Tank liters and usable mixed water are different
A storage heater holds heated water that can be blended with incoming cold water before use.
The resulting volume depends on the inlet, storage and delivery temperatures, so a tank label alone cannot determine shower duration.
A shower and washing-up session running together consume stored energy faster than either alone, while a gap between uses allows reheating.
Use this calculator when planning a replacement heater, comparing a household morning schedule or examining pauses between repeated small-business tasks.
It helps identify potential shortages and prepare specific questions for the manufacturer.
A sufficient modeled result should be checked against the actual model, operating mode and performance data before selecting equipment.
Included scopeThe outputs cover theoretical mixed volume, interval supply and shortages, first shortage time, full recovery after the schedule and conditional electricity consumption.
This is a global physical model using liters, Celsius, minutes and kilowatts; it applies no national installation or temperature standard.
Default temperatures and outputs are fictional calculation inputs, not recommended settings.
Find the right equipment and usage inputs
Volume and temperatures
Use the documented usable storage volume.
Inlet temperature means the incoming cold supply, storage temperature means the fully heated tank, and delivery temperature means the blended water needed for the task.
Use measured conditions or documented test conditions, and make separate seasonal scenarios where appropriate.
Thermal output and losses
Heating output is the rate of heat delivered to water in kW.
Additional loss is your constant deduction while heating, not a universal equipment loss rate.
If the documented net output already includes the same loss, enter zero to avoid subtracting it twice.
Electrical input
Electrical input and thermal output are separate quantities.
A heat pump transfers heat from its surroundings, so thermal kW cannot simply be treated as electrical kW.
Enter the confirmed electrical input for the relevant heating mode; zero means unknown and suppresses electricity results.
Do not substitute a label UEF as instantaneous heating efficiency.
Manufacturer FHR
First-hour rating describes tested hot-water delivery over one hour beginning with a fully heated tank.
Enter the published value in liters and record its source date, inlet and delivery temperatures, and operating mode.
Zero means absent; the rating is never added to or subtracted from theoretical mixed volume.
Mixed volume and heating equations
Convert stored heat into mixed liters
Vmixed = Vtank × (Tstorage − Tinlet) / (Tdelivery − Tinlet)
Qstored = Vtank × 4.184 × (Tstorage − Tinlet) / 3600
Pnet = Pheating − Ploss
Cold-start heating (minutes) = Qstored / Pnet × 60
The model approximates water density as 1 kg/L and specific heat as 4.184 kJ/(kg·K).
Heat is expressed in kWh; dividing kW by sixty gives kWh per minute.
Delivery temperature must exceed inlet temperature because their difference is the denominator.
Delivery above storage temperature is rejected because cold-water blending alone cannot achieve it.
Each mixed liter requires e = 4.184 × (Tdelivery − Tinlet) / 3600 kWh.
Dividing net heating output by e gives mixed liters per hour, then dividing by sixty gives liters per minute.
When delivery and storage temperatures match, theoretical mixed volume equals tank volume.
Build a demand schedule step by step
- Check usable tank volume and thermal output in the manufacturer data.
Leave unconfirmed electrical input and FHR at zero and describe missing information in the source note.
- Choose elapsed minute zero as the scenario start.
For each use, enter its start minute, duration and mixed-water flow; separate simultaneous fixtures into separate rows.
- Use measured fixture flow and actual running time.
Separate pauses from continuous flow instead of inferring everything from household size.
- Calculate and inspect unmet liters, the first shortage and full recovery after the final row.
Change flow or spacing and recalculate to compare practical schedules.
- Save the inputs and results as TXT for a manufacturer or installer discussion.
Record the model, data date and operating mode so different test conditions are not confused.
Worked example: a fictional 100 L tank
Assume 100 L usable volume, 15°C inlet, 60°C storage, 40°C delivery, 2 kW thermal output and zero additional loss.
These are calculation examples, not temperature settings or product recommendations.
Separately assume 2 kW electrical input and a draw of 8 L/min for thirty minutes starting at elapsed minute zero.
Mixed volume, shortages and recovery for the fictional 100 liter tank| Output | Calculated value |
|---|
| Initial theoretical mixed volume | 180 L |
| Initial stored heat | 5.23 kWh |
| Total demand / supplied / unmet | 240 / 214.42 / 25.58 L |
| First shortage / shortage duration | 26.27 / 3.73 min |
| Final recovery / cold-start heating | 156.9 / 156.9 min |
| Schedule + final recovery electricity | 6.23 kWh |
Initial mixed volume is 100 × (60 − 15) / (40 − 15) = 180 L.
Heating during the draw adds the equivalent of approximately 34.42 L, allowing supply to exceed the initial mixed volume.
The 6.23 kWh electricity result includes reheating during the schedule and the final refill, but excludes the separate 5.23 kWh used to heat the initial cold tank.
