The Tank on the Hill: How High for Gravity Distribution From a Solar Pump

The short answer: height is a currency, and the exchange rate is fixed by physics — every 10 metres of elevation buys about 1 bar of pressure. A gravity system works when you spend that currency only where it is needed: a tank 15 to 20 m above the house for real showers, 5 to 10 m above a field for drip irrigation, and almost never high enough for sprinklers. Every metre you ask of the tower is also a metre you ask of the pump, and the pump asks the array — which is why half the height costs a quarter of the energy, and why the smartest layout often lifts less and boosts locally. This guide walks the full arithmetic: the pressure menu, the tower bill, the tank on the slope, and the pipes downhill — every model verified against our live catalogue.

Most solar pumping guides, including several on this site, treat the storage tank as the end of the story: the pump fills it, done. But on a real farm the tank is usually the beginning of a second system — pipes that carry water downhill to a house, a trough line, a drip field — and that second system runs on one fuel only: elevation. Pumped pressure stops when the sun does; height pressure is there at 2 a.m., on cloudy weeks, ten years from now, with no electronics in the path. Getting that elevation right is a design decision with three sides: enough height for the fixtures you actually have, not so much that the pump and array drown in the bill, and a tank sized for the slope it stands on.

What the height is actually for

Water in an open tank has no pressure. Open the outlet and it just dribbles over the rim. Pressure appears only when water falls — or is pushed — through a height difference, and the numbers are fixed:

• 1 metre of elevation ≈ 0.098 bar ≈ 1.42 psi of pressure at the bottom.
• Practical reading: 10 m of height ≈ 1 bar ≈ 14.5 psi.

This is static pressure, set by geometry alone — pump brand, pipe diameter and flow rate change nothing about it while no water moves. Two things then modify it before water reaches the tap:

• Elevation drops: every metre the fixture sits above the tank bottom subtracts from the budget. The pressure at a tap is the height of the water surface above that tap, not above your feet.
• Friction losses: once water flows, the pipes eat roughly 0.5 to 2 m per 100 m of run at household flows in 25 mm pipe — more in narrower pipe. Our pipe-sizing guide has the full table.

So the design question is never “how high is the tank” in isolation. It is: height of water surface above the highest fixture, plus friction, minus nothing — is that enough bar for what the fixture needs? The next section is that menu.

The pressure menu: what each fixture really needs

Appliances and emitters differ wildly in what they demand. The numbers below are the same ones used across our irrigation guides, and they are the vocabulary of every decision in this article:

Typical ranges — always check the fixture’s own sheet. 1 bar ≈ 10 m of head.

Two conclusions fall straight out of the table. First, drip irrigation is gravity’s best customer: a tank on a 6 m hill feeds a drip field all afternoon and all night with no pump, no controller and no battery anywhere in the delivery path. Second, sprinklers are gravity’s worst customer: 2.5–4 bar means a 25–40 m column — you are building a municipal water tower on a farm budget. The menu, not the catalogue, decides which camp your system is in.

The arithmetic of buying pressure with height

The transaction itself is brutally simple. To hold N bar at a fixture, the water surface must sit 10×N metres above it. Everything else is bookkeeping:

• House on flat ground: tank stand 12 m, water surface 13 m → ~1.3 bar at ground-floor taps → every fixture works, showers are modest. Tank stand 15 m with water surface 16–17 m → 1.6–1.7 bar → genuinely good showers.
• House halfway up a slope, tank at the top: a 10 m tower on ground that is itself 8 m above the house gives 18 m of surface height → ~1.8 bar. The hill is doing free work no tank stand could economically provide.
• Drip field below the tank: water surface 6 m above the emitters → 0.6 bar at the head of the field — inside the drip band of the menu. On undulating ground, place the tank so the far end of the field still has 5 m of head; pressure-compensating drippers hold their output down to about 0.5 bar and forgive the gentle slope.
• Trough line on a ridge: cattle troughs need almost nothing — a floating inlet valve closes at 0.2–0.5 bar. Any tank standing higher than the troughs by 3–4 m feeds a whole ridge line of drinking water without a single powered part.

Notice what the menu punishes and what it rewards. The middle of the table — showers, micro-sprinklers, pop-ups — is exactly the band where a tower becomes expensive. The bottom of the table — drip, troughs, toilets — is where gravity is nearly free. The layout trick, when your fixtures sit in different rows of the menu, is in the next section.

