Four people. One shower each. A few loads of laundry. The dishwasher. Maybe the garden hose on Sunday afternoon. That adds up to 460 liters every single day - or 13.8 cubic meters per month at $4.50/m3 on the bill. Most households have no idea the number is that high because the bill shows m3 and nobody converts it.
Here is the part that stings: a 120 m2 roof in a moderate climate (800 mm annual rainfall) catches 76,800 liters per year. For free. It just runs down the gutters and into the storm drain. A $150 tank and a downpipe diverter redirect that water to the garden and cut the bill by $144/year. And if the garden uses drip lines instead of a hose, another 35% of outdoor water disappears from the meter.
Three tools. Three angles. One water system. The numbers below are not averages from a government report - they are what the calculators produce when you plug in real inputs.
Where 460 liters actually go
The breakdown for a household of four (1 shower each, 4 baths/week, 4 laundry loads, 1 dishwasher cycle, 500 L/week garden watering):
| Source | Daily liters | Share |
|---|---|---|
| Baseline (toilet, cooking, hand-washing) | 120 L | 13% |
| Showers (4 x 75 L) | 300 L | 32% |
| Baths (4/week x 175 L) | 400 L/day avg | 43% |
| Laundry (4/week x 60 L) | 34 L/day avg | 4% |
| Dishwasher (1/day x 12.5 L) | 13 L | 1% |
| Garden (500 L/week) | 71 L/day avg | 8% |
| Total | 938 L/day | 100% |
That 938 liters per day translates to 28.14 m3 per month and $126.63 at $4.50/m3.
Baths dominate. Not showers - baths. One bath uses 175 liters. One shower uses 75. Replacing two of those four weekly baths with showers saves 200 liters per week or 800 liters per month. That alone drops the bill by about $3.60/month, $43/year. Not life-changing, but it adds up.
The dishwasher, on the other hand, is a bargain. At 12.5 liters per cycle it uses less than hand-washing dishes under a running tap (40-60 liters). Running the dishwasher is not laziness - it is water efficiency.
Quick reference - water usage by household size
- Water usage for 1 person - 105 L/day, 3.15 m3/month
- Water usage for 2 people - 230 L/day, 6.9 m3/month
- Water usage for 3 people - 345 L/day, 10.35 m3/month
- Water usage for 4 people - 460 L/day, 13.8 m3/month
- Water usage for 5 people - 575 L/day, 17.25 m3/month
- Water usage for 6 people - 690 L/day, 20.7 m3/month
- Water usage for 7 people - 805 L/day, 24.15 m3/month
- Water usage for 8 people - 920 L/day, 27.6 m3/month
All values assume 1 shower/person/day, 2 baths/week, 4 laundry loads, 1 dishwasher cycle, no garden. Add garden watering for summer months.
The roof nobody thinks of as a water source
Rain falls on the roof. The roof has gutters. The gutters have downpipes. The downpipes empty into the ground. What if they emptied into a tank instead?
A 120 m2 roof at 800 mm/year rainfall collects: 120 x 0.8 x 0.8 = 76.8 m3 (76,800 liters). The 0.8 is the runoff coefficient - about 20% of rain evaporates or splashes off before reaching the gutter.
That 76,800 liters is more than enough to cover a 100 m2 garden for the entire 16-week growing season. The garden needs 2,000 L/week x 16 weeks = 32,000 L. The roof supplies 76,800 L. Coverage: 100% with surplus.
The economics: a 2,000 L cistern costs $200-600 depending on type. Annual savings at $4.50/m3: $144. Payback in 1.5-4 years. After that, garden water is free.
What changes the numbers:
| Roof area | 400 mm/year (dry) | 800 mm/year (moderate) | 1200 mm/year (wet) |
|---|---|---|---|
| 60 m2 | 19,200 L | 38,400 L | 57,600 L |
| 100 m2 | 32,000 L | 64,000 L | 96,000 L |
| 150 m2 | 48,000 L | 96,000 L | 144,000 L |
| 200 m2 | 64,000 L | 128,000 L | 192,000 L |
Even a 60 m2 roof in a dry climate generates 19,200 liters - enough for a 30 m2 garden with room to spare.
