Solar Pump for a Garden or Small Farm: Size It to Your Soil, Not Your Plot

The short answer: a garden or smallholding solar pump is not sized by plot area. Two numbers decide it. Your flow ceiling comes from the soil: infiltration rate in mm/h multiplied by the area you are wetting at that moment, divided by 1,000. Your flow floor comes from the sun: daily water requirement divided by your effective pumping hours. Pick any model whose flow at your total dynamic head lands between those two numbers and the system works. Pick one above the ceiling and the water runs off the beds and takes your topsoil with it. Pick one below the floor and you will never keep up, and the cheapest crop you lose costs more than the pump.

I get asked for “a pump for my garden” almost every week, and nearly every enquiry arrives with the wrong two facts: the plot size and a budget. Neither of those picks a pump. This article gives you the two numbers that do, using FAO soil data, real measured curves from our own catalogue, and a worked example you can copy.

Two numbers decide everything, and neither is your plot size

The reason small systems fail more often than large ones is not that they are harder. It is that the margins are narrower. On a 20 hectare field, a pump that is 30 percent oversized is an inefficiency. On a 120 square metre vegetable bed, a pump that is 30 percent oversized is a mudslide down the path.

So write down two numbers before you look at any catalogue.

	What sets it	Formula	What happens if you get it wrong
Ceiling	Your soil	infiltration (mm/h) x wetted area (m2) / 1000 = Q max in m3/h	Water ponds, then runs off. Topsoil and nutrients leave with it, and the bed gets a fraction of the water you paid to lift
Floor	Your solar day	daily requirement (m3/day) / effective pumping hours = Q min in m3/h	The tank never fills. You water shallow, roots stay near the surface, and the crop collapses in the first dry week
Your pump	The head you have	read the H-Q curve at your total dynamic head	Must land between the two. There is no other criterion

Two definitions matter here, because both are where people go wrong.

The wetted area in the ceiling is not your plot. It is the area you are wetting at this moment: one sprinkler head’s circle, one drip zone, one block of beds with the valve open. Split a 1,000 m2 garden into four blocks with four valves and your ceiling for each block is a quarter of the whole-plot number. This single reframing solves most “my pump is too strong” complaints.

The effective pumping hours in the floor is not 24, and it is not the number of daylight hours either. It is the number of hours your panel array can hold the pump near its rated power, which is roughly 4.5 to 5.5 hours in a tropical or subtropical growing season and 3 to 3.5 in winter at higher latitudes.

The arithmetic between millimetres and litres is the easiest part, because 1 mm over 1 m2 is exactly 1 litre:

Depth applied	Over 10 m2	Over 100 m2	Over 1,000 m2	Over 1 ha
1 mm	10 L	100 L	1,000 L	10 m3
5 mm	50 L	500 L	5 m3	50 m3
25 mm	250 L	2.5 m3	25 m3	250 m3

Memorise the middle row. A garden event is measured in millimetres, a garden tank is measured in cubic metres, and this table is the only converter you need.

Your soil sets the flow ceiling

This is the number nobody calculates, and it is the one that decides whether your beds get watered or flooded. The FAO publishes basic infiltration rates, which is the steady rate at which a soil swallows water after the initial fast phase is over. It is measured as the depth in millimetres that enters in one hour.

Soil type	FAO basic infiltration rate	USDA-NRCS maximum application rate	What that means in practice
Sand	very high	25 mm/h (1.0 in/h)	You will never drown a sand bed by pumping too hard
Loamy sand	high	18 mm/h (0.7 in/h)	Very forgiving, but dries out fast
Sandy loam	20-30 mm/h	13 mm/h (0.5 in/h)	The easiest garden soil to irrigate
Loam	10-20 mm/h	10 mm/h (0.4 in/h)	The design target for most gardens
Silt loam	5-10 mm/h	8 mm/h (0.3 in/h)	Crusts easily when bare, mulch it
Clay loam	5-10 mm/h	-	Slow. Split into zones and cycle the valve
Clay	1-5 mm/h	-	The binding constraint on the whole design

The rule that goes with the table comes from the USDA Natural Resources Conservation Service, and it is blunt: the maximum sprinkler application rate must fall below the soil intake rate. If the application rate exceeds it, water runs off or redistributes within the bed, leaving some plants drowned and some dry. The NRCS adds a second condition that matters for gardens: once the net application depth exceeds half an inch, 12.7 mm, the application rate has to come down further, because the soil’s intake has slowed by the time you are that deep into the event.

