3.7 kW, 5.5 kW or 7.5 kW? Choosing the right DAFI power for your sink, shower—and zero guesswork  - A practical guide: how many kW you really need for a basin, kitchen sink, and shower so the water stays warm even in winter.

3.7 kW, 5.5 kW or 7.5 kW? A no-guesswork guide to DAFI power sizes

What “kW” really means—and why the water sometimes feels only lukewarm

Power in kW is the speed of heating. The higher the power, the more litres per minute you can bring up to a comfy temperature. Real-life rule of thumb: the bigger the outlet (shower > sink > basin), the colder the inlet (hello winter), and the longer/cooler the pipe run—the more it pays to choose a higher power. Typical performance under common conditions (inlet ~15°C, ~200 kPa):
– 3.7 kW ≈ 1.9 l/min to ~40°C;
– 4.5 kW ≈ 2.4 l/min to ~40°C;
– 5.5 kW ≈ 2.9 l/min to ~40°C;
– 7.3–7.5 kW ≈ ~4.0 l/min to ~40°C;
– 9 kW ≈ 4.7 l/min, 11 kW ≈ 5.8 l/min to ~40°C.
That’s the hard data, but life adds a twist: even a 1% voltage drop at the outlet can shave roughly 2% off heating effectiveness—winter can be the difference between “great” and “meh.” So the electrical installation and cable run matter more than most people think.

A down-to-earth mini-calc

If you like quick maths: P [kW] ≈ 0.07 × Q [l/min] × ΔT [°C]. Want to go from 10°C to 40°C (ΔT=30°C) and dream of 5 l/min? 0.07×5×30 ≈ 10.5 kW. That’s what you truly need for a “mixer-like” shower without compromises. With 7.5 kW you lower the flow (e.g., ECO showerhead at ~4 l/min) and suddenly—boom—it works. In summer, when inlet water is ~15–18°C, the very same power feels more generous than in winter.

Kitchen sink—comfort at low flow + the aerator trick

What actually works for dishwashing—and why small kW wins here

At the sink, temperature and “spray quality” matter more than sheer flow. For washing dishes and hands you usually need 2–3 l/min. Here 3.7–4.5 kW performs brilliantly—if you add a spray/aerator. Manufacturers explicitly recommend spray nozzles for 3.7 and 4.5 kW models because they “widen” the stream and boost perceived warmth; you genuinely feel more heat at lower water use.
If you often rinse large trays or want a brisker stream, 5.5 kW gives around 2.9 l/min to ~40°C, which is that sweet spot—slightly higher power draw, noticeably snappier comfort.

Kitchen realities: distance and… limescale

The closer the heater is to the tap, the less heat you lose in the pipes. Mount it as close as possible to the point of use—free comfort points. And clean the aerator regularly (yep, those rubber nubs are for “pushing out” limescale). A clogged aerator means lower flow, wobblier temperature, and the feeling the heater is “weak” when it’s perfectly fine.

Bathroom basin—3.7 kW vs 5.5 kW and when it’s truly worth paying extra

Shaving, makeup, toothbrushing—instant comfort

For a basin, comfort already starts at 3.7 kW. With a short run and an aerator you get warm water fast, no waiting. If winters hit hard and you like your stream properly warm, 5.5 kW gives extra headroom—less fiddling with the tap and lower risk of that “lukewarm surprise” after a minute. On paper the jump is clear: 1.9 vs 2.9 l/min to ~40°C—practically it’s a step from “fine” to “ahh, nice.”

Two outlets? Not ideal on the lowest power

In theory one unit can serve two outlets (say, basin + sink). In practice, low powers aren’t built for that. Every extra tee and metre of pipe eats temperature. If your plan is “one DAFI—two basins,” sense starts at 5.5 kW, preferably 7.3–7.5 kW—and still only with short runs. Honestly, “one heater per point” is the most frustration-free path.

Shower—when 7.3–7.5 kW is enough, and when to reach for 9–11 kW

The reality under a rain head

A classic, full shower likes 6–8 l/min of warm water. That typically means 9–11 kW, especially in winter with a bigger ΔT. 7.3–7.5 kW works if you meet three conditions: ECO showerhead at ~3.5–4.0 l/min, short pipe run, and solid electrics. Numbers don’t lie: around 7.3–7.5 kW you get ~4 l/min up to ~40°C (inlet 15°C)—that’s a “minimalist comfort” shower. Want a rain head? Go higher: 9 kW ≈ 4.7 l/min, 11 kW ≈ 5.8 l/min to ~40°C, which lets you use larger heads without “cold dips.”

