Although all sites suitable for tidal energy conversion are located on shelf seas, few studies have characterised shelf sea systems based on the phase relationship between tidal elevations and tidal currents, and the implications on the suitability for bottom-fixed versus floating turbine technologies. Here we examine this phase relationship across five shelf sea regions. We use the TPXO tidal atlas to characterise systems as progressive (peak currents at high/low water) to standing (peak currents at mid-flood/ebb). We estimate variations in power density for respective turbine hub heights where bottom-fixed devices remain at constant height above seabed, and floating devices track tidal elevations. We hypothesise greater progressive power asymmetry in floating turbines, where peak power loss at low water outweighs peak high water gain. Mean system differences were up to 0.5% between technologies; however the tidal range to water depth ratio was more significant. Bottom-fixed devices outperformed floating in all regimes, but variation reduced from 0.5% to 0.3%, progressive to standing. Fixed devices outperformed floating in shallower, progressive regimes with large tidal ranges. Floating technologies were more suited to deeper, standing regimes with small ranges. Large variance suggests vertical power variability is an important consideration; variation over the rotor swept area and wave–current interactions may also be significant.