Irrigation that runs on daylight

A borehole has three running costs: the fuel or the electricity, the servicing, and the time somebody spends going out to it. The first is the largest and the only one that grows with how much you water.

Start with what the pump draws. A modest borehole pump is a few kilowatts and a large one on deep water is more, and the plate on the motor will tell you. Multiply by the hours you run it in a day, then by the days you water in a year, and you have the energy your season needs.

If the pump runs on the meter, that figure is already in kilowatt hours and you can price it against your own bill. If it runs on an engine, divide by about two and a half. A small diesel engine turns roughly two and a half kilowatt hours of useful work out of a litre and the rest leaves as heat and noise, so a season of thirty thousand kilowatt hours is around twelve thousand litres. Multiply by what you actually pay at the pump.

That annual figure is what a solar system removes rather than reduces, and it is also what sets the payback: the cost of the system against a fuel bill that stops. Every quotation we write shows both, worked from your own hours rather than from an average, because a borehole that runs eight hours a day and one that runs two are not the same proposition.

The panels make direct current. An ordinary installation would convert it to the alternating current a building runs on, and that conversion is why an ordinary installation needs an inverter.

A solar pump either skips the conversion entirely, with a motor that runs on direct current, or uses a solar drive that makes what an existing motor expects. Either way the controller does the same job: it holds the panels at the point where they give the most power and sends it to the pump.

Which route is right depends on what is in the borehole today. A sound alternating current pump with a drive in front of it is the cheaper job; a direct current pump is more efficient and usually the better answer where the existing one was due for replacement anyway.

Both answers carry the pumping past sunset. The difference is what you are paying to store: a cubic metre of water in a tank, or the energy it took to lift it, held in cells that have a life and a price.

The tank wins on almost every site that has somewhere to put one, and it wins by a wide margin. It also fails gracefully. A battery that ages gives you less every year; a tank that is too small simply means watering earlier in the day.

A battery is the right answer where there is no room or no height for a tank, or where something else on the same system needs power after dark. We price it when that is the case and not before.

Head height is the first. Lifting water is most of the work a pump does, and a borehole twice as deep is roughly twice the energy for the same volume.

Flow is the second, and it comes from the crop and the schedule rather than from the pump's catalogue figure. The useful question is how many cubic metres a day the land needs in the heaviest month, not what the pump can do at its best point.

Distance and pipe size are the third, and the one most often left out of a quotation. Friction in a long run can cost as much as a few extra metres of depth, and it is cheaper to solve with pipe than with panels.

The exact numbers come from the survey. A quotation that has not asked for all three has guessed at least one of them.

A solar pump runs slower when the light drops rather than stopping. Slow pumping still fills a tank, and a tank sized for a run of dull days is what turns a variable supply into a reliable one.

The seasons help rather than hinder. The short days are the months the crop needs the least, and the long ones are the months it needs the most, so the two curves move together instead of against each other.