Heads-up about 400 V supply

7.5 / 9 / 11 kW models require two-phase 400 V (2/PE, no neutral) and hard-wired connection with the right circuit protection. Important: if your home only has single-phase, the realistic ceiling “without refit” is usually 7.3 kW (230 V). Check your breaker panel and cable gauges before you buy the “dream power”—your installation has to carry it, safely.

Bathtub? Better not—numbers don’t lie

Why an instantaneous heater and tub filling don’t mix well

A 120–180 L tub needs high flow at high temperature for a long stretch. Even 11 kW at ~5–6 l/min to ~40°C will fill painfully slowly, and you’ll feel temperature sag when house pressure plays games. Realistically—tubs love storage tanks or hybrid setups. If you must go tankless, accept compromises: very low flow (patience!) or summer-only optimism. These power classes are built for basins, sinks, and showers—not for dumping 140 litres in one go.

Pre-purchase checklist—pressure, electrics, distance, install and quick hacks

1) Water pressure

Most compact units play nicely around 1–6 bar. If you have spikes/high pressure, add a pressure reducer. Too low? You’ll get a feeble stream and the heater may not engage reliably.

2) Electrical installation

– Above 3.7 kW: hard-wired with protective earth (TN-S/TN-C-S), breakers matched to nominal current;
– 7.5–11 kW: 400 V, 2/PE, no neutral; dedicated breaker and proper cable gauge;
– Remember: each 1% voltage drop costs ~2% of heating effectiveness. Short runs and tight connections are your free “power boost.”

3) Distance to outlet

Mount as close as possible to the tap/shower. Every metre of pipe is lost heat and a “moving front” of warm water. In kitchens and basins it’s crucial; for showers it often decides whether 7.5 kW is “fine” or “frustrating.”

4) De-aeration and no-go zones

Always bleed the unit before powering on (open hot tap, let it run ~1 minute). Never mount inside the shower enclosure. Tiny habits, big savings—for your element and your nerves.

5) Aerator/ECO showerhead and filter

For low powers—use a spray/aerator (especially 3.7–4.5 kW). For showers—ECO head ~3.5–4 l/min. Add a mechanical filter before the heater: less scale, steadier flow, longer life. Clean the aerator and inlet screen regularly—warning signs are weak stream, overheating, or failure to engage.

Quick sizing cheat sheet (real-world scenarios)

What to pick today so you don’t regret it tomorrow

• Basin (short run, aerator 2–3 l/min): 3.7 kW (comfortable), 5.5 kW (extra comfy in winter). Why: to ~40°C at 1.9–2.9 l/min; the aerator does wonders.
• Kitchen sink (aerator 2.2–2.7 l/min): 4.5 kW (versatile), 5.5 kW (snappier). Why: more margin for rinsing.
• Shower (ECO head 3.5–4.0 l/min): 7.3–7.5 kW (minimum comfort), 9–11 kW (full comfort, light rain head). Note: 7.5 kW+ requires 400 V.
• Bathtub: not recommended for this power class; consider a storage tank or other setup.
• Two outlets on one unit: keep it short and start from 5.5 kW—but “one heater per outlet” is smarter.

Plain-English wrap-up: sizing without the facepalm

A simple plan that saves nerves and money

1. Pick the outlet: basin / sink / shower.
2. Set the flow: aerator 2–3 l/min (basin/sink), ECO showerhead 3.5–4 l/min (shower).
3. Estimate ΔT: winter (e.g., 8→40°C = 32°C), summer (15→40°C = 25°C).
4. Calculate: 0.07 × l/min × ΔT → you’ll get your “dream power.”
5. Check the electrics: 230 V (up to ~7.3 kW) vs 400 V (7.5–11 kW), breakers, cable gauges.
6. Add the helpers: aerator/ECO head + filter; mount close, bleed before first power-on, never inside the shower area.

When pressure, electrics, short runs, and sensible power line up, that dreaded sudden “brrr” under the shower disappears. That’s the whole point.

Conclusion

That’s everything you need to size your DAFI without guessing—and without those “lukewarm surprises.”