Engineering-Led Solar Sizing · Watermain Supply · Houston, TX
Solar Water Pump Sizing CalculatorGrundfos SQFlex · CRFlex · RSI
Enter how much water you need a day, where the water is and where it goes. The calculator works out the flow, the total head, the solar array and the storage tank, then reads the Grundfos curves to name the pump that covers it.
25 PumpsOn Grundfos Curve Data
820 ftDeepest SQFlex Head
141 GPMLargest CRFlex Flow
1-888.510.4036Talk to a Person
A solar pump is sized on gallons per day, not gallons per minute, because the sun sets. This calculator turns a day's water into the flow a pump has to make in full sun, adds up the total dynamic head (how high and how far the water has to be pushed, in feet), and checks it against the published Grundfos flow-versus-power curves for all 16 SQFlex submersibles and 9 CRFlex surface pumps. It returns the best match with alternates, the array watts, the tank size and the controls, and when one Flex pump cannot do the job it shows how to close the gap, up to an RSI inverter on a three-phase pump.
Print the result on one page, or send it to us from the results panel: we confirm every system in the Grundfos sizing tool before it ships. Questions go to sales@watermainsupply.com or 1-888.510.4036. Genuine Grundfos equipment, factory-new in original packaging — sold and supported by Watermain Supply, an independent industrial supplier.
The Short Version
Add up a day's water on the hottest day of the year: animals, people, irrigation.
Divide by sun hours × 60 to get the flow in full sun. 1,560 gallons at 5.5 hours is 4.7 GPM.
Add up the head: pumping water level + rise to the tank + pipe friction + outlet PSI × 2.31.
Let casing size and head pick the family, then the curve picks the model, ideally at 55 to 90 % of what it can do.
Array = pump input power + 25 %; tank = three days of water. That is the system.
Solar Pump Sizing Calculator
Estimating tool for the Grundfos solar range. It is not a selection: we confirm every system with the Grundfos sizing tool before it ships.
Sizing received — thank youWe have your numbers and will come back with the pump, the array, the controls and a quote. Anything urgent: 1-888.510.4036.
Step 1 · How Much Water a Day
Enter gallons directly, head counts, or both — they add. Head counts use hot-weather planning figures: 30 gal per cow-calf pair, 20 per stocker, 15 per horse, 3 per sheep or goat, 75 per person.
Irrigation, tank fill or a known figure
Step 2 · Where the Water Is
A well means a submersible; surface water within suction lift means a CRFlex.
Pump off for an hour or more
Leave 20 if unknown, then test it
Lowest water level of the year
Step 3 · Where the Water Goes
Rise is measured to the highest point of the fill line, usually the top of the storage tank.
Zero for an open tank or trough
Step 4 · Sun, Storage and Backup
Peak sun hours are yearly averages. If you need full water in winter, pick one line lower.
If nothing happens when you press the button, email the numbers to sales@watermainsupply.com and we will run them.
One letter page on our letterhead: your inputs, the head build-up, the result and the Grundfos models that cover it. The detailed analysis below explains each figure.
Your Results
Daily Water—Gallons per day
Design Flow in Full Sun—GPM
Total Dynamic Head—ft
Array, Estimating—Watts of panel
Storage Tank—Gallons
Detailed Analysis
Head term
Value
Where it comes from
Candidate
Listing
Max flow at this head
Your flow, % of max
Input power, est.
Method: design flow = gallons per day ÷ (peak sun hours × 60). Head = lift + rise + Hazen-Williams friction in plastic pipe with a 20 % fitting allowance + outlet PSI × 2.31. Capability and input power come from the Grundfos Product Center flow-versus-input-power curves — nine points on every head Grundfos charts — for all 16 SQFlex models and 8 of 9 CRFlex models (CRF 10-2 from the data booklet curves): the calculator reads the two charted heads either side of yours and the two curve points either side of your flow, so the watts shown are an estimate between published points, not a published figure. Array = input power plus 25 %, rounded up to 50 W. Well yield, water quality and shading are not modelled.
Send This Sizing to Us
We run it in the Grundfos sizing tool and come back with the pump, the array, the controls and a quote. Your inputs and results go with the message; you do not need to retype them.
How the Calculator Sizes It
The same six steps we run by hand, in order. Each one feeds the next.
1. Daily water
Everything the site uses on the hottest day: head counts, people, irrigated area, tank top-ups. The tank covers the peaks, so the pump only has to make the daily total.
2. Design flow
Peak sun hours are the hours a day the site gets the equivalent of full sun. Design flow = gallons per day ÷ (sun hours × 60). The common mistake is dividing by daylight hours, which undersizes the pump by half or more.
