Every sizing calculation in this book starts from the same place: how much water does the household actually use, for what purposes, and when? This chapter provides a structured method for quantifying residential water demand at the fixture level — the only reliable foundation for system design.
Rule-of-thumb estimates (e.g., “150 L/person/day”) are useful for early-stage planning but inadequate for system design. Two households with identical occupant counts can have radically different demand profiles depending on appliance age, behavior, and outdoor use. More importantly, the split between potable-required and non-potable-acceptable uses determines which systems are worth building.
A fixture-level analysis gives you:
Toilets are typically the single largest non-potable consumer in a home, accounting for 25–30% of total indoor use.
| Toilet type | Volume per flush (L) | Typical flushes per person per day |
|---|---|---|
| Old (pre-1995) | 9–13 | 5 |
| Standard (1995–2010) | 6–9 | 5 |
| Dual-flush (full) | 6 | 2 |
| Dual-flush (half) | 3 | 3 |
| Low-flow (modern) | 3–4.5 | 5 |
Daily toilet demand per person (dual-flush, modern):
(2 × 6) + (3 × 3) = 21 L/person/day
All toilet flushing is a Tier 3 (non-potable) use.
| Shower type | Flow rate (L/min) | Typical duration (min) |
|---|---|---|
| Power shower | 12–20 | 8 |
| Standard mixer | 8–12 | 8 |
| Low-flow head | 6–8 | 8 |
| Aerated (eco) | 4–6 | 8 |
Daily shower demand per person: 8 min × 8 L/min = 64 L/person/day (standard)
Showering requires Tier 2 (hygienic non-potable) or Tier 1. Untreated rainwater with basic filtration can qualify for Tier 2 in many jurisdictions.
A full bath uses 150–200 L per event. With a frequency of roughly 1–2 baths per person per week:
175 L × 1.5/7 = 37.5 L/person/day
Bathing requires Tier 2.
| Machine type | Water per cycle (L) | Cycles per household per week |
|---|---|---|
| Top-load (old) | 100–150 | 5 |
| Front-load (modern) | 40–60 | 5 |
| Front-load (eco cycle) | 30–45 | 5 |
Daily washing machine demand: 50 L × 5/7 = 35.7 L/day for a household (not per person)
Washing machines can operate on Tier 3 water. Using non-potable water for laundry is permitted in some jurisdictions (check local regulations on fabric contact standards).
| Type | Water per cycle (L) | Cycles per day |
|---|---|---|
| Old model | 15–20 | 1 |
| Modern (A-rated) | 9–12 | 1 |
| Eco mode | 7–10 | 1 |
Daily dishwasher demand: ~10 L/day for a household
Dishwashers require Tier 1 (potable) because water contacts food and utensils.
Kitchen sink use includes food preparation, washing, and hand-washing. Average flow rate from a standard tap: 6–12 L/min.
| Activity | Volume per event (L) | Events per day (household) |
|---|---|---|
| Hand-washing | 1–2 | 10 |
| Food prep/rinsing | 5–10 | 3 |
| Pot washing | 5–15 | 2 |
Daily kitchen sink demand: approximately 30–50 L/day (household)
Kitchen sink use requires Tier 1 (potable).
Bathroom hand-washing and tooth-brushing:
| Activity | Volume (L/event) | Events/person/day |
|---|---|---|
| Hand-washing | 1–2 | 8 |
| Tooth-brushing (tap off) | 0.3 | 2 |
| Tooth-brushing (tap running) | 5 | 2 |
| Shaving | 3–5 | 1 |
Daily bathroom sink demand: ~15–20 L/person/day
Tooth-brushing and face-washing require Tier 1 for direct ingestion risk. Hand-washing can use Tier 2.
Garden demand is highly variable and dominates seasonal peaks. It is also the most flexible — irrigation can be interrupted without household impact.
| System type | Application rate | Area (m²) | Events/week |
|---|---|---|---|
| Sprinkler | 5–10 mm/event | variable | 2–3 |
| Drip irrigation | 2–4 mm/event | variable | 3–4 |
| Hose (manual) | 10–20 L/min | — | variable |
Irrigation demand rule of thumb: 3–5 L/m²/week during the growing season.