Now apply it to the thing most gardens actually use, one sprinkler head.

Soil	Infiltration	One head wetting 80 m2	Flow that head typically wants	Verdict
Sandy loam	25 mm/h	2.0 m3/h	0.5-0.8 m3/h	Comfortable
Loam	15 mm/h	1.2 m3/h	0.5-0.8 m3/h	Fine, with margin
Clay loam	6 mm/h	0.48 m3/h	0.5-0.8 m3/h	Already over the limit
Clay	3 mm/h	0.24 m3/h	0.5-0.8 m3/h	Three times over the limit

Read that bottom row again. On clay, a single ordinary sprinkler head is already applying water three times faster than the soil can take it. Nothing is wrong with the sprinkler, and nothing is wrong with the pump. The soil is the constraint, and it was there before you bought anything. The three fixes, in order of cost, are: run the head in short cycles instead of continuously, switch to drip or soaker line spread over more of the bed, or add organic matter for a few seasons and raise the infiltration rate itself.

Your solar day sets the flow floor

The floor is simpler arithmetic but it is where people under-buy. Your daily requirement is fixed by the crop and the area. The pump’s total daily output is its flow at your head multiplied by effective pumping hours, not by 24. So a smaller number of hours means a proportionally larger flow is needed from the same daily volume.

Daily requirement	5.5 h of pumping	4.5 h	3.5 h
0.6 m3/day (120 m2 garden)	0.11 m3/h	0.13 m3/h	0.17 m3/h
3 m3/day (600 m2 garden)	0.55 m3/h	0.67 m3/h	0.86 m3/h
5 m3/day (0.1 ha smallholding)	0.91 m3/h	1.11 m3/h	1.43 m3/h
12 m3/day (0.25 ha)	2.18 m3/h	2.67 m3/h	3.43 m3/h

This is the table that tells you why a garden is genuinely easier than a field. A 0.1 hectare smallholding needs only about 0.91 m3/h to stay ahead, and our smallest 24 V submersible already does more than that at a working head. The difficulty is not the flow. The difficulty is the ceiling above, and the head, which is the next two sections.

What a garden actually drinks each day

There is one more number to get right, and it is the one that decides your watering rhythm.

Crop water use is reference evapotranspiration multiplied by a crop coefficient: ETc = Kc x ETo. For the FAO-56 “small vegetables” group, which covers cabbage, carrot, onion and most leafy and root crops, the mid-season Kc is 1.05. Tomato is 1.15, cucumber and lettuce are 1.00. In a warm growing season ETo runs 4 to 6 mm/day in a temperate summer and 6 to 8 in a hot dry one. Take a mid figure of 5 mm/day and Kc of 1.05, and your design number is about 5 mm/day of net crop use for a mixed vegetable garden. That is the figure I use, and it is honest for planning.

The second half of the rhythm is how deep you water when you do. That comes from soil water-holding capacity, which the FAO gives as millimetres of water per metre of soil depth at field capacity:

Soil	FAO available water capacity	Water in a 30 cm root zone	Refill at 50% depletion	Days between events at 5 mm/day
Coarse sand	80 mm/m	24 mm	12 mm	2-3 days
Sand	150 mm/m	45 mm	22 mm	4 days
Sandy loam	180 mm/m	54 mm	27 mm	5 days
Loam	180 mm/m	54 mm	27 mm	5 days
Silty loam	200 mm/m	60 mm	30 mm	6 days
Clay loam	180 mm/m	54 mm	27 mm	5 days
Clay	180 mm/m	54 mm	27 mm	5 days, if infiltration allows

Notice what this table does and does not say. Storage barely differs between loam, clay loam and clay. What differs enormously is the infiltration rate from the previous section. So the two soil families fail for opposite reasons and need opposite answers:

• On clay, you are limited by how fast water goes in. The soil can store plenty, but it cannot accept it quickly. Cycle the valve, split the bed, or run soaker line, and you will get there. Buying a bigger pump makes clay worse, not better.
• On sand, you are limited by how much water stays. The soil accepts anything you give it, then loses it below the roots within days. Water smaller amounts more often, add compost to raise water-holding capacity, and do not oversize the tank on the assumption that a big reservoir helps. It does not, if the soil under it cannot hold anything.