Every metre of tower is a metre you ask of the pump

Here is the part most tank discussions skip: static pressure and lift merge into one number at the pump. The solar pump does not know that 40 m of its total dynamic head is the well and 12 m is the tower on the hill. It lifts against the sum — and it pays for every litre, every day, forever.

The catalogue makes this concrete. Consider the same household, same well, two tower decisions:

• Version A — 10 m tower. Water surface 13 m above ground, well lift 30 m, friction 4 m → the pump works against 47 m of head. The 2DPC1.5-35-24-200 (200 W, 24 V, 1.5 m³/h at 35 m max head) is already past its ceiling; the next rung, the 2DPC1.7-45-24-300 (300 W, 24 V, 1.7 m³/h at 45 m), holds about 1.2–1.4 m³/h here — 8 to 9 m³ on a five-hour day, array sized by the 1.3-times rule.
• Version B — 15 m tower. Same well, water surface 18 m → the pump now works against 52 m. On the same 300 W rung the curve has slid further down; daily volume drops again, and if the well were deeper the whole selection would have jumped a voltage rung — a 110 V controller, heavier cable, a bigger array.

Five metres of vanity cost real flow every single day. The relationship is not even linear: since power at a duty point grows roughly with flow × head, doubling the height roughly quadruples the energy per litre. Half the tower, a quarter of the bill. That steepness is the whole economic argument behind the next section.

One more mechanism hides in the same ledger. The pressure-compensating trick of adding a booster pump at the tank outlet is not an extra cost stacked on top of the tower — it is an arbitrage. A booster and a tower buy pressure at nearly the same energy rate per bar; the booster buys it only for the litres that need it (shower time at the house), while the tower buys it for every litre the system ever moves, including the trough line that needs none. Our booster pump guide walks the DQB family for exactly this job.

When not to buy height at all

The cheapest metre of elevation is the one the land gives you. Before pricing a single tank stand, walk the parcel:

• Natural shelves. A farm is rarely flat. A shelf 10 m above the house is a free 15 m tank stand; a hill 30 m above the drip field can gravity-feed a whole irrigation block from a tank nobody even sees from the house. The survey costs an afternoon; the tower it replaces costs real money forever.
• Springs above the field. If you have a spring, our spring-water guide’s first move applies before any pump is bought: a 63 mm pipe running 300 m on a 0.4 m drop already delivers 2 m³/h by pure gravity. Pump only the litres gravity cannot reach, into the tank gravity cannot fill.
• Split the system instead of stacking it. Where one tall tower would have to serve both a shower band and a trough line, put the troughs on their own low tank (fed from the same well pump through a second float, or by gravity overflow) and keep the tall tank for the house only. Two modest tanks beat one heroic one on cost, on wind loading, and on what a single failure takes down.
• Boost at the point of use. When the house pressure is genuinely too soft, the arithmetic of the previous section says: buy the missing bar at the house, not at the well. A small booster on the tank outlet runs off AC mains or the same array, works only when a tap is open, and leaves the solar pump’s duty point — and therefore its whole selection — untouched.

Sizing the tank: three layers on a slope

The tank-sizing guide gives the full three-layer arithmetic — the night layer, the cloudy layer, the float dead zone. On a gravity-distribution layout, two of the three layers change shape:

• The dead zone grows with height. Every litre the pump lifts to the overflow, it lifts — including the litres that will never drain, sitting below the outlet. Design the outlet at the very bottom of the wall (a drainable sump if the tank allows it), because that dormant volume is height you paid for at the well and never spend.
• Usable storage is measured from the outlet, not from the nameplate. A 5,000 L tank with its outlet 0.5 m off the base and a float dead zone of another 0.3 m holds roughly 10–20% less than the sticker says. Size up accordingly.
• The cloudy layer rides the same slope. A family that draws 0.6 m³ a day through gravity pipes needs the same two-to-three-day reserve as any other family — but on a hill site the reserve tank is often the cheap part (the land is free) while the tower is the expensive part. Where the site allows it, over-size the volume on free terrain rather than the height on purchased steel.

The failure mode to design against is quiet: a gravity system that is slightly too small never announces it. The tank simply draws down below the outlet line on the second cloudy day, pressure fades at the far tap, and the household learns to carry water by hand. The three layers exist so the second cloudy day is a non-event.

