Almost every instant shower sold in Kenya is either 5.5kW or 7.5kW, and people choose on price. That is the wrong way round: choose on the supply you have, then on price.
A 5.5kW unit draws about 24 amps. It needs a dedicated 6mm cable back to the board and a 32A breaker of its own. That is the standard fitting for a family bathroom and it is what most houses here are already wired for.
A 7.5kW unit draws about 33 amps. It needs 10mm cable and a 40A breaker. If you fit one on the 6mm cable that fed the old shower, the cable runs hot, the breaker nuisance trips, and in the worst case the insulation goes. Check the board before you buy, not after.
Pressure matters as much as power. An instant shower is not a pump. If your tank sits on the roof directly above the bathroom, there is often not enough head to open the flow switch, and the unit will not fire at all. Two fixes: a shower with an integral booster pump, or a separate pressure pump on the line.
One more thing people miss. Salty or hard water shortens the life of the heating element. If you are on borehole water, buy the salty water version. It costs more and lasts several times longer.