Updated: 24-May-2012

Saving Water

Rainwater Storage

It all sounds so simple - buy a water barrel connect it to your guttering down pipes and save money and water - not necessarily true. In practice while it will save water and reduce the water bill (provided you are on a meter) it may not save you money. Before rushing out and buying a barrel think about the following factors:

How much rain water comes from the guttering or how long will the barrel(s) take to fill up?
What size barrel(s) do you need and how much is the barrel going to cost?
What else do you need to buy before it is all up and running?
How are you going to connect to the down pipes?
Over what period do you want to recover the cost?
How long will the barrel(s) last?

The Benefits Of Using Rainwater

Apart from saving water one not so obvious benefit of using rainwater to water garden plants is they seem to like it better than tap water. Providing it is clean it can also be used for washing the car. In order to use rainwater for washing the car it is best to install a fine filter in the water inlet and made sure the barrels are clean inside, this removes any grit in the water. One downside of installing this type of filter is that it tends to clog which means regular cleaning. The advantage is that in the winter when very little water is needed in the garden it is being used for washing the car.

How Much Water Comes From The Guttering Or How Long Do The Barrels Take To Fill Up?

Below is a real example backed up by eight years practical experience:

Roof area water collected from 15m * 7m = 105 square metres
For each 25mm of rain this equates to a volume of 105 * 0.025 = 2.65 cubic metres of water
This equals 2.65 * 1000 = 2650 litres of water

Sounds really good BUT this figure assumes the rain is falling straight down (how often does that happen?), all the water goes into the guttering (unlikely), there is no evaporation (as will happen with light rain), the guttering never overflows and there is no absorption by the tiles. In practice use a figure of half of this (1325 litres).

This means that for every 25mm of rain five 250 litre barrels could be filled. Even this sounds really good BUT it assumes all the water goes into the barrels (depends on how the barrels are connected - using standard connectors probably no more than 50%), the barrels never overflow (unlikely since most of the rain comes at a time of year when water is not needed for the garden) and there are no leaks or losses due to evaporation from the barrels.

Taking all of the above into account the real figure is likely to be nearer the equivalent of filling four 250 litre barrels five times a year or 5000 litres for 27 inches (686mm) of rainfall, which is typical in East Anglia (UK), this is still a lot of water. In practice in spring the barrels are full and are occasionally topped up by rain during spring and summer which means the barrels gradually empty during the summer as the usage is higher volume than the topping up. To put this into some sort of context a rough guide for watering a vegetable patch is 11 litres per square metre or 2 gallons per square yard. This means that assuming 70 days when you need to water a 3m * 2m vegetable garden (not very large) will need around 4600 litres.

What Size Barrels Are Needed And How Much Is The Barrel Going To Cost?

The obvious thing is to go for as big a barrel or tank capacity as possible but that takes no account of cost, whether there is space and in the case of large barrels and tanks whether proper foundations and a properly constructed stand are needed. Water is heavy with 1 litre weighing 1kgm (2.2 pounds), this means a 250 liter barrel will hold 250kgm (550 pounds) of water. It is possible to get barrels with a flat back which will sit tight against a wall but they are more expensive. For practical reasons it is usually better to go for tank sizes of 100 to 250 litres chaining them together to get the required capacity. It also makes it easier to go for the minimum to start with and add capacity based on experience hence minimising the cost. Without a stand a 250 litre barrel will cost around £50 at 2012 prices.

What else do you need to buy before it is all up and running?

Apart from the barrels to store the water you will need the following

Pipes and fittings to connect to the down pipe
Pipes and fittings for the overflow
A stand for the barrel, either built or purchased to raise it off the ground
A tap to get the water out
A filter to stop grit and debris getting into the tank
Pipes to connect barrels if more than one

How Are You Going To Connect To The Down Pipes?

How the barrels are connected to the rain water down pipes has a large bearing on how much water is collected. The simple inline connections for round and square pipes found in the large DIY stores and garden centres are a waste of time and relatively expensive. The reason is they simply have a small tray around the outside of the fitting to collect water running down the walls of the down pipe which means when there is heavy rain a lot of the water misses the trap. Times of heavy rain are when water collection needs to be the most efficient. They also partially block the down pipe reducing the ability to carry large volumes of water. The most effective method is to feed the down pipe into the top of the barrel and then have an overflow of the same diameter of the down pipe to carry away excess water when the barrel is full. If more than one barrel is fed from one down pipe then just connect the bottom of the barrels with a flexible pipe of about half to a third the diameter of the down pipe. The down pipe feeding the barrel should be terminated with a fine mesh filter to trap debris - this needs to be easy to remove for cleaning.

Over What Period Do You Want To Recover The Cost?

It is all very well spending large sums of money on a project like this but unless you recover the money in a reasonable time there is not much point in doing it in the first place. A sensible recovery time is between 3 and 4 years. This means the cost of the water saved over this period is greater than the money initially invested. When working out the recovery time it is logical to take into account the likely inflation in water costs. If installing more than one collection system it is best to look at each one separately when working out the payback time.

How Long Will The Barrels Last?

It is difficult to predict how long plastic water barrels will last because it depends on the plastic used and how much sunlight they get - ultra violet radiation from the sun does degrade the plastic over time. Usually these barrels are made of recycled plastic which is not of the highest quality. From experience it is likely they will last in excess of ten years but they will need replacing eventually.

The example water cost savings give an example costing for this type of installation.