For a loam garden the practical rhythm is therefore about 25 mm every four to five days, not 5 mm every morning. If you have been watering lightly every day with a hose, you have been growing shallow roots, and the first hot wind will show you.

Same pump, four very different flows

Now the part that most garden buyers never see: a pump does not have a flow. It has a curve. Here are the measured H-Q points for four small DC models in our catalogue. These are bench measurements, not catalogue estimates, and each of these four curves belongs to a single model, so every point can be attributed to it.

Total head	3DSS0.5-28-12-80 (80 W, 12 V)	3DSS1.2-56-24-120 (120 W, 24 V)	2DPC1.5-35-24-200 (200 W, 24 V)	HJQB2-30-24-280 (280 W, 24 V)
0.5-1 m	0.47 m3/h	1.42 m3/h	1.52 m3/h	1.98 m3/h
5 m	-	-	1.37 m3/h	1.51 m3/h
8-10 m	0.40 m3/h at 8 m	1.20 m3/h at 9.6 m	1.24 m3/h at 8 m	1.21 m3/h at 9.7 m
14 m	0.30 m3/h	1.01 m3/h	1.09 m3/h at 12.4 m	-
20-22 m	0.20 m3/h	0.80 m3/h at 21.5 m	0.87 m3/h at 17.7 m	0.61 m3/h at 20.5 m
24-26 m	0.10 m3/h at 24 m	0.60 m3/h at 24 m	0.65 m3/h at 24.7 m	0.31 m3/h at 26.2 m
28 m and above	0 at 28 m	0.41 m3/h at 34.6 m	0.47 m3/h at 26.5 m	0 at about 27 m

Three things to take from that table.

The smallest pump is not a toy, but it is a trickle. 3DSS0.5-28-12-80 is a screw pump, and screw pumps trade flow for pressure: 400 litres an hour at 8 m of head, which is 6.7 litres a minute, and it will still be producing water at 24 m where every centrifugal model in this class has given up. If your garden is a few raised beds and your water level is deep, that is a completely serious pump. If you expect it to fill a 2,000 litre tank, it will take five hours.

Doubling the head costs you more than doubling the pump. Going from 8 m to 24 m cuts 2DPC1.5-35-24-200 from 1.24 to 0.65 m3/h, almost exactly half. The same doubling costs 3DSS0.5-28-12-80 three quarters of its output. Head is the expensive variable in a garden system, and every metre of it comes from somewhere you can usually find: burying the pipe deeper, moving the tank, shortening the hose, or lifting the water level by putting the pump further into the well.

Read the curve at your head, not the “Max. Flow” box. 2DPC1.5-35-24-200 is sold with a maximum flow of 1.5 m3/h. You will see 1.5 m3/h only if your total head is under a metre, which for a garden with any lift at all is never. At a realistic 10 m it is 1.17 m3/h. If you are sizing from the summary table instead of the curve, you are oversizing by roughly a third, and in a garden a third is the difference between a bed and a puddle. This is worth reading once properly: how to read a pump curve.

The ladder: what is actually available from 80 W to 900 W

Here is the real product range for small plots, from 12 V up to 48 V, with the specifications exactly as they appear in our catalogue. Outlet size matters more than you would think, because it dictates the smallest sensible pipe and the friction you will live with for the next fifteen years.