3. Total dynamic head
Pumping water level (static plus drawdown) + rise to the top of the tank + pipe friction + any outlet pressure × 2.31. Size on the pumping level, never the static level: a well that draws down 40 ft asks 40 ft more of the pump.
4. Family, then model
A 3 in casing allows the helical rotor and high-speed SQFlex; a 4 in casing adds the 4 in centrifugal models; surface water within 19.68 ft of lift means a CRFlex. Inside the family the curve decides, and the best match covers your flow with 10 to 45 % to spare.
5. Array
The pump's input power at your duty point, read from the Grundfos curve, plus 25 % for heat, dust and wiring, rounded up to 50 W. Wire the strings for 120 V or more and keep the cold-morning open-circuit voltage under 300 VDC.
6. Storage and controls
Three days of water in the tank is the default battery. A CU 202 with a level switch stops the pump when the tank is full; an IO 101 lets a generator take over in a long cloudy spell.
When one pump cannot do itThe calculator tries the cheaper fixes first: one pipe size up, which cuts friction; a 4 in well, which opens the larger SQFlex models; a few hours a day on a generator through an IO 101. Past that it sizes the step up to an RSI solar inverter on a three-phase Grundfos pump, from 2 to 250 HP.
Worked Examples
Three common jobs, run through the calculator. The first is the default case, so pressing Size It without changing anything reproduces it.
Worked example — stock water from a deep well50 cow-calf pairs and 4 horses: 50 × 30 + 4 × 15 = 1,560 gallons a day. At 5.5 peak sun hours: 1,560 ÷ 330 minutes = 4.7 GPM. Static level 180 ft plus 20 ft of drawdown puts the pumping level at 200 ft. Add 25 ft up to the tank and 2.6 ft of friction in 400 ft of 1-1/4 in poly plus 200 ft of drop pipe: 200 + 25 + 2.6 = 228 ft, or 99 PSI. A 4 in casing allows every SQFlex family. At 228 ft the Grundfos curves put the 6 SQF-2 helical rotor at 5.7 GPM, so 4.7 GPM is 82 % of its maximum. That flow takes about 370 W into the motor; plus 25 %, rounded up, is a 500 W array wired for 120 V or more. Tank: 3 days × 1,560 = 4,680 gallons. The alternates are the 6 SQF-3 at 81 % on about 440 W and the 11 SQF-2 at 40 % on about 400 W.
Job
What was entered
Flow
Head
Best match
Array, est.
Stock tank, deep well
50 cow-calf pairs and 4 horses; 4 in well, pumping level 200 ft; 25 ft rise; 400 ft of 1-1/4 in pipe; 5.5 sun hours
Results as the calculator returns them, rounded. They are estimates between published Grundfos curve points; we confirm the selection before anything ships.
Planning Figures
The numbers behind the calculator's inputs. All three tables are estimating figures, not measurements — replace them with your own when you have them.
Water per head per day — hot-weather planning figures used in Step 1.
Use
Gallons a day
Cow-calf pair
30
Stocker or dry cow
20
Horse
15
Sheep or goat
3
Person in the household
75
Peak sun hours — yearly averages by region used in Step 4.
Region
Hours a day
Desert Southwest
6.5
Texas, Southern Plains, California
5.5
Gulf Coast, Southeast, Mountain West
5.0
Midwest, Mid-Atlantic
4.5
Northeast, Great Lakes
4.0
Pacific Northwest
3.5
Pipe friction per 100 ft of plastic pipe — feet of head, calculated with Hazen-Williams at C = 150, before the calculator's 20 % fitting allowance.
Flow
1 in
1-1/4 in
1-1/2 in
2 in
3 GPM
0.6
0.16
0.07
0.02
5 GPM
1.5
0.40
0.19
0.06
10 GPM
5.5
1.5
0.69
0.20
20 GPM
20.0
5.3
2.5
0.74
40 GPM
72.2
19.0
9.0
2.7
Calculated values for Schedule 40 inside diameters (1.049, 1.380, 1.610 and 2.067 in). Friction climbs with flow to the power 1.85, so doubling the flow nearly quadruples the loss: on a long run to a far tank, one pipe size up is often cheaper than the extra panels.
What We Check Before It Ships
The calculator handles the hydraulics. These six things come from your site, and we go through them with you on every enquiry.
Well yield
The pump can only deliver what the well gives. A drawdown test, or the driller's log, tells us whether the daily volume is there. The water level electrode stops an SQFlex before it runs dry either way.
Water quality
SQFlex takes sand to 50 ppm and CRFlex to 20 ppm, pH 5 to 9. Past that we talk about screens, setting depth or a different pump end.
