Solar pump or diesel pump
We are not going to tell you the payback period. It is decided by your delivered fuel price, your daily volume and your head, and those vary more between two neighbouring farms than between any two suppliers.
What we can give you is the part that is the same everywhere: how much diesel it takes to lift a given amount of water to a given height. Put your own fuel price against it and you have your answer in five minutes.
Fuel scales with head
Lifting water to 100 m costs roughly four times the diesel of lifting it to 20 m. Head, not volume alone.
The trip is often the cost
On a remote borehole, getting somebody out to refuel can exceed the fuel itself.
Opposite cost shapes
Solar is capital with near-zero running cost. Diesel is the reverse. That is the whole comparison.
Five steps, about five minutes
| 1. Your daily water, and your head | Cubic metres a day and total dynamic head. If you do not know the head, the depth of the hole is not it. |
| 2. Litres of diesel for that water | Read it off the table below, then scale. The table is per 100 m³. |
| 3. Your delivered fuel price | Not the pump price at the depot. Include the trip, the drum, and what goes missing. |
| 4. Everything else diesel costs a year | Oil, filters, the refuelling trip, and the overhaul fund. Most people forget all four. |
| 5. Compare against one equipment purchase | Solar is a capital cost with near-zero running cost. Diesel is the reverse. The crossover is where your numbers say it is, not where a brochure says it is. |
How much diesel it takes to pump 100 m³
This is the number that does not change between countries. It comes from physics and two efficiency bands, not from anybody's price list.
| Total head | Hydraulic energy | Shaft energy | Diesel |
|---|---|---|---|
| 20 m | 5.4 kWh | 9 – 12 kWh | 3 – 5 L |
| 40 m | 10.9 kWh | 18 – 24 kWh | 5 – 9 L |
| 60 m | 16.3 kWh | 27 – 36 kWh | 8 – 14 L |
| 80 m | 21.8 kWh | 36 – 48 kWh | 10 – 18 L |
| 100 m | 27.2 kWh | 45 – 61 kWh | 13 – 23 L |
| 120 m | 32.7 kWh | 54 – 73 kWh | 15 – 28 L |
| 150 m | 40.9 kWh | 68 – 91 kWh | 19 – 35 L |
Hydraulic energy is Q × H ÷ 367. Shaft energy assumes a pump and motor efficiency of 45 to 60%, lower on small units. Diesel assumes a small generator burning 0.28 to 0.38 litres per kWh. Ranges, not fixed figures.
A worked example you can copy
20 m³ a day at 60 m of head. From the table, 100 m³ at 60 m needs 8 to 14 litres, so 20 m³ needs roughly 1.6 to 2.8 litres a day. Over a 300 day season that is around 480 to 840 litres.
Multiply by your delivered fuel price, not the depot price. Then add the six items in the next section. That total, repeated every year, is what a solar system is being compared against.
What diesel costs besides diesel
Six items that are real money and rarely appear in a comparison. On a remote site the third one is frequently larger than the fuel.
- •Engine oil and filter changes, typically every 200 to 250 running hours
- •Fuel filters, air filter and coolant
- •The trip to refuel, and whoever makes it
- •Fuel losses between the depot and the tank
- •Overhaul or replacement, usually measured in running hours rather than years
- •Downtime when the pump stops and nobody is there to restart it
A solar system has none of these. What it has instead is a purchase, and the honest counter-items on the solar side are array cleaning, an occasional surge cartridge, and the fact that it produces nothing at night.
Where each one actually wins
We sell solar equipment, so treat the next table as what it is: our view, written so you can argue with it.
| Diesel still wins on | Water at night or on demand regardless of weather, very small occasional duties, and situations where nobody can guard an array. |
| Solar still wins on | Daily irrigation through a dry season, remote sites where the refuelling trip is the real cost, and anywhere fuel goes missing. |
| Neither wins on | A well that cannot sustain the flow. Changing the power source does not change what the aquifer gives you. |
| The honest hybrid answer | Solar for the daily duty, keep the existing generator as standby. Costs more than solar alone and far less than running diesel every day. |
The comparison people get wrong
Comparing a solar system against fuel only makes solar look worse than it is, and comparing it against fuel plus a brand new generator makes it look better than it is. If you already own a working generator, its purchase cost is spent and should not be in the comparison at all. What belongs in it is fuel, servicing, the trip and the overhaul fund from here on.
The option most people should price
Not solar instead of diesel. Solar for the daily duty, with the generator you already own kept as standby. The pump runs on sun every day it can, and the generator covers the nights and the weeks it cannot.
That needs a cabinet with an AC input, which costs more than a solar-only one. Three arrangements, three prices:
| Solar only | Simplest and cheapest. The pump runs when the array can run it. |
| Automatic transfer | The cabinet switches between solar and the generator as conditions require, one source at a time. |
| Standby only | The generator is switched in manually when needed. Less equipment than automatic transfer. |
Say which one you want at enquiry. It is the single answer that moves the cabinet price most, and it is the one buyers most often leave out.
Have a diesel pump you want to keep?
Send the nameplate and the site location. Building the solar side around a pump that already works is almost always cheaper than replacing it, and we can price the generator as standby at the same time.
Solar vs diesel FAQ
Is a solar water pump cheaper than a diesel pump?
Over a few years, usually yes on a daily irrigation duty, but the crossover depends on your delivered fuel price and how far somebody has to travel to refuel. Solar is almost all capital cost with near-zero running cost; diesel is the reverse. Work it with your own fuel price rather than trusting a payback figure from a brochure.
How much diesel does it take to pump water?
It depends on head. Pumping 100 m³ needs roughly 3 to 5 litres at 20 m of head, 8 to 14 litres at 60 m, and 13 to 23 litres at 100 m. Those figures come from hydraulic power divided by a pump and motor efficiency of 45 to 60%, and a small generator burning 0.28 to 0.38 litres per kWh.
What is the payback period on a solar water pump?
We will not publish a number, because it is decided by your fuel price, your daily volume and your head, and those vary more between two farms than between two suppliers. What we can give you is the diesel consumption per cubic metre at your head, so you can put your own fuel price against it in about five minutes.
What does a diesel pump cost besides fuel?
Oil and filter changes every 200 to 250 running hours, fuel filters, air filter and coolant, the trip to refuel and whoever makes it, fuel lost between depot and tank, an overhaul fund measured in running hours, and the downtime when it stops and nobody is there to restart it. The refuelling trip is often the largest of these on a remote site.
Can I keep my diesel generator as a backup for a solar pump?
Yes, and it is often the sensible answer. A hybrid drive cabinet switches between solar and the generator automatically, one source at a time, or keeps the generator on manual standby. Both cost more than a solar-only cabinet and far less than running diesel every day, so say which you want at enquiry.
Does solar pumping work through the dry season?
The sun is usually better in a dry season, so the pumping side improves exactly when you need it. What gets worse is the well: the water level drops, the head rises and the yield falls. Size against the dry season water level rather than the level on the day you measured it.
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