Calculate Your Water Storage Needs

To determine the amount of storage that is appropriate for you, you need to know how much rainwater you can collect from your roof (supply) and the amount of water you will need (demand).

How Much Water Can I Collect?

Your total potential water capture can be calculated using the following simple formula:

1″ of rain on 1 square foot of roof area produces 0.52 imperial gallons of water
or 1 mm of rain on 1 square metre of roof area produces 1 litre of water

The total amount of water you can collect from your roof depends on 3 things:

  The annual rainfall in your area. You can check this here.
  The size of your roof (the area measured as the horizontal plane under the roof including overhangs as shown in the sketch below).
  The proportion of the actual rainfall you can collect. The collection efficiency of a roof is based on factors such as evaporation loss from prevailing winds, tree cover, type of roofing, and the sizing of the pipes for storm events, etc. Well designed systems can capture 75-85% of the total rainfall during the winter, but less during the summer months.

The Mediterranean climate (wet winters and dry summers) of coastal British Columbia varies dramatically. For example a house with 1000 square feet of roof area in a dry area such as Saturna Island would collect 12,900 imperial gallons (58,643 litres) in a year with average rainfall, while a similar house in the Sooke area would collect more than double that amount.

How Much Water Do I Need?

When calculating the amount of water you need, seriously consider what you want to use the water for, and what conservation measures you can incorporate that fit your lifestyle. Consider your indoor and outdoor water requirements separately.

Indoor Water Use

It is best to calculate water use on a monthly basis. As a guide, most of the Rainwater dependent households with systems from the Rainwater Connection average less than 30 imp. Gal (136L) per person per day, and some report use levels as low as 25G/P/D (114L). Cottage/cabin users are typically in this range largely because of lower clothes washing volumes.

When estimating indoor water use consider:

  How many residents?
  Is it a full time residence or part-time?
  If part time, when are you mostly there? Are there plans to live there full time in the future?
  How many visitors? For how long? At what times of the year?

Indoor water conservation is an important part of any rainwater system because of the cost of water storage. For a rainwater dependent household, water saving appliances can pay for themselves in one or two years because they reduce the size and the capital cost of the cistern.

Low flush toilets (3 to 6 litres per flush), can lower indoor water use by as much as 15% and pay for themselves in a year by reducing storage requirements. The new water efficient clothes washers use as little as 4 gal (18L) for small loads and 15 gal (70L) for large multi rinse washes. These washers can reduce household water use by 20%. Other water-saving appliances include – faucet aerators and efficient shower heads, low water use dishwashers, on-demand hot water units and composting toilets.

Outdoor Water

Outdoor water use can be double your indoor use during the summer. The following guidelines will help determine how much water you may use outdoors.

  1 watering can = 3.3 gal. (15 L)
  3 ft. shrub in hot weather (1 week) = 7 gal. (32 L)
  18 in. pot in hot weather (1 week) = 1.8 gal. (8 L)
  40 deck pots – drip water (1 week) = 50 gal (227 L)
  1 sprinkler full flow (for 1 hour) = 240 gal. (1100L)
  Car washing (1/2 hour) 120 gal. = (550 L)
  Pressure washing (1/2 hour) = 40 gal. (180 L)

Also consider other outdoor water uses:

  Large vegetable gardens or fruit trees
  Flower pots, border planting and shrubs
  Car washing and outdoor power washing or deck cleaning
  Hot tub refills
  Extra water reserve for fire protection and earthquakes

The greatest savings (up to 50%) are achieved by selecting drought tolerant and native plants and setting up drip watering systems.

How Much Water Do I Need to Store For Summer?

Water storage is the heart of a rainwater system – especially given our dry summers. To determine your minimum water storage requirements combine your monthly demand with the monthly rainfall in your area and the size of your roof. Storage size must be sufficient to get you through the dry summer months. The drier the summers and the higher the summer use – the larger the cistern required.

How to Calculate Your Storage Needs

The Rainwater Connection uses a Water Balance Table computer program that helps determine the minimum cistern size required to get through the summer in your area. This table combines your estimated monthly rainwater capture, and water usage, and calculates the amount of water in different sizes of cistern at the end of each month.

For a simple garden system the storage capacity could be calculated by totalling your estimated water demand for the summer months and deducting the potential rainwater catchment for those same months. The size of cistern would be equal to the difference of these two. For example, if you estimate a demand of 4,000 gallons from June through September, and the rainfall catchment is 1,000 gallons over that period, you would need a full cistern with a capacity of 3,000 gal at the beginning of June.