Add that separate cold-start figure only if your scenario also includes initial heating.
Overlapping draws and idle gaps
The calculation splits time at every demand start and end, then sums all active flows in each interval.
For example, 6 L/min from minutes zero to ten and 4 L/min from minutes five to fifteen create a five-minute overlap at 10 L/min.
The same daily liters can produce an earlier shortage when more of the demand occurs simultaneously.
Next stored heat = min(capacity, max(0, previous heat + net heating − demand heat))
Unmet heat = max(0, demand heat − net heating − previous heat)
Gaps allow reheating but storage stops increasing when the tank reaches full energy capacity.
A long wait therefore cannot create unlimited hot water in a small tank.
Unmet demand belongs to its original interval and is not queued into a later use.
The model does not allocate shortage among individual fixtures or predict actual outlet temperatures.
Shortage duration is not recovery time
Shortage onset and duration
First shortage is the elapsed time when stored energy plus current heating can no longer meet the entire requested flow.
Shortage duration sums all intervals with unmet demand.
It is not a guaranteed time when a shower feels cold; read it with the interval supply and shortage figures.
Full recovery after use
This is the no-draw wait after the final entered row needed to restore initial stored energy.
A partly charged tank takes less time to recover than a completely cold tank.
If another use begins during the wait, add it to the schedule and recalculate.
With zero net heating, initially stored energy remains available but consumed energy cannot be replenished.
The result shows recovery unavailable instead of infinity or a negative duration.
With no draw and an already full tank, additional recovery is zero minutes.
Three useful comparison scenarios
- Morning household demand: enter consecutive showers and overlapping washing-up.
Keep total demand unchanged while varying spacing, then check whether the resulting schedule is practical for the household.
- Seasonal inlet changes: keep the tank and heating output fixed and compare confirmed cold-water temperatures.
Colder inlet water requires more heat per mixed liter, so a summer-only estimate can hide winter shortages.
- Repeated business tasks: enter measured flow, running time and pauses between washing operations.
Prioritize peak-period records over broad daily averages and show the shortage intervals to the manufacturer.
A theoretical energy model, not a delivery test
The tank is treated as an energy store whose heat is fully available, with newly generated heat immediately usable for delivery.
Actual stratification, cold-inlet mixing, heater position, thermostat delays and water left in pipework are not reproduced.
Results are optimistic planning bounds and cannot replace a certified first-hour rating.
The loss input is only a constant deduction from output during heating.
It does not simulate passive cooling while idle or tank losses varying with room temperature.
The entire tank starts fully heated; a partly heated initial condition is not inferred.
Output and electrical input may vary with operating mode, ambient temperature, defrost and backup heaters.
Check data for the relevant conditions and do not use differences between theoretical and tested figures to diagnose faults or rank products.
Do not raise a storage setpoint based on this calculation; use manufacturer instructions and qualified advice for operation.
Frequently asked questions
Why is mixed volume larger than tank volume?
Cold water is blended with hotter stored water.
The mixing equation assumes temperature and energy balance; it does not guarantee usable delivery.
Can I enter electrical input as heating output?
Check the thermal output delivered to water first.
Heat pumps especially have different thermal and electrical inputs, which belong in separate fields.
Can temperatures be equal?
Delivery may equal storage temperature.
Delivery equal to or below inlet temperature is rejected because the mixed-volume denominator is invalid.
What happens with zero heating output?
Initially stored mixed water is still available.
If energy is consumed, recovery is marked unavailable because there is no useful reheating.
How do I enter simultaneous uses?
Add a separate row for each use.
Overlapping flow is summed, but fixture priority and individual outlet temperatures are not modeled.
Can I directly compare this result with FHR?
Storage, ongoing heating, test temperature and cutoff conditions differ.
Send the scenario and test-condition notes to the manufacturer instead of simply subtracting the values.
Does the electricity result give my bill?
It only multiplies confirmed electrical input by modeled heating time.
Standby electricity, tariffs, billing tiers and other appliances are excluded.
Are my inputs saved?
Calculation runs in the browser and the save button downloads inputs and results as a text file.
There is no automatic restoration after refreshing the page.
Sources and verification date
Physical references were checked on 2026-09-22.
They support the heat balance and distinction between storage and tested output; no statutory temperature or tank-size rule is applied.
Prices are not fetched, and manufacturer revisions and operating conditions must be checked for each use.
Take your scenario to the performance sheet
Save a scenario using the actual tank volume, seasonal inlet temperature and peak usage schedule.
Ask the manufacturer whether the stated delivery and recovery are supported at matching temperatures and operating modes.
For a separate comparison of replacement quotes and operating costs, use the related cost calculator.
Compare boiler repair and replacement costs