads In The Shed Can Reduce Your Solar Losses

Say you’ve got a 6mm² cable to the shed, and it will cost a motza to upgrade it. You were hoping to put 6 kW of inverter and solar panels on the shed roof to help charge your EV.

Some installers may say you can only install a 3kW solar system to stay under a 2% loss in that cable. The pressure will be high when there’s lots of sunshine, because drag from an undersized cable will choke off the flow from your 6kW solar system.

The more enlightened installer may offer you the full 6 kW, knowing an EV on a level 2 charger can soak up all the power those panels produce, until it’s fully charged and a 3kW export limit brings things under control.

It’s a great example. The saving grace for solar is that it isn’t so much an extra load on the network, it’s actually unload⁵.

What’s The Proper Solution For Solar Cable Thickness?

First, we measure, then calculate. It’s distance versus load, with adjustments for cable type, installation method and temperature de-rating.

To go longer distances, you increase the cable size. Jumping from 6 to 10 to 16mm² (or even more) costs a few bucks in copper, but will likely cost a lot of bucks in conduit and a new trench. However, that presents an opportunity. I always insist we put some data cables in (because you’ll need them later anyway) and I have even seen a pipe to the hot water service added for a little shed luxury. The wisest of all customers will bury a 90mm stormwater pipe to use as a duct to pull other services through later.

Now that solar systems are getting much bigger, network operators are insisting on export limits. To achieve technical compliance, an inverter smart meter ⁶ is installed at your main switchboard so your solar power system can see what’s coming and going.

A trap that befalls some is this smart meter must have an RS485 communication link to the inverter. There are some wireless solutions available, but a hard-wired connection is always best; so it’s handy you’ve already installed a few extra data cables while you had a trench open for the new shed supply.

And The Upshot Is?

There are lots of reasons to install solar panels on your shed:

• It’s often much easier to install more solar onto a straightforward shed instead of a quintuple-fronted tile-roofed house.
• Many people prefer solar out of sight.
• Often your outbuildings have a different aspect or better sun access without tree shade.
• It’s valuable extra space for those who love big solar systems.

So, if you have the option, and you can get good connectivity for electricity and data then sheds are a great place for solar panels. Especially when I don’t have to climb through this nightmare:

Yes, I crawled through there because the apprentice is too young, tall and handsome to fit.

A Better Alternative: Long DC Cables

Covering the distance with high voltage DC is the answer if you have the right solar design. It will still involve a trench or overhead conduit but potentially less copper and more efficiency, which makes a large system feasible where it would otherwise just be too far away.

You see, the magic of electricity is high voltage and low current will do the same amount of work as low voltage and high current. The AC side of your system is limited to 250ish volts, but the DC side is legal up to 600 volts. Sadly, conflicting Australian standards prevent us from using 1000 volts; which everything is designed for internationally.

So, for example, for 3 kW of power pushed through a 6mm² copper cable:

• 240 V AC and 12 A, standard AC cable at 75ºC = 27m with 1% loss
• 600 V DC and 5 A, in better quality DC cable rated for 90ºC = 152m with 1% loss

In the first example, you could have a shed around 10 metres from your switchboard. However, in the second example, the difference is dramatic: higher voltage means your outbuilding can be 75 metres away.

There you have it. Solar power really can go the distance when you get the design right.