Instant Water Heater and Electric Shower Calculator

The thermal equations are derived here from steady-flow conservation of energy:

Use explicit conversions from L/min to m³/s and W to kW. Water is approximated by 1000 kg/m³ and 4180 J/(kg·K). The heat capacity is consistent with NIST-JANAF liquid-water data near room temperature; density is a nominal engineering approximation.

Maximum thermal flow at the target is $Q_{max}=P_{available}/(\rho c_p\Delta T_{need})$. Simultaneous actual heater-fed outlet flows are added; downstream cold bypass flow is excluded. Rinnai defines one Japanese go as 1 L/min heated by 25 K; the corresponding useful heat is about 1.742 kW. Required capacity is $N=Q\Delta T/25$ when Q is in L/min.

For the full selected electric setting, derive $R_h=V_n^2/P_n$ and $R_c=2\rho_c L/A$. Then $I=V_s/(R_h+R_c)$, $V_h=V_s-I R_c$, $P_h=I^2R_h$ and $P_s=V_s I$. Useful heat is $\eta P_h$. Copper resistivity 0.0225 Ω·mm²/m is the normal-service approximation in Prysmian's 2024 guidance, printed page 34. Both loaded conductors have equal area and length.

Screen with $I_b=\max(P_n/V_n,V_s/R_h)$, deliberately giving no credit for cable voltage drop reducing current. Compare $I_b\le I_n\le I_z$, using the installer's corrected cable ampacity. The continuous drop-only area is $A_{drop}=2\rho_c L I_b/\Delta V_{allow}$. It does not size a protective conductor or establish ampacity. Energy at circuit origin is $P_s t$ and cost is energy times the entered tariff.

Electric outlet temperature is unregulated at the selected power. Useful-heat and go modes assume modulation to $\min(P_{available},P_{need})$. The animation changes flow through steady operating points and never changes the sizing cards. Flow curves stay below 90°C and span the target-flow crossing. Their lower bound exceeds the full-power flow at 90°C by 1%; their upper bound is 1.5–3 times the target capacity, adjusted for entered demand. This keeps the target crossing readable even when entered demand is far from capacity. A demand outside that plot span remains included in the sizing results and winter curve. Above, near and below target are distinguished with a 1 K comparison band, not a safety threshold. Stiebel Eltron's attached page 1 supplies independent thermal and current reference values; Triton's attached page 9 supplies a separate voltage/power comparison.

• Clean liquid water, constant properties, negligible pipe heat loss, storage, kinetic energy and elevation effects. The 1000 kg/m³ density approximation introduces a few percent error toward the hot end of the range.
• Every flow input is actual water passing through the heater, before downstream cold mixing. All heater-fed streams share one heater outlet target. Final mixed flow must not be entered here. A mixed-delivery energy balance may be used separately for demand sizing, but it cannot predict the heater outlet temperature or validate the 90°C model limit. This calculator does not model mixing streams.
• Electric mode is one two-wire, single-phase resistance load on a full selected heating setting: no pump, phase-angle control, electronic constant-power regulation or thermostat cycling. Multi-feed heaters require a separate electrical design for each feed.
• Non-electric inputs are useful heat at the water, or Japanese go capacity. Ideal target modulation is assumed; minimum modulation, activation flow, pressure, temperature limits and appliance controls are omitted. No gas consumption or gas installation sizing is calculated.
• Cable resistance is a bulk normal-service approximation. Contacts, source impedance, upstream drop, reactance and unequal conductors are excluded. Use manufacturer cable resistance when a detailed design is required.
• The 3% branch-drop allowance and 0.25 currency/kWh tariff are editable illustrations, reviewed 27 October 2026. No regional electricity price is implied. The monthly estimate uses 30 days.
• The electrical screen is incomplete until corrected ampacity is supplied. Standards ampacity tables are not reproduced. No wire or breaker is automatically approved.
• The viewer converts temperature readings with their offset when switching units. Flow capacity is shown in both L/min and US gal/min because the viewer does not automatically switch L/min; type an explicit gal/min value in a flow field. Diagram and electrical summary labels retain the SI units written beside their numbers.

Electric shower installation belongs to a qualified electrician. Mains electricity and water can cause fatal shock. Use a dedicated circuit, protective earthing, appropriate RCD/DR protection, safe isolation, suitable terminals and all required bathroom-zone protections. The cited Triton manual requires 30 mA RCD protection for its UK installation. RCD/DR protection does not replace earthing. Do not bypass a safety cut-out or restrict a vented shower outlet.

Regulatory context reviewed 27 October 2026: Brazil, ABNT NBR 5410:2004 (corrected 2008); UK, BS 7671:2018+A4:2026 as identified by the IET. These are installation references, not a compliance calculation. Check against the standard in force, current local rules, supply conditions and the exact appliance manual. Have the installer verify cable installation method, grouping, ambient temperature, insulation, protective-device characteristic and breaking capacity, fault/disconnection protection, service capacity and manufacturer minimum conductor requirements.

A generic breaker guide can disagree with a model-specific manual: Lorenzetti's general FAQ and Acqua Century manual illustrate this. The exact model manual and qualified local circuit design govern; a numerical $I_b\le I_n\le I_z$ result is only one necessary screen.

Thermal sizing is intended for inlet water 1–40°C and target water 5–65°C with positive total flow and positive rise. An electric heater-outlet result above 90°C is held back because boiling and cut-out behavior are outside the model. Use actual heater flow for this check: downstream cold mixing does not reduce the temperature inside the heater and cannot validate an out-of-range heater state. Non-electric output assumes ideal modulation; a large full-power envelope is not an operating recommendation. Hot water can scald: a target value or a “near target” status within 1 K is not a safe-temperature certification. Verify delivered temperature and product-specific minimum flow, pressure and controls. Gas installation and flue work also require appropriately qualified professionals.

This tool does not authorize appliance voltage mismatch, circuit modifications, adding outlets to a single-outlet shower, or selecting a breaker merely by rounding current up.