Quick reference - rainwater by roof size
- Rainwater from 60 m2 roof - 38,400 L/year at 800 mm
- Rainwater from 80 m2 roof - 51,200 L/year
- Rainwater from 100 m2 roof - 64,000 L/year
- Rainwater from 120 m2 roof - 76,800 L/year
- Rainwater from 150 m2 roof - 96,000 L/year
- Rainwater from 200 m2 roof - 128,000 L/year
- Rainwater from 250 m2 roof - 160,000 L/year
All values at 800 mm annual rainfall with 0.8 runoff coefficient. Dry climates halve these numbers, wet climates add 50%.
One thing people forget: a single moderate rain event (10 mm) on a 120 m2 roof dumps 960 liters into the gutter. A 300 L rain barrel overflows three times over. If you are collecting rain seriously, go big on the tank.
Drip vs sprinkler - the 35% gap nobody advertises
A 200 m2 mixed garden (lawn plus beds) needs 2,000 liters per week during summer. Over a 16-week season that is 32 m3 of raw plant demand. But the water that leaves your tap is not the water that reaches the roots.
Sprinklers deliver 70% of what they spray. The rest evaporates in the air, drifts onto pathways, or sits on leaf surfaces until it evaporates. That 32 m3 demand becomes 45.7 m3 at the meter.
Drip irrigation delivers 90%. Water goes through tubes directly to the base of each plant. Almost nothing evaporates. That same 32 m3 demand becomes 35.6 m3 at the meter.
The difference: 10.1 m3 per season. At $4.50/m3 that is $45.45 saved every year.
| System | Efficiency | Actual use (200 m2) | Season cost | Install cost |
|---|---|---|---|---|
| Manual (hose) | 70% | 45.7 m3 | $206 | $30 |
| Sprinklers | 70% | 45.7 m3 | $206 | $500 |
| Drip | 90% | 35.6 m3 | $160 | $700 |
| Automatic | 70% | 45.7 m3 | $206 | $1,000+ |
Drip costs $200 more to install than sprinklers. It saves $45/year in water. Payback: roughly 4 seasons. After that, it is pure savings. And the plants grow better because they get consistent, targeted moisture instead of a periodic soaking.
Automatic systems are the luxury option. Same 70% efficiency as sprinklers - you are paying for convenience (the timer waters at 6 AM while you sleep), not efficiency.
The best combo for most gardens? Drip for beds (vegetables, flowers, borders) and sprinklers for lawn (drip lines on 1000 m2 of grass are impractical). Zone each system separately.
When to water
Timing matters more than most people think. Watering at noon on a hot day loses up to 50% of water to evaporation before it hits the soil. Morning (6-8 AM) is ideal - the lawn dries before evening, reducing fungal disease risk. Evening (6-8 PM) works too, but wet grass overnight invites mildew in humid climates.
The verdict - mains water, rainwater or drip?
All three. They stack.
Mains water covers indoor needs: showers, laundry, kitchen. No alternative there. But knowing your usage (the water calculator) reveals which habits burn cash - like the 175 L bath that costs 2.3x more than a shower.
Rainwater replaces mains water for garden irrigation. A 120 m2 roof generates 76,800 L/year - enough for a 100 m2 garden with surplus. The tank pays for itself in 2-4 years.
Drip irrigation makes every liter count. Whether the water comes from a tap or a rain tank, drip delivers 90% to roots versus 70% for sprinklers. That 20-percentage-point gap translates to 10 m3 per season on a 200 m2 garden.
Stack all three for a household of four:
| Without optimization | With optimization |
|---|---|
| Indoor: 938 L/day ($126/month) | Indoor: 680 L/day ($91/month) - shorter showers, fewer baths |
| Garden: sprinklers, 45.7 m3/season ($206) | Garden: drip + rainwater tank, $0 (covered by roof) |
| Total: ~$1,777/year | Total: ~$1,092/year |
| Savings: $685/year |
That $685/year comes from three changes: shorter showers (-$180), rainwater for garden (-$206), and drip efficiency (-$45). Plus the $254 from replacing two baths per week with showers. No sacrifice in comfort - just better hardware and timing.
Tools discussed in this article
Water Usage Calculator - calculate daily, monthly and yearly household water consumption. Break down usage by source (showers, baths, laundry, dishwasher, garden) with cost estimates at your local water rate.
Rainwater Harvesting Calculator - estimate how much rainwater your roof collects per year. Get tank size recommendations, seasonal garden coverage and annual savings on your water bill.
Garden Irrigation Calculator - calculate weekly watering volume and seasonal cost. Compare hose, sprinkler, drip and automatic systems by efficiency, water use and installation cost.
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