Methodology & sources
Every number on the site has a traceable source and an open formula. Here are all the inputs and calculations. Whatever you calculate here, you can recalculate yourself.
Three rules we stick to
- We don’t mix “per year” and “per 10 years”. The calculator and tips always show yearly values. The reference figure of CZK 46,732 is the saving over 10 years (Berger).
- We count water and energy separately. Cutting the flow doesn’t mean the same cut in heating energy — only part of the water is hot and heating has losses.
- We cap the totals. Water savings can’t exceed what a household actually spends on water (roughly CZK 17,000/year for a 4-person family).
Input values
| Quantity | Value | Source |
|---|---|---|
| Water price (water + sewage) | 133.5 CZK/m³ | Czech average 2026 (MoA/CZSO) |
| Energy price for heating hot water | 3.06 CZK/kWh | Weighted heating mix (ENERGO 2021) |
| Water consumption per person | 88.3 l/person/day | MoA/CZSO 2024 |
| Hot-water heating share of household energy | 16 % | dena 2024 |
| Specific energy to heat shower water | 29.1 kWh/m³ | Berger et al. 2017 |
| Heating emission factor | 0.254 kg CO₂/kWh | Berger et al. 2017 |
| Shower flow, ordinary → low-flow | 15 → 8 l/min | Berger et al. 2017 |
| Hot-water share in the shower | 65 % | Estimate (rest is cold mix) |
| Correction “water ≠ energy 1:1” | × 0.6 | Hadengue 2023 |
| Aerator — real water saving | 15 l/person/day | Corrected (not vendor figures) |
| Leaking toilet — typical yearly loss | 220 m³/yr | Upper estimate (not vendor figures) |
Formulas
Shower saver (regulator 15 → 8 l/min)
Based on the reference study Berger 2017 (2-person household, 10-year horizon). Converted to per person and year:
water = 10,500 l/person/yr × people
(= 210 m³ over 10 years ÷ 10 ÷ 2 people)
energy = (water in m³) × 29.1 kWh/m³
CZK = water × 133.5 + energy × 3.06
CO₂ = energy × 0.254 kg/kWhFor a 2-person household this gives 210 m³ of water, 6,106 kWh and 1,550 kg CO₂ over 10 years — i.e. a locked-in 21,000 l of water/year and ≈ CZK 46,732 over 10 years (water + heating).
Shorter shower and the calculator
The calculator computes the saving from swapping the showerhead, scaled by your usage. Water = saved flow × duration × number of showers × people. We take energy only from the hot part of the water (65 %) and apply a correction of × 0.6, because cutting the flow doesn’t save energy 1:1.
Sources
- Berger, M. et al. (2017) — LCA of water and energy savings from shower regulators.
- dena (2024) — Deutsche Energie-Agentur: structure of household energy consumption.
- Hadengue (2023) — correction of the link between water savings and heating energy.
- ENERGO 2021 — energy mix for heating hot water in the Czech Republic (CZSO).
- MoA / CZSO 2024 — water tariffs and per-person consumption in the Czech Republic.
The constants live in a single shared module (savings.ts), used by both the website and the quiz. When a source changes, the number changes in one place.