Model	Voltage	Power	Max. flow	Max. head	Outlet	Typical garden use
3DSS0.5-28-12-80	12 V	80 W	0.5 m3/h	28 m	0.75 in	A few raised beds, deep water level
3DSS1.2-56-24-120	24 V	120 W	1.2 m3/h	56 m	0.75 in	Small garden, deep borehole
2DPC1.5-35-24-200	24 V	200 W	1.5 m3/h	35 m	0.75 in	Classic 0.1 ha homestead, to a tank
3DPC3.5-25-24-200	24 V	200 W	3 m3/h	25 m	1.25 in	Shallow well, bigger flow, no pressure
DQB2.0-25-24-210	24 V	210 W	2 m3/h	25 m	1 in	Direct to drip, shallow water
DQB2.0-30-24-280	24 V	280 W	2 m3/h	30 m	1 in	As above with more head margin
3DPC3.5-35-24-300	24 V	300 W	3 m3/h	35 m	1.25 in	Half a hectare of beds in zones
3DSC4.5-35-24-300	24 V	300 W	4 m3/h	35 m	1.25 in	Stainless impeller, sandy water
3DSC4.5-50-48-400	48 V	400 W	4 m3/h	50 m	1.25 in	Deeper well, stainless impeller
4DSC3.5-50-48-400	48 V	400 W	3.5 m3/h	50 m	1.25 in	Narrow bore, more head
4DPC4.5-40-48-500	48 V	500 W	4.5 m3/h	40 m	1.25 in	You want sprinklers at pressure
3DPC5-45-48-500	48 V	500 W	5 m3/h	45 m	1.5 in	Large garden, low head
4DSC6-45-48-500	48 V	500 W	6 m3/h	45 m	1.25 in	Big flow off modest head
DLP15-14-48-500	48 V	500 W	15 m3/h	14 m	2 in	Surface pump, pond or canal only
DLP20-19-72-900	72 V	900 W	20 m3/h	19 m	2 in	Flood or furrow, very low head

Two notes that will save you money.

The stainless impeller is worth it if your water is sandy or slightly salty. 3DSC and 4DSC are the stainless lines, 3DPC and 4DPC are plastic. In a garden the difference matters most where you draw from a pond, a canal or a sandy shallow well, because abrasion, not corrosion, is what wears a garden pump out. There is a full comparison of when plastic is genuinely fine in plastic vs stainless steel impeller.

Panel sizing is a rule, not a guess. Our catalogue states it directly: the solar array should be at least 1.3 times the pump’s rated power. A 200 W pump therefore wants 260 W of panels or more, which in practice means two 150 W panels or one 400 W panel. And the controller sets a hard ceiling on how those panels may be wired, because its maximum open circuit voltage is fixed by the model’s voltage class:

Controller voltage class	Maximum panel VOC	Roughly how many nominal-12 V panels in series
24 V models (2DPC, 3DPC, 2HJPC, 3HJSS)	under 60 V	2 (about 45 V); three would be about 67 V and is over the limit
48 V models (3DSC, 3HJSC 48 V)	under 105 V	4
72 V models (3DPC, 3HJSC 72 V)	under 155 V	6
110 V models (3DSC, 3HJPC 110 V)	under 205 V	9

This is the single most common wiring mistake on small systems. Someone buys a cheap 60-cell panel with a VOC of 45 V, adds a second one to get more power, and ends up at 90 V on a 24 V controller that stops at 60 V. And cold mornings push VOC up, not down, so a winter dawn is when it happens. If your site freezes, leave more margin than the table shows.

Hose, not pipe: the head that small systems forget

On a field system, friction loss is a design line item. On a garden it is usually ignored, and it is the reason a pump can be perfectly sized and still deliver a thin trickle.

Here is what 30 metres of hose costs you at a modest 0.5 m3/h, calculated with the Darcy-Weisbach equation using a roughness of 0.007 mm for smooth polyethylene and hose:

Hose or pipe	Internal diameter	Friction over 30 m at 0.5 m3/h	Against a 20 m total head
25 mm (1 in) PE pipe	about 21 mm	0.4 m	2 percent
20 mm (0.75 in) PE pipe	about 16 mm	1.4 m	7 percent
1/2 inch garden hose	about 10 mm	13 m	65 percent

That last row is the whole problem. A 1/2 inch garden hose on the end of a pump with a 1.25 inch outlet throws away most of your head, and it does it invisibly, because the hose looks fine and the water does come out. The field rule of thumb of roughly 1 m of head per 10 m of pipe is calibrated for field flows of 1 to 2 m/s. At garden flows it over-reads slightly for proper pipe, and under-reads by a factor of four for a thin hose. If your pump works when you hold the hose straight and dribbles when you unreel it, you have just measured your friction loss. The general principles are in what size pipe do I need.

