Walk into your shed and listen. There’s the hum of a fridge, maybe a cold room compressor kicking in. A few lights. An air compressor in the corner, a welder that gets used twice a week and a bore pump running somewhere out the back.

Now picture all of that running at once on a hot afternoon. That’s what a farm shed asks of a power system, and it’s why a standard house-sized solar setup rarely fits. Sheds don’t use power the way homes do. They’re heavier, lumpier and harder to predict.

Get the size wrong and you’ll feel it. Too small, and you’re still paying full price for power. Too big, and you’ve spent money on panels you can’t use. This guide shows you how to size a farm solar system for a shed, workshop or cold room, in plain steps, so you can have a sharper conversation with an installer.

A good rule is to size solar around what your gear actually uses and when it uses it. Start with 12 months of power bills. List your big loads. Work out how much runs in the daytime and how much runs at night. Then check that the inverter can handle your biggest motors starting up.

What Makes a Shed, Workshop or Cold Room Different?

A house has a steady pattern. Lights in the evening, a fridge all day, the oven at dinner. A shed has spikes.

A welder might pull a huge amount of power for ten minutes, then sit idle for a week. A cold room runs day and night, all year. A bore pump might fire up morning and afternoon. Big motors, like compressors, need a hard kick to get going, much more than their normal running power.

That’s why rural systems tend to be bigger than city ones. Most rural homes sit somewhere between 10kW and 25kW, because of pumps, sheds, workshops and cold rooms. A typical suburban system is usually much smaller.

How Do You Start Sizing?

Start with your power bills, the last 12 months if you can find them. They show your total use across all seasons, so you can see the summer peaks and the quiet months.

Next, a quick word on two units, because they trip people up. kW tells you how fast something uses power. kWh tells you how much it uses over time. Think of speed and distance. A 2kW compressor running for 3 hours uses 6kWh.

Now walk around and write down every big machine. Look for the label on each one, which usually shows its power rating. Note roughly how many hours a day it runs. Don’t forget the small stuff, like lights, fans and chargers. They add up.

What Does a Sample Calculation Look Like?

Here’s a simple example. These numbers are made up to show the method, not to quote a system. Your real numbers will be different.

• Cold room compressor, about 1.5kW, running roughly 12 hours a day: 18 kWh

• Workshop gear (welder, grinder, compressor), about 4kW for 3 hours: 12 kWh

• Bore pump, about 1.1kW for 4 hours: 4.4 kWh

• Lights, fans and small items: about 3 kWh

That’s around 37 kWh a day. A rough rule of thumb is to divide your daily use by the number of good sun hours, which inland Queensland and NSW often get at around 5 on an average day. So 37 ÷ 5 gives about 7.4kW. Add a bit extra for losses from heat, dust and wiring, and you land somewhere near 9 or 10kW.

That’s just a starting point. A proper design uses your real bills, your roof or ground space and your shading. But it shows why the answer is rarely “a standard 6.6kW system.”

Does It Matter When You Use the Power?

A lot. Solar only makes power while the sun is up. If most of your use happens in the daytime, panels cover it directly. You get the biggest savings that way.

Cold rooms are the tricky one, because they run all night too. A compressor that’s 12kWh of overnight use means your panels can’t help while it’s dark. You’ve got two choices. Keep buying cheaper night power from the grid, or add battery storage to carry the load through the evening.

A battery should be sized by what you want it to do. Covering just the cold room overnight is a smaller job than running the whole workshop. Be honest with your installer about what you actually need, and ask whether it makes financial sense on your site.

Often it helps to shift what you can. Run the welder and the air compressor during daylight, when the panels are working hardest.

What About Big Motors and Start-Up Surges?

This is where many sizing jobs go wrong. Motors, like those in compressors, pumps and cold rooms, need a big burst of power for a second or two to start up. That surge can be several times their normal running load.

If your inverter can’t handle it, the system trips out or struggles. So the inverter has to be chosen with those surges in mind, not just your daily total.

Phase matters too. Many farm sheds run on three-phase power, and your inverter and system need to match. On top of that, some rural lines, like SWER connections, have limits on inverter size and how much you can send back to the grid. That can shape what you’re allowed to install, so it’s worth knowing early.

Where Should the Panels Go?

A shed roof is often the best spot. It’s big, it’s flat and the building is already there. Good sheds have no trees nearby, and the panels are close to the load, which keeps cable runs short.

But check the roof first. Is it in good shape? Is the sheeting strong enough? Does it face a decent direction? If the roof is old or small, a ground mount can be a smarter choice, and it’s often easier to clean in a dusty country.

The best plan looks at the house roof, the sheds and the open ground together. A well-planned farm solar system often uses a bit of each. And if you want an experienced hand, a proper solar installation starts with a site visit, not a standard package.

Should You Plan for Growth?

Yes, and it’s the one thing people forget. Farms change. You might add a second cold room, buy a bigger pump or put in an electric ute charger. A system sized for today can feel tiny in three years.

Leave a bit of headroom. Ask whether the inverter can take extra panels later, and whether your switchboard can cope. If you’re expanding an older setup, system upgrades can add panels or swap parts without starting over.

For properties a long way from the grid, there are hybrid and off-grid options too. These combine panels, a battery and sometimes a backup generator, and they can make sense where a new grid connection would cost a fortune.

Once it’s all running, monitoring and optimisation lets you see what your shed actually uses, hour by hour. It’s the best way to check your sizing and catch a problem early.

Common Mistakes to Avoid

Sizing by your house bill alone. The shed may use more than the house. Look at all your meters and loads.

Ignoring start-up surges. A system that looks big enough on paper can still trip out when the compressor starts.

Forgetting night loads. Cold rooms don’t switch off at sunset.

Putting the inverter somewhere hot. Sheds can bake in summer. An inverter in direct sun may cut back its output just when you need it most.

Cheaping out on cables. Long runs from panels to a far-off shed can lose power. Thicker cable helps.

Skipping approvals. Your network may need to approve the system, and that can take time. Start early.

Quick Questions

Can I run a welder on solar?
Yes, but the inverter has to handle the surge, and it helps if the welder runs in the daytime. Tell your installer exactly what welder you have.

Do I need a battery for a cold room?
Not always. The grid can cover night use. A battery makes sense if night power is expensive, your supply is unreliable or you want backup in an outage.

Will my solar keep working in a blackout?
Standard grid-connected systems shut off during an outage, for safety. A hybrid system with a battery can keep chosen circuits running.

Can the shed roof take the weight of panels?
Most good steel sheds can, but it depends on the roof structure and age. An installer should inspect it before quoting.

Is it better to oversize a little?
A small buffer is usually smart, because loads tend to grow. But there’s a limit, so match the extra size to real plans, not guesswork.

The Bottom Line

Sizing solar for a shed isn’t hard, but it does take a bit of thought. Know what you use, know when you use it, allow for the big motors and leave a little room to grow. Do that, and your farm solar system works hard for years.

Want a number you can trust? Call us at Rural Solar and we’ll go through your bills and your gear with you.