The Pressure Tank and Pressure Switch: Running a Solar Pump on Demand, Not Just on Storage

The short answer: a pressure tank does not store water and it does not store energy — it stores compression. About a third of its volume is usable between the pressure switch cut-in and cut-out, and that slice is what protects the pump motor from the one thing that kills it fastest: starting. Every start hammers the motor with several times rated current, and every motor lives under a start-count budget measured per day. Size the tank from the pump’s real duty flow — roughly three minutes of it — pick the switch band off the pump curve so the pump can actually reach cut-out, keep the precharge set 0.14 bar below cut-in, and the pressure switch layout runs a house with tap-level pressure for years. Skip any of those steps and the same hardware short-cycles the pump to death within a season.

Most solar pumping content — including several guides on this site — treats the storage tank plus float switch as the only way to control a pump. For irrigation and stock watering it is. But a house that wants shower-pressure water from a tap often cannot build a 15 m tower, and that is where the pressure tank and pressure switch earn their place: the pump fills a sealed tank against an air cushion, the switch watches the pressure, and water is available the moment a tap opens — no height, no float, no waiting. Used correctly it is a compact, reliable control layer. Used with the folklore sizing most websites repeat, it becomes the fastest pump-killer in the industry. This guide does the real arithmetic: what the tank actually holds, what a start costs, how to size from duty flow, and where the pressure layout fits next to the storage tank in a solar design.

What a pressure tank actually does

A pressure tank is a sealed vessel with a flexible bladder (or diaphragm) dividing it into two chambers: water on one side, pressurized air on the other. The pump fills the water chamber, compressing the air; the switch cuts the pump off at the upper set point; and when a tap opens, the compressed air pushes the stored water out — no pump running at all. At the lower set point (cut-in) the switch starts the pump again.

The number that matters is drawdown: the volume the tank actually delivers between cut-in and cut-out. It is never the tank’s nameplate size:

• The air cushion must keep some volume even at cut-out — water cannot push air to zero.
• With the precharge correctly set about 0.14 bar (2 psi) below the cut-in pressure, usable drawdown works out to roughly 30–35% of the tank volume — about a third.
• The rest is permanently occupied by the air cushion.

So a 300 L tank holds about 90–100 usable litres. That is the whole working volume the household lives on between pump starts, and it is why the folklore of “one litre per 10 W of pump” sizes nothing correctly.

Two adjustments complete the physics:

• Precharge is a maintenance item, not an installation detail. Air permeates slowly through the bladder and escapes through the charge valve. A tank that loses its precharge still holds water but delivers almost nothing per cycle — the classic short-cycling complaint that gets blamed on the pump.
• The tank must be drained to zero pressure to check it. Measuring precharge on a connected, pressurized tank reads the system pressure, not the air charge.

Pressure switch or float switch: two different brains

The two control devices answer two different questions, and confusing them is the root of most bad pressure-tank advice:

The storage-tank layout is the standard for solar because it doubles as the energy battery — the pump works a five-hour solar day, the household works a 24-hour water day, and the float switch starts the pump only a handful of times per day, which is exactly what pump motors want. The pressure-switch layout inverts this: the pump responds directly to consumption, so every shower, every toilet flush, every garden tap becomes a potential start event. Whether that is acceptable depends entirely on the arithmetic in the next section.

The start-count arithmetic: starts are the real budget

Starting is the most violent thing a pump motor experiences. Inrush current runs several times the rated value, the splines and thrust bearings take a mechanical shock, and the motor heats without cooling airflow. The industry numbers are explicit — Franklin Electric’s AIM manual, the reference for submersible motor practice, publishes maximum starts per 24-hour period:

Franklin adds two conditions that matter even more for solar systems: every run should last at least one minute to dissipate the starting heat, and motors of 6-inch frame and larger need at least 15 minutes between starts. For the longest system life the same manual recommends keeping starts below about 10 per day.

Now put a pressure tank under a pump and watch the clock. The cycle works like this: a tap draws at, say, 10 L/min; the pump supplies 20 L/min at that pressure; the surplus 10 L/min fills the drawdown band until cut-out. When the tap closes with the pump off, the next start waits only for the band to empty. With a drawdown of just 20 litres, a 10-minute shower empties it twice, a garden hose can cycle the pump every couple of minutes, and a family of five can easily log 50–100 starts before lunch — inside the table’s ceiling for a small motor but far beyond the 10-per-day ideal, and each run far shorter than one minute.

The fix is arithmetic, not a bigger switch: drawdown ≥ about 3 minutes of duty flow turns every run into a comfortable multi-minute job and keeps daily starts in single digits for a household. For a pump delivering about 20 L/min at the working pressure, that is ≥60 L of drawdown — roughly a 200 L tank once the one-third factor is applied. For 40 L/min, 120 L of drawdown — a 400 L class tank. This is the entire sizing method; wattage never enters it.

Sizing the tank from the pump’s real duty flow

Duty flow depends on the working pressure, so read it from the pump curve at the pressure band you will actually run — not from the maximum-flow figure on the nameplate. The table below takes typical duty flows for household pressure service (roughly 20–35 m of system head) and applies the rule above:

Two conclusions fall out of the table. First, the small end is very affordable: a 200–300 W pump needs only a 150–200 L tank — the smallest standard size. Second, the big end is a category error: pumps in the 4/6DSC and 6DSC/8DSC class move irrigation-scale flows, and no realistic pressure tank buys them three minutes of run time — they would need multi-thousand-litre vessels. Big pumps belong on storage tanks with float control, as our tank-sizing guide argues. The pressure-tank layout is for household and smallholding pressure, and it fits it beautifully.