Worked example: a 0.1 ha homestead garden

Let me put all of it together on a real site, because the numbers interact and this is where people discover that a “too small” pump is actually correct.

The site. 0.1 ha, 1,000 m2, mixed vegetables for a household plus a little to sell. Loam soil. Static water level 4 m below ground, and it drops 2 m when pumping. The tank sits on a 1.5 m stand. 25 m of 20 mm pipe runs from the wellhead to the tank. Provision is made for 1.3 times pump power in panels.

Step 1, the floor. At 5 mm/day of net crop use over 1,000 m2 the garden needs 5.0 m3/day. Add 15 percent for application losses and it is 5.9 m3/day gross. Over 5.5 effective pumping hours that is 1.07 m3/h.

Step 2, the ceiling. Loam infiltrates 10-20 mm/h, so design on 12 mm/h. If the garden is split into four blocks of 250 m2, the ceiling per block is 12 x 250 / 1000 = 3.0 m3/h. If you irrigate all 1,000 m2 at once the ceiling is 12 m3/h, but you now have an application rate problem with no pump in the world able to help, because the daily volume is only 5.9 m3. The four-block split is not a compromise, it is the design.

Step 3, the head. Water level plus drawdown, 4 + 2 = 6 m. Lift to the tank inlet on the 1.5 m stand, 2.5 m. Friction in 25 m of 20 mm pipe at just over 1 m3/h, about 1.6 m including fittings. Total dynamic head is 10.1 m, call it 10 m.

Step 4, the model. 2DPC1.5-35-24-200 at 10 m of head delivers 1.17 m3/h. That sits between the floor of 1.07 and the ceiling of 3.0. It matches the floor almost exactly, which is what you want on a garden pump: no wasted capacity, no wasted panel.

Check	Number	Verdict
Floor (daily need / solar hours)	1.07 m3/h	-
Ceiling (infiltration x wetted block)	3.0 m3/h	-
Pump flow at 10 m TDH	1.17 m3/h	Inside the window, 9 percent above the floor
Daily output (1.17 x 5.5 h)	6.4 m3/day	Against 5.9 m3/day needed, 8 percent margin
Application rate on a 250 m2 block	4.7 mm/h	Against loam’s 10-20 mm/h, comfortable
Panel requirement (1.3 x power)	260 W	Two 150 W panels, or one 400 W
Controller VOC limit	under 60 V	Two nominal-12 V panels in series, about 45 V

Step 5, the schedule. The daily volume is 5.9 m3, which at 25 mm per event covers 236 m2. So the garden is watered in four blocks, one block a day, each block getting 25 mm once every four days. That is precisely the rhythm the soil data called for in the earlier section, arrived at from the other direction. Each block takes about 5.5 hours at 1.17 m3/h, which is your whole solar day, which is why the four-way split is not optional.

Step 6, the tank. Size it at 1.5 to 3 times the daily requirement, so 2,000 to 3,000 litres for this site. But note what the tank is actually doing here. The pump delivers 1.17 m3/h into a block that can only absorb 4.7 mm/h. On loam that gap is harmless. On the same garden with clay loam at 6 mm/h the ceiling drops to 1.5 m3/h per block and the gap starts to matter, which brings us to the second example.

A heavy-soil variant. Take 120 m2 of clay loam beds, 25 mm per event, so 3.0 m3 of water. The soil ceiling at 6 mm/h is 120 x 6 / 1000 = 0.72 m3/h. The pump 3DSS1.2-56-24-120 at 11 m of head delivers 1.18 m3/h, which is above the ceiling. Two options, and both are legitimate:

1. Step the pump down to the 80 W 3DSS0.5-28-12-80, which gives 0.35 m3/h at 10 m. It respects the ceiling, but 3.0 m3 then takes 8.6 hours, which is more than a solar day. You would apply the event over two days.
2. Keep the 120 W pump and add a tank, which is what I would do. The pump fills a 3,000 litre tank at 1.18 m3/h while the tank drains to the beds at 0.72 m3/h through soaker line. Over 5.5 hours the pump delivers 6.5 m3, the beds receive 4.0 m3 at an application rate of 6 mm/h, right at the soil’s limit instead of double it, and 2.5 m3 stays in the tank for the evening or for a cloudy day.

