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    COAX consumption calculator: how it works

    Assumptions, formulas and limitations in the calculation

    Overview

    The COAX consumption calculator estimates energy use, water use and running costs for a flow-through heater and a conventional tank. The result depends on the values you enter. This guide explains the formulas, default values and limitations.

    Basic calculation formula

    The energy needed to heat water is calculated as follows:

    Required energy = (Volume × Specific heat × Temperature rise) ÷ Efficiency

    Where:

    • Volume: Total number of litres of hot water used per day
    • Specific heat: 1,162 Wh/L/°C (energy needed to heat 1 litre of water by 1 °C)
    • Temperature rise (ΔT):
      • COAX: Target temperature - Inlet temperature (40 °C - 10 °C = 30 °C)
        • Heats water directly to the temperature needed at the tap
      • Tank: Storage temperature - Inlet temperature (65 °C - 10 °C = 55 °C)
        • Water is stored at a higher temperature (60-70 °C), then mixed with cold water to reach the temperature needed at the tap
    • Efficiency: How much of the electricity supplied is converted into heat

    The calculation step by step

    1. Daily water volume

    For each system, the calculator adds up water used for showers, kitchen taps, washbasins and pipe flushing:

    Shower volume = Number of showers × minutes per shower × flow rate (L/min)

    Kitchen volume = Kitchen uses × (Duration in seconds ÷ 60) × flow rate

    Bathroom basin volume = basin uses × (Duration in seconds ÷ 60) × flow rate

    Pipe flushing = Total uses per day × Pipe length (m) × 0,15 L/m × Insulation factor

    Total daily volume = The sum of all the volumes above

    Pipe flushing is the cold water that runs out of the pipes before hot water reaches the tap.

    2. Waiting time and wasted water

    Water that goes to waste while you wait for hot water:

    Water wasted while waiting = Total uses per day × (Waiting time in seconds ÷ 60) × flow rate (L/min)

    This water goes down the drain while you wait. Adjust the waiting time to match what you measure at home.

    3. Daily energy consumption

    COAX (flow-through heater):

    Daily energy (kWh) = (Daily volume × 1,162 × ΔT_use) ÷ 0,98 ÷ 1000

    Where:

    • ΔT_use = Target temperature - Inlet temperature (40 °C - 10 °C = 30 °C)
    • COAX heats water directly to the temperature needed at the tap (40 °C)
    • Heats water only when it is used
    • No standby loss

    Tank system:

    Mixing ratio = (Target temp - Inlet temp) ÷ (Storage temp - Inlet temp)
Required hot water volume = Daily volume × Mixing ratio
Daily energy (kWh) = (Required hot water volume × 1,162 × ΔT_storage) ÷ 0,93 ÷ 1000

    Where:

    • ΔT_storage = Storage temperature - Inlet temperature (65 °C - 10 °C = 55 °C)
    • Mixing ratio = (40 °C - 10 °C) ÷ (65 °C - 10 °C) = 30 ÷ 55 ≈ 54,5 %
    • Tanks store water at 60-70 °C (default 65 °C) to prevent bacterial growth
    • Hot water is mixed with cold water to reach the required tap temperature (40 °C)
    • Only the required volume of hot water has to be heated to storage temperature
    • Plus standby loss (water is kept hot 24/7)

    4. Adjustment for standby loss

    Tank systems lose heat continuously. The calculator adjusts for room temperature:

    Adjustment factor = 1 + (20°C - Room temperature) ÷ 10
Adjusted standby = Base standby × Adjustment factor

    Examples:

    • 15 °C room (cellar): 1,5× base standby (more heat loss)
    • 20 °C room (typical): 1,0× base standby
    • 21 °C room (kitchen/bathroom): 0,9× base standby (less heat loss)

    5. Annual calculations

    Annual energy (kWh) = Daily energy × 365 + Annual standby loss
Annual cost (NOK) = Annual energy × Electricity price
Saving = Annual cost of the tank - Annual cost of COAX

    Default assumptions

    Household use in the default example

    • 3 people in the household
    • 1 shower per person per day (6 minutes each)
    • 4 kitchen tap uses per person per day (37 seconds each)
    • 6 handwashing sessions per person per day (20 seconds each)

    Water temperatures

    • Inlet temperature: 10 °C (typical Norwegian cold water) (8 °C on winter days, 12 °C on summer days)
    • Target temperature: 40 °C (comfortably hot water at the tap)
    • COAX heating temperature: 40 °C (heats directly to the required tap temperature)
    • Tank storage temperature: 65 °C (stored at a higher temperature, then mixed with cold water)
      • A high storage temperature (60-70 °C) prevents bacterial growth (Legionella)
      • Mixed with cold water at the tap to reach 40 °C
      • Mixing ratio: ~54,5 % hot + 45,5 % cold = 40 °C

    Physical constants

    • Pipe capacity: 0,15 L/m (typical 15 mm pipes)
    • Pipe insulation: Optional (reduces heat loss by 50 % if enabled)

    Economic assumptions

    • Electricity price: 1,5 NOK/kWh (change this default to match the price you pay)

    Default values for each system and why they are used

    Default values for COAX

    Flow rate: 6 L/min

    The example assumes the heater is installed near the outlet and used with water-saving taps. Waiting time is set to 3 seconds, compared with 10+ seconds for tanks. A lower flow rate reduces calculated consumption, but it must suit your shower head and how you use water.

