This article first appeared in the Irish Farmers Journal.

Solar PV is a new subject for most farmers, but the concepts are straightforward when explained.

Panels run on daylight, not heat

There is a common belief that solar only works on bright, sunny days, and that Ireland does not get enough of them. This is not the case.

Solar panels do not run on heat. They run on daylight. In fact, the amount of electricity a panel generates falls as the surface of the panel gets hotter, by around 0.4% for every degree above 25°C. A bright, cold morning in spring can generate more power than a warm afternoon in July.

Cloud is not the enemy either. On an overcast day, panels keep working from daylight reflected off the clouds. Under heavy cloud, a system can still produce 15% to 25% of its peak output. Panels generate power far more often than most people expect.

So generation is rarely the problem. The more important question is when that power is generated.

A unit used beats a unit exported

A unit of electricity you use on the farm is worth more than a unit you export. Export rates are currently around 18c to 20c, less than the roughly 30c you pay to buy power in. That gap will widen. As more homes, farms and businesses install solar, there will be sunny days when everyone is exporting and not enough demand to take the power. This can seem hard to believe with all the talk of rising demand from data centres, but it has already happened in other markets, where prices fell below zero and people were charged for putting power into the grid. The value of your system depends on how much of its power you use on-site.

Use the power when you make it

Solar panels generate most in the middle of the day. On a dairy farm, the biggest demand comes at morning and late-afternoon milking. The answer is to line up your electrical loads with the times you have spare power. Timing water heating, the ice builder for cooling milk, and other flexible loads to run in the middle of the day uses that power on the farm. A battery does the same job by storing the midday surplus for the evening.

None of this takes away from the basic case. What other investment pays for itself in around three years? But it does mean a farm needs a system designed around how and when it uses power, not a cookie-cutter system off a shelf.

The app in your pocket

There is one more benefit that is easy to overlook. Solar comes with an app in your pocket showing your power use in real time. Our customers have used it to find money leaking away quietly, an immersion left on around the clock being a common one.

Thinking about solar for your farm? Start with our guide to the TAMS 3 and SEAI grants, see our agricultural solar services, or call 064 770 2000 for a free farm survey — designed around how and when your farm uses power.