Combine hot-water demand from separate fixtures
A four-person home estimates 80 L of heated water for showers and 20 L from the hot supply for washing up. It wants to compare a typical day with one using only 10 L for washing up.
Open CalculatorInputs and calculation
| Tool input | Value |
|---|---|
| Currency | USD |
| Volume or billing unit | liters |
| Volume (L) | 100 |
| Temperature rise (°C) | 30 |
| Heating method | Electric resistance |
| Efficiency (%) | 100 |
| Energy price (USD per input kWh) | 0.3 |
| Water price (USD/L) | 0 |
Heat delivered = 100 × 30 × 0.001163 kWh. Divide by 1 to get about 3.489 input kWh. Multiply input energy by 0.3, add 100 × 0 for water, then round to USD 1.05.
Tool result · USD 1.05
| Tool input | Value |
|---|---|
| Currency | USD |
| Volume or billing unit | liters |
| Volume (L) | 90 |
| Temperature rise (°C) | 30 |
| Heating method | Electric resistance |
| Efficiency (%) | 100 |
| Energy price (USD per input kWh) | 0.3 |
| Water price (USD/L) | 0 |
Heat delivered = 90 × 30 × 0.001163 kWh. Divide by 1 to get about 3.1401 input kWh. Multiply input energy by 0.3, add 90 × 0 for water, then round to USD 0.94.
Tool result · USD 0.94
What the comparison shows
The shower contribution stays fixed. The 10 L reduction saves 0.3489 kWh, about 0.10 before comparing separately rounded display values.
Assumptions and limits
Add volumes only when the same temperature rise and efficiency apply. The sample covers these two activities and no other household draws.
Related guides and examples
- What it costs to heat water
- Hot water from the tank is not the same as mixed shower volume
- Why a water heater uses energy between hot-water draws
- Build a four-person household water estimate
- Estimate incremental shower water for visitors
- Heating 100 liters for a family
- Count full and reduced toilet flushes