    Higher flow rates (9-12 L/min) are also possible. Measure your flow rate and enter it before comparing the systems.

    Pipe length: 2 metres

    The example places the heater near the outlet, for instance under the kitchen sink or in a bathroom cabinet. Short pipes reduce heat loss and waiting time. Separate heaters can serve different parts of the building.

    Room temperature: 21 °C (implicit in the comparison)

    Examples of rooms where the heater can be placed:

    • Kitchen: Room temperature or warmer (20-22 °C)
    • Bathroom: Heated rooms (20-24 °C)
    • Living areas: Heated rooms

    COAX loses little heat from the unit itself because it only heats water while you use it. Room temperature therefore has less effect than it does on a tank that stays hot.

    Waiting time: 3 seconds

    This assumes the heater is close to the tap. Cold water in the pipe must still run out before hot water arrives. The default reflects the installation in this example.

    Efficiency: 98 %

    This is the heating efficiency used in the calculation. The calculator adds no standby loss for COAX because it does not store water in a hot tank.

    Default values for the conventional tank

    Flow rate: 9 L/min

    The tank example uses a higher flow rate than the COAX example. That affects both the cost of water and the energy needed for heating. If you want to compare using the same shower head and usage pattern, set the same flow rate for both.

    Pipe length: 10 metres

    This assumes a tank in a utility room, cellar or plant room supplies the taps in the house. Pipe length is the distance from the tank to the outlet. Yours may be shorter or longer.

    Room temperature: 15 °C

    Typical locations:

    • Cellar/utility room: Typically 10-15 °C
    • Plant rooms: Often unheated or barely heated
    • Garage/outbuilding: Exposed to the outdoor temperature

    The colder the room, the greater the temperature difference between the tank and its surroundings. More heat is lost, so the tank uses more energy to keep the water hot.

    Waiting time: 10 seconds (default, adjustable 5-45s)

    Waiting time depends on the distance to the tap and how much cold water is in the pipe. Measure how long the water takes to run hot and enter that time in the calculator.

    Efficiency: 93 %

    This value describes heating efficiency. Heat lost from stored water is calculated separately as standby loss. Keep the two figures separate when interpreting the result.

    Storage temperature: 65 °C (default, adjustable 55-75 °C)

    Rationale:

    • Health requirements: Water must be stored at 60-70 °C to prevent bacterial growth
    • Mixing at the tap: Stored water (65 °C) is mixed with cold water (10 °C) to reach 40 °C
    • Mixing ratio: Approx. 54,5 % hot + 45,5 % cold = 40 °C
    • Energy: The tank has to heat water to 65 °C, not just 40 °C
    • Calculation: Based on the storage temperature (65 °C - 10 °C = 55 °C rise)

    Standby loss: 1000 kWh/year (base, adjusted for room temp)

    Rationale:

    • 24/7 operation: The tank keeps water hot continuously
    • Heat loss: The water cools down and has to be reheated
    • Temperature-dependent: A lower room temp increases the loss

    How the assumptions affect the result

    1. No standby loss for COAX

    Assumption: COAX has 0 kWh/year compared with the tank's 1000+ kWh/year

    COAX heats water when the tap is on. The tank example keeps water hot 24/7, so the calculation adds the energy needed to replace heat loss. The tank's insulation and location affect how much extra energy is needed.

    2. Waiting time

    Assumption: COAX 3 sec compared with tank 10 sec

    Rationale:

    • COAX: Close to the outlet, short pipes = minimal waiting time
    • Tank: Centralised, long pipes = considerable waiting time
    • Wasted water: Longer waiting time = more cold water down the drain
    • Adjustment: The tank's waiting time can be adjusted (5-45s)

    3. Efficiency

    Assumption: COAX 98 % compared with tank 93 %

    These values are fixed in the calculator. The difference gives COAX lower calculated electricity use for the same heating demand. They are model assumptions, not measurements of your installation.

