The tank is the battery of a solar pump system — except it costs a tenth as much. Getting its size right is about balancing three numbers: how much water you use per day, how fast your pump can refill, and how many cloudy days you want to survive. This calculator works all three out together so the pump, tank and farm fit each other instead of fighting.
The sizing rule in plain terms
Take your daily water use and multiply by the number of buffer days you choose — 2 days is the typical safe choice, 3–4 for climates with frequent cloud or dust storms. Add a 10% reserve on top and you have your total tank capacity. A 5,000 L/day farm with a 2-day buffer needs about 11,000 liters of tank. Bigger is cheaper insurance than any other option in the system.
The refill check nobody does
A big tank is useless if the pump cannot refill it within the sun hours available. Divide the tank volume by the pump's hourly flow at your real total dynamic head: that is the refill time. If it is longer than your sun hours, the tank never fills, and the buffer you paid for does not exist. The calculator flags this immediately — and the chart shows where larger tanks start to break the balance.
Gravity pressure from tank height
Every 10 meters of tank height gives about 1 bar of pressure. A tank on a 3 m stand gives gentle gravity flow — enough for irrigation channels and animal troughs. For household pressure (showers, taps) you want 10–15 m of elevation, or a small pressure pump after the tank. Do not put the tank on the ground and expect pressure; the height is free energy that works while the sun is down.
A worked example
A small farm uses 5,000 L/day, has a pump giving 1,000 L/h and 6 sun hours. With a 2-day buffer the tank should be ~11,000 L (the calculator suggests the nearest practical size, e.g. 10,000–12,000 L). Refill takes 11 hours — longer than the sun hours — so the sensible answer is either a 5,000 L tank (fills in 5 h) with a 1-day buffer, or a faster pump. That trade-off is exactly what the chart shows.
Evaporation and losses
In hot climates an open tank loses 0.5–2% of its volume every day to evaporation. Over a 10,000 L tank that is 50–200 liters lost daily — meaningful for tight budgets. Cover the tank, and losses drop near zero. The calculator includes your loss rate when it checks whether the daily refill covers both demand and losses.
Frequently asked questions
How big should my water tank be for a solar pump?
A practical rule is one to two days of your water demand as storage. A farm using 5,000 liters per day should aim for a 5,000–10,000 L tank. Because water storage is far cheaper than battery storage, going bigger is almost always the right move with solar pumps — it buys you cloudy-day reserves and lets the pump run fewer hours.
Why is a tank better than batteries for a solar pump?
Because storing water costs a fraction of storing electricity. A 10,000 L concrete or plastic tank costs roughly $500–1,500 and lasts 15–30 years, while batteries delivering the same "energy reserve" cost several times more and degrade. The tank turns every sunny hour into stored water automatically.
How many hours does the pump need to run to fill the tank?
Divide the tank volume by the pump's hourly flow. A pump delivering 1,000 L/h fills a 5,000 L tank in about 5 hours — which fits a typical sunny day with 5–6 peak sun hours. If the refill time is longer than your sun hours, you need a bigger pump or a smaller tank strategy.
What if some days are cloudy?
That is what the storage buffer is for. If your tank holds 2 days of demand, one cloudy day uses only half the reserve and you recover the next sunny day. For areas with frequent cloudy stretches, increase the buffer to 3–4 days — it is the cheapest insurance available for a solar pump system.
Does tank size affect the pump size I need?
Indirectly, yes. A larger tank means more volume to refill within your available sun hours, which pushes the required pump flow rate up. The calculator shows both sides of the balance: tank volume against refill hours, so you can size the tank and the pump together instead of guessing.
How high should I place the tank?
High enough for gravity flow to your point of use. Every 10 m of tank height gives roughly 1 bar of pressure. A tank on a 3–5 m stand gives gentle gravity pressure for irrigation and animal troughs; for household pressure you typically want 10–15 m of elevation or a small pressure pump after the tank.