Every well in a district has two bosses. The first is the production target: so many acre-feet this month, out of this well, because that is what the annual plan, the allocation or the adjudication says it should carry. The second is the electric tariff, which says some hours of the day cost three times what others do, and that a single fifteen-minute slip into the wrong window can set a charge for the whole month.
Most districts manage the first boss carefully and let the second one manage itself. The well runs when the tank calls for it, the bill arrives, and somebody notices in August that the energy line is over budget.
It does not have to work that way. The target tells you how many hours the well has to run. The tariff tells you which hours are cheap. Put the two together and the schedule very nearly writes itself. This note is the arithmetic.
Step one: turn the target into hours
An acre-foot is 325,851 gallons. So the hours a well needs to make one acre-foot is simply:
hours per acre-foot = 325,851 ÷ (flow in gpm × 60)
Take a well that makes 1,200 gpm against 600 ft of total dynamic head and draws 200 kW at the meter. That is 181.8 water horsepower, or about 68% wire-to-water efficiency — a healthy, ordinary municipal turbine. It needs 4.53 hours per acre-foot, and at 200 kW that is 905 kWh per acre-foot.
Now give it a target: 120 acre-feet in September.
120 AF × 4.53 h = 543 hours, or 18.1 hours a day across 30 days
That one number is the whole budget. Everything after this is deciding which 543 hours.
Use the flow the well actually makes, not the design flow on the plans. A well that has lost 15% of its capacity since it was equipped needs 15% more hours to make the same water, and a schedule built on the old number comes up short on the last day of the month. If you have a recent pump test, use its flow. If you have a meter, use the meter. The wire-to-water calculator will give you kWh per acre-foot from a test in about a minute.
Step two: know which hours are which
Time-of-use tariffs split the day into periods and price each one differently. The shape matters more than the exact prices, so learn the shape of yours.
On Southern California Edison’s business time-of-use schedules, which include the agricultural and pumping rates most water districts in its territory are on:
- Summer (June through September): on-peak is 4 p.m. to 9 p.m. on weekdays. On weekends and holidays the same hours are mid-peak. Everything else is off-peak.
- Winter (October through May): 4 p.m. to 9 p.m. is mid-peak every day, and 8 a.m. to 4 p.m. is super off-peak every day — the cheapest hours of the year, in the middle of the day, because that is when the grid is full of solar.
- Agricultural and pumping customers can choose an option that narrows the peak to 5 p.m. to 8 p.m., with super off-peak running to 5 p.m. in winter.
On Pacific Gas and Electric’s agricultural schedules, the peak is 5 p.m. to 8 p.m. every day, including weekends and holidays — so there is no weekend relief to lean on.
Notice what the winter pattern does to the old rule of thumb. “Pump at night” was right for decades. On today’s tariffs, for eight months of the year the cheapest water in Southern California is pumped at noon.
Summer weekday
Run 9 p.m. to 4 p.m.: 19 hours available, the example needs 18.1
Winter day
Cheapest first: all eight super off-peak hours, then off-peak
Step three: fill the cheapest hours first
Now count. September 2026 has 21 working weekdays (Labor Day is a holiday) and 9 weekend and holiday days. Every one of those days has 19 hours outside the 4-to-9 window.
30 days × 19 hours = 570 off-peak hours available
543 hours needed → it fits, with 27 hours to spare
So this well can make its entire September target without running a single on-peak hour. The order to fill hours in is always the same:
- The cheapest period first — off-peak in summer, super off-peak then off-peak in winter.
- Then the middle period on weekends and holidays, which on SCE’s schedules carries no time-related demand charge.
- Weekday peak hours last, and only when the target cannot be met any other way — and then on purpose, knowing what it costs.
Put illustrative prices on it to see why the order matters. Say off-peak is $0.11 per kWh, weekend mid-peak $0.17, weekday on-peak $0.31, and a time-related demand charge of $22 per kW for the month. These are round numbers for the example, not a quote from any tariff — your rate schedule has the real ones.
| September, 120 AF from the same well | Scheduled | On tank-level control |
|---|---|---|
| Hours run | 543 | 543 |
| Energy, kWh | 108,617 | 108,617 |
| Share of hours in the weekday peak | 0% | about 15% |
| Energy cost | $11,948 | $15,523 |
| Time-related demand charge | $0 | $4,400 |
| Total | $11,948 | $19,923 |
| Cost per acre-foot | $100 | $166 |
The tank-level column assumes the run hours land evenly around the clock, which is roughly what a well on tank-level control does. Real wells on level control can do worse, because the evening peak is exactly when demand draws the tanks down and calls the wells on.
The fifteen minutes that cost more than the month’s water
Look at the demand charge line again, because it is the part people miss.
Demand is not energy. Energy is how much you used. Demand is the highest rate you drew power at, metered as an average over a short interval — fifteen minutes on PG&E’s agricultural schedules. On SCE’s schedules there are two kinds. The facilities-related demand charge is set by the highest demand in the month at any hour, and no schedule avoids it. The time-related demand charge is set by the highest demand during the summer on-peak and winter mid-peak hours on weekdays, and scheduling avoids it completely.
Which means this:
One weekday afternoon, the well starts at 4:15 p.m. and runs fifteen minutes before someone catches it.
Energy: 200 kW × 0.25 h × $0.31 = $15.50
Time-related demand for the month: 200 kW × $22 = $4,400
Fifteen minutes. The energy is a rounding error; the demand charge is set for the month, and the well can run through every peak hour for the rest of the month for nothing extra, because the damage is done. That is why the schedule has to be a hard rule in the controls, not a preference, and why a SCADA lockout of the peak window on every well that does not strictly need it is one of the cheapest energy measures a district will ever make.