    4. Storage temperature

    Assumption: COAX 40 °C compared with tank 65 °C

    Rationale:

    • COAX: Heats directly to the required tap temperature (40 °C)

      • No need for a high storage temperature
      • Energy: (Volume × 1,162 × 30 °C) ÷ efficiency

    • Tank: Stores at 60-70 °C (65 °C) for health reasons

      • Prevents bacterial growth
      • Mixed with cold water down to 40 °C
      • Energy: (Required hot water × 1,162 × 55 °C) ÷ efficiency
      • Impact: The tank's temperature rise is 83 % greater (55 °C vs 30 °C), but it heats only ~54,5 % of the volume

    The tank heats a smaller volume to a higher temperature, then mixes it with cold water. COAX heats all the water you use directly. The calculation must account for both the temperature rise and the mixing ratio.

    5. Pipe length

    Assumption: COAX 2 m compared with tank 10 m

    Rationale:

    • COAX: Decentralised = several units with short pipes
    • Tank: Centralised = one unit with long distribution pipes
    • These pipe lengths reflect the installation in the example.
    • Adjust them to the distance between the heater and the taps in your home.

    6. Flow rate

    Assumption: COAX 6 L/min compared with tank 9 L/min

    Different flow rates explain part of the difference in calculated consumption. The flow rate depends on the taps, shower head and usage pattern. Adjust both values if you want to compare the heaters with the same water consumption.

    Calculation limitations and simplifications

    Included:

    • Running costs (heating)
    • Standby loss for the tank
    • Pipe heat loss
    • Water wasted while waiting
    • The effect of room temperature on standby loss
    • Several types of use (shower, kitchen, basin)
    • Annual estimates (365 days)

    Not included:

    • Installation costs
    • Maintenance costs
    • Differences in service life
    • Seasonal variations in use
    • Peak demand scenarios
    • Water heating using energy sources other than electricity
    • Environmental impact beyond energy and water

    Simplifications:

    • Average daily use: A constant pattern, without seasonal variations
    • Constant inlet temperature: Assumes 10 °C throughout the year
    • Fixed efficiency: Assumes no decline over time. Conventional tanks can lose efficiency as they age, partly due to deposits and sludge building up in the tank.
    • Linear standby adjustment: A simplified temperature formula

    How the default values affect the result

    These assumptions determine the result when you use the default settings:

    1. Standby loss

      • Tank: 1000+ kWh/year
      • COAX: 0 kWh/year

    2. Heating temperature and mixing

      • COAX: 40 °C (30 °C rise)
      • Tank: 65 °C (55 °C rise), but only ~54,5 % of the volume is heated

    3. Efficiency

      • COAX: 98 %
      • Tank: 93 %
      • 6 % difference

    4. Waiting time

      • COAX: 3 sec = less wasted water
      • Tank: 10+ sec = more wasted water

    5. Flow rate

      • COAX: 6 L/min
      • Tank: 9 L/min
      • Reduces water consumption

    6. Pipe length

      • COAX: 2 m
      • Tank: 10 m
      • Less heat loss and flushing

    7. Room temperature around the heater

      • COAX: Kitchen/bathroom (21 °C)
      • Tank: Cellar/utility room (15 °C)
      • Lower ambient temperature increases standby loss

    The estimated 30-50 % energy saving depends on how you use hot water and how the heater is installed. Enter your own settings before deciding whether to switch.

    How to use the calculator

    1. Check which default values the calculation uses.
    2. Change the number of people, shower frequency and electricity price to match your situation.
    3. Open "Advanced settings" to adjust pipe length, water consumption and other values.
    4. Compare annual energy use, cost and water consumption.

    Interpreting the result

    The default settings assume different installations and water use, which affects the comparison:

    • COAX is close to the tap and is used with a lower flow rate.
    • The tank supplies several taps through longer pipes.

    The result is an estimate based on your chosen settings. Use measurements from your installation where possible, and try a range of values where you are unsure. Include installation costs when deciding whether a replacement will pay off.

    Other considerations when choosing a heater

    COAX benefits (not calculated):

    • Space saving
    • Hot water without waiting for a tank to reheat
    • Water is not stored in a hot tank
    • Several units can be installed so one remains available if another is out of service
    • Potentially lower installation costs in new buildings

    Tank benefits (not calculated):

    • Several taps with a high flow rate at the same time
    • Straightforward replacement where a tank is already installed
    • Lower load on the electricity grid
    • Can be used on electrical circuits with lower capacity
    • Established technology

    Efficiency: 98 % (COAX) vs 93 % (Tank)

    Rationale:

    • COAX 98 %: Modern direct heating, minimal heat loss
    • Tank 93 %: Includes heating loss (not standby loss, which is calculated separately)

    How the values are used:

    • Heating efficiency only
    • Tank standby loss is calculated separately (1000 kWh/year)
    • Total efficiency loss for the tank = lower heating efficiency + standby loss
    • COAX efficiency reflects the conversion of electricity to heat in the element

    Stated comparison figures:

    • Modern flow-through heaters: 98-99 %
    • Modern tanks: 90-95 % (heating only)
    • Total tank efficiency (including standby): often 60-75 %