Daily water output is the pump’s flow rate multiplied by the useful sun hours, then reduced for head, cloudy periods and storage losses. A pump that delivers a high flow at noon does not deliver that flow all day, so size from daily volume, not from the peak flow figure alone.
What Daily Water Output Means
Daily output is measured in cubic metres per day. Litres per hour at one duty point is not a daily figure, because the pump runs only while the sun powers it.
Why Sizing by Peak Flow Fails
Undersizing by peak flow is the common mistake: the system falls behind in the hottest week, exactly as the agriculture irrigation article warns about sizing on an average instead of peak demand.
When to Use This Calculation
Use this calculation before choosing a pump, an inverter step, or a tank size; revisit it when head rises as a water level falls in the dry season.
Where This Calculation Applies
It applies to borehole, well and surface pumping for irrigation and livestock watering, wherever the pump runs only while the sun powers it.
How to Calculate Daily Water Output
- Take the required daily volume in cubic metres.
- Take the pump flow at the real total dynamic head, not at zero head.
- Multiply flow by useful sun hours (the agriculture article’s 6 to 9 productive hours is the planning range; use the peak-season month, not the annual average).
- Apply a derating allowance for head growth, dust and cloudy days, and check the result against the required volume.
- Size the tank to carry the difference between the solar day and the watering schedule.
- If the result is short, increase sun hours by storage, not by assuming a higher flow.
Worked examples (flow x sun hours = gross daily output, before derating):
| Pump flow | Sun hours | Gross daily output |
|---|---|---|
| 2 cubic metres per hour | 6 hours | 12 cubic metres per day |
| 5 cubic metres per hour | 7 hours | 35 cubic metres per day |
| 10 cubic metres per hour | 8 hours | 80 cubic metres per day |
What reduces the gross figure:
| Factor | Effect | What to do |
|---|---|---|
| Higher head as the water level falls | Flow drops at the same power | Size from the dry-season head, not the resting level |
| Dust and heat | Array output falls | Clean the array and keep hot-day voltage in the MPPT window |
| Cloudy days | Hours at useful flow fall | Carry 2 to 3 days in tank storage for livestock, and a tank buffer for irrigation |
Related reading: the sizing article, the deep-well article, and the QB100-PV solar pump inverter product page.
Frequently Asked Questions
What is the simplest daily-output formula?
Flow rate multiplied by useful sun hours, then derated.
Should I use peak-season or average sun hours?
Peak-season: the system must meet demand in the hottest week.
Does a bigger array always raise daily output?
No: once the pump is at its duty point, extra array power cannot push more water than the pump curve allows at that head.
If my pump is rated 10 cubic metres per hour, will I get 240 cubic metres a day?
No: that multiplies a peak flow by 24 hours, but a solar pump only runs at useful flow during sun hours, and flow falls as head rises.
How does tank storage change the calculation?
It does not raise daily output; it lets water pumped in sun hours be used on the crop or herd schedule, and covers cloudy days.
To get a sized system, send daily volume, head and irrigation method for a system sized to those figures.