That is the idea worth taking away: at garden scale a tank is not really storage, it is a flow transformer. It lets a pump that is faster than the soil become an irrigator that is slower than the soil, without buying a smaller pump or accepting a two-day watering. The size you need is the difference between the pump’s flow and the soil’s ceiling, multiplied by your pumping hours, plus a working reserve. It is also the reason a direct-drive, battery-free system suits a garden so well, as covered in battery vs tank, and the reason a garden is one of the few places where drip is the natural default rather than a premium choice, as covered in solar pump for drip irrigation design.

Three mistakes that end small systems early

1. Choosing the pump from the plot area. The denominator is the area you are wetting at that moment, not the whole plot. If you have ever stood in a garden watching water sheet off a bed while the far end stays bone dry, you have seen the ceiling being ignored. Split the plot into zones on paper before you split it with valves, and check each zone against the soil table.

2. Running the last 30 metres in a garden hose. A 1/2 inch hose costs 13 m of head where 20 mm pipe costs 1.4 m. If your total head is 20 m, that hose has just taken two thirds of it, and it will be doing so for the life of the system. Use the same diameter as the pump outlet, or one size up, and buy the pipe before you buy the pump so you cannot talk yourself out of it. This matters twice over at low head, and a garden is all low head.

3. Wiring more panels than the controller’s VOC allows. The 24 V class stops at 60 V per the catalogue. Two nominal-12 V panels in series is about 45 V and safe; three is about 67 V and instant failure. The trap is that you add the second panel to fix a weak flow problem that is actually a head problem, and the extra panel is not the cure. Before adding panels, measure your head honestly with the total head calculation, and read the sizing method through once in how to size a solar water pump.

What to buy, in order

Buying in the wrong order is how garden systems end up with the wrong pump. Do it this way.

1. Measure your water level, your drawdown and your outlet height. These three numbers, plus friction, are your total dynamic head, and they decide the pump, not the plot.
2. Work out the daily volume from area, crop and climate: area in m2 x 5 mm = litres per day, adjusted for your own season.
3. Test or look up your soil’s infiltration rate. A ring infiltrometer test takes half an hour and costs nothing, and the FAO’s published values in the table above are a perfectly reasonable starting point.
4. Compute your floor and your ceiling, and check they overlap. If the floor is above the ceiling, plan zones or a tank before you plan a pump.
5. Only now pick a model, reading its measured curve at your head, and confirm the flow lands inside the window.
6. Size the tank at 1.5 to 3 times the daily volume, or at the flow gap if your soil is heavy, and put it on a stand of 1.5 m so gravity does the distribution.
7. Size the panels at 1.3 times pump power or more, then check the series wiring against the controller’s VOC ceiling.
8. Add a float switch at the tank and a dry-run setting at the controller. These two items cost very little and they are what stop a garden pump from being destroyed by its own success on the first full tank.
9. Buy the pipe at the pump’s outlet diameter, and buy it before the pump arrives.

Everything else about a garden system, from sprinkler choice to frost protection to panel angles, is a detail you can adjust after the first season. Head, floor and ceiling are not adjustable, and they are the reason one garden thrives on an 80 W pump while the neighbouring one fails with a 500 W pump and the same plot size. If you would rather have the arithmetic done for you, put your water level, lift and plot size into the sizing tool, or send me your numbers directly.

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Planning a solar pump for a garden or smallholding? Put your water level, your lift and your plot size into the sizing tool to see your real head, or message me on WhatsApp with your water level, your lift, your plot size, your soil type and your crop, and I will come back with a model, a panel count, a tank size and a watering schedule that fits your soil rather than your plot.