Milk leaves the cow at about 35°C, and bacteria multiply fast until it is cooled to around 4°C. For dairy farmers and collection centres, reliable cooling means less spoiled milk, a sellable evening milking, and better prices from processors. Where the grid is weak or absent, solar can do the job, but only if it is designed around one awkward fact: cows are milked at dawn and dusk, when the sun is weakest.
How much energy cooling takes
Cooling one litre of milk from 35°C to 4°C removes about 0.035 kWh of heat. A cooling unit moves roughly two units of heat for every unit of electricity, and some cold leaks away through the tank walls. Allowing for both, cooling takes about 0.023 kWh of electricity per litre.
| Small farm | Collection centre | |
|---|---|---|
| Milk per day | 100 litres | 3,000 litres |
| Cooling energy per day | about 2.3 kWh | about 68 kWh |
| Solar for an average day | about 0.6 kW | about 17.7 kW |
| Solar for July, the weakest month | about 0.8 kW | about 25 kW |
These are estimates; the real figure depends on the cooler's efficiency, insulation, the outside temperature and how quickly the milk must be cooled.
The timing problem
Morning milk arrives around 06:00 and evening milk around 18:00. A cooler running straight from solar panels has almost no power at either time. Storing solar electricity in batteries works, but it adds cost and losses.
The better answer for most sites is thermal storage: use midday solar to freeze water into an ice bank, then use the ice to cool the milk whenever it arrives.
- Melting ice absorbs a lot of heat: cooling one litre of milk from 35°C to 4°C takes about 0.38 kg of ice.
- For the collection centre's 1,500 litres of evening milk, that is about 560 kg of ice, roughly half a cubic metre.
- Ice banks have no chemical wear and last for decades, unlike batteries.
Sizing the small farm (off-grid)
For 100 litres a day, two 620 W panels (1.2 kW) cover the cooling load with margin even in July, running a small DC compressor that builds an ice bank during the day. A small battery is useful only for controls, lights and the milking machine, not for the cooling itself.
If the farm sells evening milk that it would otherwise lose to spoilage, the value quickly adds up. For example, saving 10 litres a day at a farmgate price of KES 50 is worth KES 500 a day, about KES 180,000 a year.
Sizing the collection centre
On the grid: a collection centre on the commercial CI1 tariff pays the daytime all-in rate of about KES 22.09 per kWh for cooling at 06:00 and 18:00. An array of about 18 kW (29 panels) sized for the average day, charging an ice bank, would cover most of the cooling for most of the year and save up to about KES 548,000 a year, with the grid topping up in the cloudy months.
Off the grid: running the same cooling from a diesel generator takes about 30 litres of diesel a day. At EPRA's current Nairobi price of KES 227.76 a litre, that is about KES 2.5 million a year in fuel. There, sizing the array for July (about 25 kW, 41 panels) with a generator for backup is almost always worth it; see our diesel generator vs solar hybrid article.
Beyond cooling
- Water: dairy farms need water for animals and cleaning; a solar borehole pump often shares the same panels.
- Milking machines and chaff cutters can run directly from solar in the late morning.
- Records: a simple temperature logger on the tank shows buyers that milk was cooled in time, which helps with quality-based pricing.
What to ask a supplier
- How many litres can the cooler take at each milking, and how fast does it bring them to 4°C?
- How big is the ice bank or cold store, and how many hours of cooling does it hold without sun?
- What is the cooler's energy use per litre in your conditions?
- What happens on three cloudy days in a row?
Plan your cooling system
- The free solar calculator gives a first estimate of panels for a daily load.
- Solar by county shows the monthly sun hours in dairy areas, which can differ a lot from the national average in the cool, cloudy highlands.
- For a cooperative or collection centre, request an assessment comparing grid, solar, ice storage and generator options.