Short answerA well can supply no more than its sustained yield times the hours you pump. A well that yields 1.5 gpm gives about 450 gallons over 5 sun hours, whatever the pump is rated for, so match the pump's flow to the yield and add storage rather than a bigger pump.
Key takeaways
- Daily water is capped by yield x pumping hours, whatever the pump.
- Size lift from the pumping water level, measured while pumping.
- Add a low-water cut-off and storage rather than a bigger pump.
Four numbers to know
| Term | What it means | Where to get it |
|---|---|---|
| Static water level | Depth to water after the well has rested | Driller's log; measure with a well sounder or tape |
| Pumping water level | Depth to water while pumping at your rate | Measure during a test; size lift from this |
| Drawdown | Pumping level minus static level | From the two above |
| Yield | Flow the well sustains without the level falling to the pump | Driller's log (often a short test); your own test |
Read the driller's log first
Most states require drillers to file a well log with the state water agency, and many publish them online. The log gives total depth, casing, screened interval, static level and a test yield. Treat the yield as an indication: driller's tests are often short and at a single rate. If the log is missing, ask your state agency or county health department.
A simple drawdown and recovery test
Divide flow by drawdown for specific capacity, in gpm per foot. It lets you estimate drawdown at other rates, roughly, for the same well.
- Let the well rest overnight and measure the static level.
- Pump at a steady, known rate close to your solar flow, into a container you can time.
- Measure the water level every few minutes. Note when it stops falling: that is your pumping level at that rate.
- Keep going for at least as long as the solar pump will run on a long day.
- Stop and measure the level every few minutes as it recovers. A well that recovers to static within a few hours will refill overnight.
Matching the pump to the well
- Flow at or below yield. Solar suits low-yield wells because it pumps slowly all day instead of in bursts.
- Lift from the pumping level, not the static level. See total dynamic head.
- Pump set below the pumping level, with margin, and off the bottom to stay out of sediment.
- A low-water cut-off stops the pump before the level reaches the intake. See controllers and float switches.
- Storage closes the gap. If yield x sun hours falls short of the daily need, store more in good weeks or pump longer from a battery.
Questions
- How do I know if my well can support a solar pump?
- Compare the well's sustained yield in gpm with the solar pump's flow. If the yield is at least the pump's flow, and yield x pumping hours covers your daily gallons, the well can keep up. The driller's log gives a first figure; a drawdown and recovery test confirms it.
Put your own gallons, lift and pipe in and see every step of the working.