The trap at the top of the curve: cut-out versus shut-off head

A centrifugal pump produces its maximum pressure at zero flow (shut-off head) and its working pressure out along the curve. The pressure switch only sees pressure. Put those two facts together and a specific failure mode appears:

• If the cut-out pressure is set above the pump’s shut-off head, the pump can never reach it. The pump runs at shut-off — near-zero flow, maximum stare-at-the-gauge pressure — every time demand drops below duty flow, heating itself and wearing its thrust bearing for hours at a stretch.
• If cut-out sits only barely below shut-off, the pump reaches cut-out but crawls the last stretch of the band at very low flow — slow cycling, hot motor, wasted afternoon sun.

The catalogue makes the trap concrete. The 4/6DSC36-22-110-1500 delivers 36 m³/h at 22 m of head; its shut-off head is roughly 30 m — about 2.9 bar. A common 40/60 psi switch cuts out at 4.1 bar: this pump can never reach cut-out. On a sunny afternoon with the tank near full, it will run against shut-off indefinitely. A 20/40 psi switch (2.8 bar cut-out) is workable; a 15/30 psi (1.0/2.1 bar) is more comfortable still. Meanwhile a taller-curve pump like the 3DSC6-60-48-750 (shut-off well above 60 m) runs a 30/50 psi band with room to spare. The rule: choose the switch band off the pump curve, keeping cut-out comfortably below shut-off head — and remember the same logic applies to the storage-tank layout’s float switch, which is why float control is immune to this failure.

Where the pressure layout fits in a solar system

The pressure tank is not a battery and it is not an alternative to the storage tank — it is a different layer. Three honest layouts cover nearly every house:

• Storage tank + float switch + gravity distribution. The default. The pump fills a tank on high ground; pipes feed the house by elevation alone. Pressure is fixed by the height (10 m ≈ 1 bar) and costs zero energy at the tap. Our gravity-distribution guide works this layout in full.
• Storage tank + float switch + booster with pressure tank. Where no hill exists, a small booster pump (the DQB2.0-25-24-210 class) plus a pressure tank pressurizes the house from the storage tank. The big pump still works its solar day into the tank; the booster only runs when a tap opens. Note the booster needs power at the moment of use — from AC mains, or an A/D pump’s AC input — which is exactly what the A/D suffix exists for.
• Pump + pressure switch + pressure tank, direct. No storage tank at all. Simple, compact, and the only one of the three that dies at sunset: after the drawdown is spent, the house has no water until the sun returns. This layout is honest only when daytime demand dominates or an A/D model provides AC backup for the evening. Its start-count discipline is entirely the tank’s job — which is why the sizing sections above are not optional.

Maintaining the precharge

One habit keeps the whole layout alive: check the air precharge yearly, with the tank drained to zero pressure. Close the isolating valve, open a tap until flow stops, read the Schrader valve with a tyre gauge, and reset to about 0.14 bar (2 psi) below the cut-in setting. A tank that has lost half its precharge has lost most of its drawdown, and the symptom is always the same — a pump that has started clicking on for ten seconds at a time — routinely misdiagnosed as a failing pump. The bladder itself is a wearing part with a service life of years, not decades; if precharge holds but cycling stays fast, the bladder has burst and water has reached the air side, and the tank needs replacing.

A complete layout: the house that cannot have a tower

The scenario: a single-storey house, a shallow well with 8 m static water level, no hill for a gravity tank, and no AC mains. Daily demand 0.8 m³, with a morning peak the household wants at real tap pressure.

• Hydraulics. Lift 8 m + friction ~3 m + pressure head 28 m (cut-out 2.8 bar, a 20/40 psi band) = about 39 m of system head. The 3DPC5-45-48-500 (500 W, 48 V, plastic impeller, 5 m³/h at 45 m max head) holds roughly 2.5–3 m³/h at 39 m — call it 45 L/min at the working pressure.
• Tank. Drawdown ≥ 3 × 45 L = 135 L → a 400–500 L pressure tank, precharged to about 1.26 bar (cut-in 1.4 bar minus 0.14 bar).
• Start budget. Daily volume 800 L through 135 L of drawdown: about 6–8 starts/day, each run 3 minutes or more — inside every line of the start table, close to the 10-per-day ideal.
• Array. 1.3 × 500 W = 650 W minimum — two 550 W modules; the controller follows the wiring guide’s float-switch pattern, with the pressure switch in place of the float (a dry contact, same terminals).
• The honest caveat. After sunset this house runs on the 135 L slice and nothing else. If the family showers at night, the right fix is not a bigger pressure tank — it is adding the storage tank layer: pump to a modest tank by day, then feed the pressure set either by gravity or by a booster, exactly as the hybrid layouts above describe.

The buying checklist, in order

1. Read the pump curve at your pressure band — that is your duty flow; nothing else sizes anything.
2. Drawdown = 3 minutes of duty flow, minimum. Divide by one-third to get the tank’s nameplate size.
3. Choose the pressure switch band off the same curve — cut-out clearly below shut-off head; 20/40 psi for low-head pumps, 30/50 psi for 60 m-class.
4. Precharge 0.14 bar below cut-in, checked with the tank empty, marked on the tank, rechecked yearly.
5. Count the starts your household implies (daily volume ÷ drawdown) and hold it inside the motor’s table — ideally under 10/day.
6. Decide the night story honestly: storage tank + float for autonomy, or an A/D model with AC backup. A pressure tank alone buys an afternoon, not a night.
7. Buy the tank one standard size up when in doubt — the cost difference is trivial next to a motor.
8. Keep the plumbing honest: isolating valve before the tank, the switch sensing line free of sediment, and the check valve between pump and tank in working order — the same valve our check-valve guide dissects, doing its quiet job on the pressure side too.