It also tells you what to do when something goes wrong. If a well is down for three days — two weekdays and a weekend day — the example loses 57 off-peak hours and comes up 30 hours short. Make those 30 hours up on weekend mid-peak at about $1,000 and the month’s bill barely notices. Make them up in the weekday peak and it costs about $6,300. Same 30 hours of water.
What the schedule has to respect
A schedule that saves money and breaks the well is not a schedule. Before anybody programs one, it has to respect the things only operators know:
- Storage. Skipping the peak only works if the tanks can carry the system through it. If a pressure zone uses 1,000 gpm between 4 and 9 p.m., the reservoirs need 300,000 gallons above minimum operating level at 4 o’clock, every day. The tariff tells you which hours are cheap. Your storage tells you whether you can afford to skip the expensive ones.
- Starts. Every start is wear on the motor, the starter and the lineshaft, and a vertical turbine that has just stopped has to finish backspinning before it can start again. Run in long, contiguous blocks — one or two starts a day, not six.
- Start-up water. A potable well that pumps to waste on start-up spends the first minutes of every run making water nobody can use. Fewer, longer runs waste less of it.
- Must-run windows. Some wells have to be on at certain hours for pressure, blending or water quality, whatever the tariff says. Put those in first and schedule around them.
- The well itself. Running a well harder to fit its hours into the cheap window is fine only while it stays inside its pump curve and its drawdown stays above the top of the screen. Check specific capacity before asking a well for more.
Across a fleet, the cheapest acre-foot moves
Once every well has a kWh per acre-foot, the fleet has a ranking. A well at 905 kWh/AF and a well at 1,300 kWh/AF do not produce equally expensive water — at $0.11 off-peak the first makes it for about $100 an acre-foot and the second for about $143. When the district has room to choose where water comes from, the base load belongs on the efficient wells and the expensive ones carry the peaks of the season.
That comes with honest limits. Wells are not interchangeable: water quality, blending, pumping allocations, drawdown interference between neighboring wells and pressure-zone hydraulics all decide what can actually move. The arithmetic tells you what moving water would save. It does not tell you that you can.
Watch the line, not the month-end total
A monthly target only helps if somebody can see, mid-month, whether the well is on track. The simplest version is a straight line from zero on the 1st to the target on the last day, with the actual metered production plotted against it. Below the line and falling away is a well behind schedule — find out why on the 10th, not the 30th. Well above it is a well running more than it needs to, often through hours it did not have to.
A small tolerance keeps that honest: a couple of percent behind is noise, a few percent ahead is fine. A well 10% behind on the 15th is a conversation.
What to do with this on Monday
- Pull the rate schedule for every well’s electric account and write down its periods and its demand charges. You cannot schedule around a tariff you have not read.
- Get kWh per acre-foot for every well, from a recent pump test or from the meter and the power bill. That single number ranks the fleet.
- Turn each well’s monthly target into hours, using the flow it makes today.
- Count the cheap hours in the month and check that the target fits. If it does not fit, you know before the month starts, not after.
- Lock the weekday peak window out in the controls on every well that does not strictly need it, and record which wells are exempt and why.
- Plot production against the monthly line at least weekly, and ask about any well that falls away from it.
Quick answers
When is the cheapest time to run a well pump in California?
On SCE’s business schedules, overnight and early afternoon outside 4 to 9 p.m. in summer, and the middle of the day (8 a.m. to 4 p.m., super off-peak) from October through May. On PG&E’s agricultural schedules, any time outside 5 to 8 p.m., every day. Check your own rate schedule for the exact hours.
How many hours does it take to pump an acre-foot?
Divide 325,851 by the well’s flow in gpm, then by 60. At 1,000 gpm it is 5.43 hours; at 2,000 gpm, 2.72 hours.
What is a time-related demand charge?
A charge per kilowatt set by the highest demand the meter records during the peak window of the month. One short run in that window sets it for the whole month, which is why the window should be locked out in the controls rather than avoided by habit.
See each well’s monthly target, its hours and its cheapest schedule for your own fleet. Request a walkthrough →
Sources
- Southern California Edison. Rate Schedule TOU-PA-3 — Business Rate Basics. Source for eligibility of general water pumping, the facilities-related demand charge on the month’s highest demand at any time, and the time-related demand charge on summer on-peak and winter mid-peak weekday demand.
- Southern California Edison. Business Time-of-Use Rate Plans. Source for the summer (June 1–September 30) and winter (October 1–May 31) seasons and the agriculture and pumping alternative with off-peak from 8 p.m. to 5 p.m.
- Energized by Edison. Making New Time-of-Use Rates Work for Your Business. 2019. Source for the 4–9 p.m. peak, the 5–8 p.m. agricultural and pumping option, and super off-peak from 8 a.m. to 4 p.m. (to 5 p.m. for some agricultural and pumping customers) every day from October through May.
- Pacific Gas and Electric. Electric Schedule AG. Source for the peak period of 5 p.m. to 8 p.m. every day including weekends and holidays, and for maximum demand as the highest 15-minute average in the month.
- U.S. Geological Survey. Water Science Glossary. Source for an acre-foot as 325,851 gallons, the volume covering one acre one foot deep.
Every link above was retrieved and read on 2026-09-25. Paid standards are named by designation so you can check them against your own copy.