For a 50 m² garden: 50 × 4 = 200 L/week = 28.5 L/day (seasonal average)
Irrigation uses Tier 4. Greywater and lightly treated rainwater are both suitable.
| Use | Typical volume | Frequency | Notes |
|---|---|---|---|
| Car washing | 150–400 L | Monthly | Can use Tier 3 |
| Swimming pool (top-up) | 500–2,000 L | Monthly in season | Tier 3 acceptable |
| Outdoor tap (general) | Variable | Variable | — |
Example household: 4 persons, modern appliances, moderate outdoor use.
| Fixture / Use | Volume (L/day) | Quality Tier | Potable? |
|---|---|---|---|
| Toilets (dual-flush) | 4 × 21 = 84 | 3 | No |
| Showers | 4 × 64 = 256 | 2 | Partial |
| Baths | 4 × 37.5/7 × 2 | 2 | No |
| Washing machine | 35 | 3 | No |
| Dishwasher | 10 | 1 | Yes |
| Kitchen sink | 40 | 1 | Yes |
| Bathroom sinks | 4 × 18 = 72 | 1/2 | Partial |
| Garden (summer avg) | 30 | 4 | No |
| Misc. | 15 | — | Partial |
| Total | ~582 L/day |
Potable-required uses (dishwasher, kitchen, tooth-brushing fraction): ~120 L/day = ~21% of total
Non-potable-acceptable uses: ~460 L/day = ~79% of total
In practice, the non-potable fraction that is practically offsettable (toilets + laundry + irrigation) is roughly 35–45% of total, depending on whether shower supply is switched to treated rainwater.
# Calculate potable/non-potable split
fixtures = {
'Toilet flushing': {'daily_L': 84, 'potable': False},
'Showers': {'daily_L': 256, 'potable': False}, # Tier 2 acceptable
'Washing machine': {'daily_L': 35, 'potable': False},
'Dishwasher': {'daily_L': 10, 'potable': True},
'Kitchen sink': {'daily_L': 40, 'potable': True},
'Bathroom sinks': {'daily_L': 72, 'potable': True},
'Garden irrigation': {'daily_L': 30, 'potable': False},
'Misc': {'daily_L': 15, 'potable': True},
}
total = sum(f['daily_L'] for f in fixtures.values())
non_potable = sum(f['daily_L'] for f in fixtures.values() if not f['potable'])
print(f"Total: {total} L/day")
print(f"Non-potable: {non_potable} L/day ({100*non_potable/total:.0f}%)")
Pipe and pump sizing requires peak flow rates, not daily averages. Peak demand occurs in the morning (7–9 AM) when showers, dishwashers, and toilets all run simultaneously.
Peak coincidence factor: 0.3–0.5 (fraction of fixtures simultaneously active)
For our 4-person household, if peak hour demand is 30% of daily demand:
Peak hour flow = 0.30 × 582 L / 1 hour = 175 L/hour ≈ 2.9 L/min
For pump sizing, use a design flow rate of 1.5–2× the calculated peak as a margin.
Seasonal variation is dominated by outdoor use (irrigation, car washing, pool top-up). Indoor use is relatively constant year-round, varying ±10–15%.
| Season | Multiplier on baseline daily demand |
|---|---|
| Winter | 0.85–0.95 |
| Spring | 1.0–1.2 |
| Summer | 1.2–1.6 (irrigation dominant) |
| Autumn | 0.9–1.1 |
For storage sizing, use monthly demand figures reflecting this variation. Chapter 4 shows how to model this against monthly supply variability.
Before sizing supply-side systems, consider demand reduction. Lower demand reduces tank size, pump size, and treatment system cost.
| Measure | Typical saving | Notes |
|---|---|---|
| Low-flow showerhead (6 L/min) | 25–35% of shower use | Lowest cost intervention |
| Dual-flush toilet | 25–35% of toilet use | Significant if replacing old fittings |
| Front-load washing machine | 50–60% vs. old top-load | Also saves energy |
| Drip irrigation vs. sprinkler | 30–50% of garden use | Also reduces evaporation losses |
| Rainwater timer for garden | 20–30% of garden use | Prevents overwatering |
In general, every liter of demand reduction saves more cost than a liter of harvesting capacity.
Previous: Chapter 1 — Water in the Home
Next: Chapter 3 — Catching the Sky: Rainwater Collection Potential