Solar Water Pump Sizing Calculator

Head is where the quotes go wrong

Pump power comes from one equation that cannot be argued with: P = ρgQH. Lifting water takes what it takes, and the only variable anyone gets wrong is H.

Quotes are written against borewell depth. The real head is three things: the depth to the water while the pump is running — not the static level, because the water drops when you start drawing on it — plus the lift from ground to the tank or field outlet, plus the friction the water loses pushing through the delivery pipe.

Leave the last two out and the pump is undersized by a third before anybody switches it on. The head bar shows all three to scale, which is also the fastest way to see when the pipe is too narrow: once friction passes about 15 % of total head, a wider bore is far cheaper than the next pump size and it saves energy every hour the pump runs.

The array is sized against the pump, not the day

This is the part that separates a solar pump from every other solar calculation, and it is where systems designed on daily-energy logic fail. There is no battery. The pump runs on whatever the array is producing at that instant, so the array has to be able to hold the motor at its duty point through average irradiance — not merely add up to the right number of units by evening.

That is why the array is sized at 1.25 to 1.4 times the motor’s input power rather than matched to it. Size it one-to-one and the pump will not start in the morning, will stall under passing cloud, and will give up in the afternoon — while the annual generation figure still looks correct on paper.

Efficiency, honestly

A submersible pump is 55–70 % efficient at its best point and worse everywhere else; the motor is 80–88 %. Together they mean the electrical input is roughly double the water power. Sales figures quoting 80 % pump efficiency are quoting the peak of the curve, which your duty point will not sit on.

What agricultural pump subsidies usually cover

Standalone solar pump programmes typically target farmers with no grid connection, up to something like 7.5 HP. A common structure is 30 % central assistance and 30 % regional, leaving the farmer 40 % — of which much can be financed. Remote and hill regions are often treated more generously.

Confirm the split before you quote it. The share is an editable field because regional top-ups differ widely, allocations are capped and released in tranches, and terms are revised. Set it to zero if no scheme applies where you are. Treat the default as a starting point rather than an authority.

Sizing the cable and the starter

A solar pump driven through a VFD controller has a gentler start than a direct-on-line one, but the DC cable from array to controller still carries full current in full sun over what is often a long run, and volt drop on that run is a common and invisible loss. Size it with the cable sizing calculator, and check the protection with the motor starter calculator if the pump is grid-connected rather than standalone.