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.

The tank is the battery

A solar pump almost never has a battery, and that is a design decision rather than a cost saving. Storing energy in lead-acid or lithium to pump water later is expensive, adds a component with a five-year life to a system whose other parts last twenty, and solves a problem that a storage tank solves for a fraction of the money.

So the controller drives the pump directly from the array, varying speed to track available power. In early morning the pump turns slowly and delivers little; through the middle of the day it runs at full output; in the evening it winds down and stops. Daily yield is what matters, not instantaneous flow, and the tank absorbs the mismatch between when the sun is available and when the water is wanted.

Size the tank for at least one full day of demand, and preferably two where the crop or the household cannot tolerate a gap. A correctly sized array with an undersized tank is a system that spills water at noon and runs short at dusk — the pump met its specification and the installation still failed.

Dry running, and the protection that actually earns its place

A submersible pump is cooled by the water it moves. Run it dry and the motor overheats within minutes, and on a solar system the risk is higher than on a mains one because the pump runs unattended every day and the borewell level moves seasonally.

Overload relays do not catch it. A dry-running pump draws less current than a loaded one, not more, so every protection device set to trip on excess current sees a lightly loaded motor behaving perfectly. This is the single most common cause of a solar pump failing in its first year.

Three things work, in increasing order of reliability. A dry-run sensor in the controller watches for the current falling below a threshold, which is cheap and catches the obvious case. A float or probe in the borewell measures water level directly and is the honest solution. And a pressure switch on the delivery side catches a closed valve as well as an empty well. Whichever is fitted, confirm it during commissioning by actually testing it — a protection function that has never been proven is an assumption, and a controller menu option that was left disabled at the factory is a common finding.