Array voltage
Strings wired for 120 V or more, and open-circuit voltage on the coldest morning kept under 300 VDC. The 2500 W high-speed models need at least 100 VDC.
Cable and distance
Drop cable and the run from array to pump are sized for voltage drop at the pump's current, up to 8.4 A, or 12 A on the 2500 W motor.
Shading and mounting
Peak sun hours assume an unshaded array facing south at a sensible tilt. Trees, buildings or a north-facing slope cost water every day.
Where solar fits, and where grid power winsSolar is the right tool wherever water can be stored: stock tanks, cisterns, elevated tanks, irrigation run in daylight. Where a pump must deliver full flow at 2 a.m. with no room for storage, or where utility power already reaches the wellhead, a grid pump costs less to buy — the comparison against the grid pumps we also sell shows where each wins. The collection also covers the Grundfos warranty and backup power options in full.
Shop the Grundfos Solar Line
Every model the calculator can recommend, and everything that goes with it.
SQFlex Submersible
16 ModelsTo 820 ftTo 106 GPM3 in or 4 in Well
For a drilled well with no power line: helical rotor models for deep water and modest volume, high-speed 3 in models for more flow in a small casing, 4 in centrifugal models for volume at low head.
For a pond, creek, cistern or tank within 19.68 ft of suction lift: irrigation, livestock, transfer to distant tanks and pressure boosting from storage.
If yours is not here, call. Sizing conversations are the part of this we are good at.
How do I size a solar well pump?
Size it on gallons per day, not gallons per minute. Add up the water the site uses on the hottest day, divide by peak sun hours × 60 for the flow in full sun, add up the total dynamic head, then choose the model whose Grundfos curve covers that flow at that head with some room to spare. The array is the pump input power at that point plus about 25 %. The calculator on this page runs all five steps.
How many GPM do I need from a solar pump?
Daily gallons ÷ (peak sun hours × 60). A herd of 50 cow-calf pairs and 4 horses needs 1,560 gallons a day; at 5.5 sun hours that is 1,560 ÷ 330 = 4.7 GPM. The pump makes the day's water while the sun is up and the tank carries the rest.
What is total dynamic head?
It is everything the pump has to push against, in feet: pumping water level (static level plus drawdown) + rise to the highest point of the fill line + pipe friction + outlet pressure × 2.31. Size on the pumping level, never the static level.
How many solar panels do I need to run a well pump?
Enough to deliver the pump input power at your duty point plus about 25 %, wired in series for 120 V or more and never above 300 VDC open circuit on a cold morning. In the worked example a 6 SQF-2 at 228 ft of head takes about 370 W, so the array is about 500 W.
How big should the storage tank be?
About three days of water. At 1,560 gallons a day that is 4,680 gallons. The tank is the battery: it carries the site through the night and most cloudy spells with no fuel and nothing to replace.
What peak sun hours should I use?
The yearly average for your region, about 5.5 across most of Texas. If you need full water in winter, pick one line lower: 5.0 instead of 5.5 raises the design flow by about 10 %.
Can I use the calculator for a pond, creek or tank?
Yes. Choose pond, creek, cistern or tank as the source and the calculator switches to the CRFlex surface pumps, which need the pump inlet less than 19.68 ft above the lowest water level. Past that it suggests an SQFlex set in the water instead.
What if no pump on the page fits my numbers?
The calculator shows the fixes in order: one pipe size up, a 4 in well, a few hours a day on a generator through an IO 101, two CRFlex pumps in parallel, and finally an RSI solar inverter on a three-phase Grundfos pump from 2 to 250 HP. Send us the result and we will size it.
Is the calculator result a final selection?
No. It is an estimate read between published Grundfos curve points, and well yield, water quality and shading are not modelled. We confirm every system in the Grundfos sizing tool before it ships; use Send This Sizing to Us under the results and we come back with the pump, the array, the controls and a quote.
Ready to Size It
Run the calculator, then send the result from the results panel, or send us the well or pond details, the daily water need and the nearest town. We come back with the pump, the array, the controls and the reasoning.
Watermain Supply · Houston, Texas · 1-888.510.4036 · sales@watermainsupply.com · Free shipping on orders of $500 and above
Technical data on this page is taken from Grundfos product literature and is subject to change by the manufacturer. Confirm against the current installation manual before specifying. Figures from the calculator are estimates, not a selection. Grundfos, SQFlex and CRFlex are trademarks of Grundfos Holding A/S; RSI, CU 202, IO 101, Grundfos Connect and e300i are Grundfos product names. Used here for product identification only. Watermain Supply is a DBA of E4 Industrial LLC, an independent industrial supplier based in Houston, Texas.
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