Pipes downhill: getting the water back down without surprises

Water falling through a distribution pipe carries energy, and a gravity system spends it in three places. Design all three, or the tower you paid for leaks away or comes back as a hammer:

• Velocity. Keep distribution flow under about 1.5–2 m/s in the mains. A 25 mm pipe at 1.5 m/s carries roughly 2.5 m³/h; a 32 mm pipe about 4 m³/h; a 50 mm pipe about 10 m³/h. Oversizing downhill pipe is cheap insurance twice over — lower friction and lower velocity.
• Friction. Long runs eat the budget: 100 m of 25 mm pipe at 1.5 m/s can consume 1.5–2 m of head, which is a tenth of a bar you promised a shower. Where the run is long, jump a pipe size and pocket the pressure.
• Surge. A quick-closing tap or valve on a long downhill line converts the whole moving column into a pressure spike — the same water hammer physics our check-valve guide dissects, arriving now on the distribution side. Gravity lines are not too gentle to hammer; a 30 m column falling through 15 m of head moves with real momentum. Slow-closing taps on the house ends, and an air chamber or surge provisions near fast valves on long runs, are the cheap defenses.

One more downhill gremlin: air. A line that rises, falls over a hump and rises again traps air at the crests, and trapped air pockets throttle gravity flow dramatically — a pipe that looks full can be half air. Automatic air-release valves at high points, or at least a layout that avoids re-climbing, keep the column solid.

A complete layout: forty cattle and one house on a ridge

The scenario: a homestead with 40 head of cattle on a ridge, a 30 m static water level well, the house at the base, the trough line along the ridge 6 m below the tank site. Daily demand: about 0.6 m³ for the household and 2.1 m³ for the herd (roughly 50 L per head) — call it 2.7 m³/day.

• Demand side. Troughs close their float valves at 0.2–0.5 bar; the tank water surface 3–4 m above them is plenty. The house wants 1.5 bar at the shower — but the house sits below the well head, 8 m down the slope, so the effective height the tank owes the house is 15 m minus the 8 m the house gives back: a tank water surface about 7 m above the well head serves the house at ~1.5 bar at its own level, and the troughs automatically.
• Supply side. Total pumping head = 30 m lift + 7 m to the tank surface + ~4 m friction = 41 m at 2.7 m³/day. Over a five-hour day that is about 0.55 m³/h — comfortably inside the curve of the 3DSC4.8-95-48-750 (750 W, 48 V, stainless steel, 4.8 m³/h at 95 m max head), which holds roughly 3.5–4 m³/h at 41 m. Array: 975 W by the 1.3-times rule — two 550 W modules. The tank: 8,000 L, outlet at the base, three-layer arithmetic on 2.7 m³/day gives about 1.5 days of autonomy.
• What the height bought. Every fixture on the property — showers, kitchen, troughs, a garden hose — runs at any hour from stored water with no battery and no powered component in the delivery path. The single powered device on the farm is the well pump, on its five-hour shift.
• What the height would have cost if misjudged. Had the tank been placed 20 m up to chase sprinkler pressure nobody uses, the pumping head would be 54 m, the same 750 W pump would be delivering about 2.3–2.7 m³/h instead of 4, daily energy would roughly double, and the array would grow to three panels — all to serve a pressure band the property never draws from.

The buying checklist, in order

1. List the fixtures and read the menu. Drip band, shower band, or sprinkler band — most properties are in the first two.
2. Walk the terrain before pricing steel. Natural elevation is the only free height. Measure the shelf, the hill, the slope behind the house.
3. Fix the tank site by the highest band you actually need. 15–20 m of surface height for showers; 5–10 m for a drip field; 3–4 m over troughs. Never buy sprinkler pressure from a tower.
4. Add the tower height to the well lift and re-read the pump curve at that total head — the tower is part of the pump’s job description, and it can move the selection a whole voltage rung.
5. Size the tank by the three layers, outlet at the bottom, and remember the sticker overstates what a high outlet leaves you.
6. Design the downhill network at 1.5–2 m/s, jump a pipe size on long runs, put slow-closing taps and air valves where the profile climbs.
7. Only then, if one band still starves, add a booster at that band’s tap — bought per litre where needed, not per litre pumped.

Height is the only storage medium that never degrades, never needs a controller, and works during the blackout as well as the sunny noon. Spend it deliberately: buy the rows of the pressure menu you need, borrow the terrain’s free metres first, and let the solar pump lift against a number you chose on paper — not one the tower grew into.