Many boreholes on farms, at schools and in communities still run on a diesel generator. Solar pumping replaces most or all of that fuel, but only if the pump and array are sized for the real lift and the real daily demand. The good news is that the physics is simple, and once you know the numbers you can compare solar and diesel on the one figure that matters: cost per cubic metre of water.
Step 1: work out the total head
The pump has to lift water from the level it sits at while pumping, up to the outlet, and overcome friction in the pipe. That total is the total dynamic head (TDH):
- Static water level: depth from the surface to the water when the pump is off.
- Drawdown: how far the level drops while pumping. Your borehole test report usually gives it.
- Discharge height: from the borehole top up to the tank inlet or outlet.
- Pipe friction: small with a well-sized pipe, often under 2 m. StimaPulse calculates it with the Hazen-Williams method.
- Outlet pressure: zero for an open tank; around 10 m if you feed drip irrigation directly.
Step 2: energy per cubic metre
Lifting one cubic metre of water through one metre of head takes a fixed amount of energy. Allowing for a pump efficiency of about 70% and a motor and drive efficiency of about 90%, three typical Kenyan boreholes work out like this:
| Borehole | Daily water | Total head | Energy per cubic metre | Energy per day |
|---|---|---|---|---|
| Home or kiosk | 5 m³ | 46 m | 0.20 kWh | 1.0 kWh |
| School or community | 20 m³ | 84 m | 0.36 kWh | 7.3 kWh |
| Farm drip irrigation | 50 m³ | 66 m | 0.29 kWh | 14.3 kWh |
The examples assume a 30 m water level with 10 m drawdown and a 6 m tank stand for the home, a 60 m level with 15 m drawdown and an 8 m tank stand for the school, and a 40 m level with 10 m drawdown, 5 m lift and 10 m drip pressure for the farm.
Step 3: size the solar pump
A direct solar pump runs while the sun is up, so it has to move the whole day's water in the peak sun hours. Across Kenya that averages about 4.9 hours a day; your county's monthly values are on the solar by county pages.
| Borehole | Design flow | Pump motor power | Solar array | Storage tank |
|---|---|---|---|---|
| Home or kiosk | 1.0 m³/h | 0.2 kW | 0.3 kW | 8 m³ |
| School or community | 4.1 m³/h | 1.5 kW | 1.9 kW | 30 m³ |
| Farm drip irrigation | 10.2 m³/h | 2.9 kW | 3.7 kW | 75 m³ |
The array is sized about 25% above the motor power to cover heat, dust and wiring losses. The tank holds about a day and a half of water, so supply continues through the night and a cloudy day. Storing water in a tank is usually far cheaper than storing electricity in batteries for pumping.
Step 4: compare with diesel
A small diesel generator burns roughly 0.45 litres of fuel for every kWh it produces, and more when it runs lightly loaded, as pump generators often do. For the same three boreholes, solar avoids at least:
| Borehole | Diesel per cubic metre | Diesel per year | Fuel cost per year at KES 227.76 a litre |
|---|---|---|---|
| Home or kiosk | 0.09 litres | 164 litres | about KES 37,000 |
| School or community | 0.16 litres | 1,191 litres | about KES 271,000 |
| Farm drip irrigation | 0.13 litres | 2,349 litres | about KES 535,000 |
KES 227.76 a litre is EPRA's maximum Nairobi pump price for 15 October to 14 November 2026; prices are lower or higher in other towns and change on the 15th of every month. Multiply the litres by your local pump price from EPRA's monthly review, and add generator servicing, oil, transport to site and the risk of theft, which diesel carries and solar largely does not.
Step 5: cost per cubic metre for solar
Solar has almost no running cost, so its cost per cubic metre comes from the upfront price spread over the water it delivers in its life:
For example, if a supplier quoted KES 600,000 for the school system above, with maintenance at 2% of that a year over 20 years, the water would cost about KES 5.75 per cubic metre. Diesel fuel alone for the same borehole, at the current Nairobi price, is about KES 37 per cubic metre before servicing. Put your own quotes into the same formula to compare suppliers on a like-for-like basis.
Before you buy
- Get the borehole test report. Safe yield and drawdown decide the pump size; a pump bigger than the borehole can supply will run it dry.
- Size for the dry season. Water demand often peaks when you need it most, and solar output dips in the long rains and the overcast June to August period.
- Keep the generator as backup for extended cloudy spells if the supply is critical, such as for a clinic or livestock.
- Ask for the pump curve. Check the quoted pump delivers your design flow at your total head, not just its maximum flow at low head.
Size your own borehole
StimaPulse sizes solar pumping from your head, daily demand and site solar data, and compares the cost per cubic metre against your current diesel pump. See the solar water pumping guide for the full method, or request an assessment with your borehole test report and current fuel use.