Installing a rainwater tank is no longer just a trend for new houses. More and more owners of older homes and cottages are choosing them as well. Before selecting a specific tank, it’s good to know what size you’ll need and where to ideally place it.
Why get a rainwater collection tank?
Rainwater is free, and if you capture it wisely, you can save up to several dozen percent of your drinking water consumption. On top of that, you may qualify for a government subsidy – the Czech Dešťovka program covers part of the cost of the tank and installation (as part of the “New Green Savings” scheme).
What determines the right tank volume? 
Several basic factors influence the tank size:
- Roof or catchment area – the larger it is, the more water you collect.
- Rainfall amount – in the Czech Republic, the average is about 600 mm per year, which means you can collect around 0.6 m³ of water per 1 m² of roof area.
- Intended use – it depends on whether you want rainwater only for garden irrigation or also for household use (toilets, laundry, shower).
How to calculate the right tank size?
Don’t worry – it’s not rocket science. The calculation is simple and you only need three numbers:
- Annual rainfall in your area – in the Czech Republic it averages around 600 mm.
- Roof area used for collection – e.g., 100 m².
- Collection efficiency – how much water actually reaches the tank (count with 80–90%).
Example:
You have a 100 m² roof, live in Eastern Bohemia, and average rainfall is about 600 mm/year.
600 mm = 0.6 m × 100 m² = 60 m³, i.e. 60,000 liters per year.
At 90% efficiency, that’s about 54,000 liters per year.
It sounds like a lot. But we all know rainfall is uneven – sometimes heavy, sometimes dry. The water doesn’t come evenly. That’s why it’s recommended to size the tank to cover a few weeks of drought, not the entire year. The tank should primarily balance fluctuations. The higher and longer the consumption, the more volume makes sense. On the other hand, an oversized tank that stays empty for long periods can be unnecessarily expensive and harder to maintain.
How large should the tank be depending on use?
Now that we know how much water can be collected, the next question is how you plan to use it:
- Garden irrigation only – usually a 1,000–3,000 liter tank is enough.
- Irrigation + toilets/laundry/shower – better to plan for 5,000 liters or more.
- Large garden or business use – 10,000 liters and beyond.
Above-ground or underground tank?
The key question: visible or invisible? Rainwater tanks are divided into above-ground and underground types, each with its own pros and cons.
- Above-ground tanks – suitable mainly for garden purposes or where digging isn’t possible. Easy to install, often just connect to the downpipe and add a simple filter. But in winter they must be protected from frost – either drained or winterized.
- Underground tanks – save garden space and are protected from weather. Suitable not only for seasonal irrigation but also for extended household use. Selection must consider soil type, groundwater level, and connection method. Most plastic tanks are self-supporting, meaning they don’t require concrete casing and can be installed directly in an excavation.
Where to place the tank
Several factors matter: distance from downpipes, pump connection options, and ground slope. For underground tanks, allow access for machinery and handling during installation. Ideally, water distribution can continue by gravity, or with a simple pump.
Also consider overflow. The tank should have an overflow system to divert excess water into infiltration, drainage, or sewerage, preventing overfilling during heavy rain.
A quality tank saves both money and worries
The right rainwater collection tank is both eco-friendly and practical, reducing your drinking water use by up to dozens of percent. Whether you’re planning just seasonal garden watering or more complex household use, it pays to choose a quality tank with suitable volume and installation method.
On our CZ PLAST e-shop you’ll find a wide range of underground plastic tanks – for example, VARIO or VENICEmodels, ideal for most family homes. These self-supporting tanks don’t need concrete casing and thanks to their low profile, are suitable even where deeper excavation isn’t possible. For larger needs, there’s also the ORCA – a large-capacity version with 10 m³.
CZ PLAST tanks are designed for long service life, easy installation, and connection to pumps or filtration systems.
