Efficiency governs the size of a panel not its output.

i.e. A 20% efficient panel will be smaller in size than a 16% efficient panel. It needs less silicone per sqm to generate the same amount of power.

When space is limited we like LG 300 watt panels.
These are 1640mm by 1000mm which is more or less then same area than a standard 250 watt panel.

However, because their efficiency is 18.30% they are capable of generating that extra 50 watts over the same area of panel. The module’s nominal power is interesting, as is its efficiency. But the single most important factor for PV panel operators is yield: how many kilowatt-hours per kW of installed power flow from the PV system to the inverter?

Another aspect of efficiency is temperature. Contrary to popular belief, panels don’t work more efficiently in hotter conditions.

Estimated losses due to temperature and low irradiance: 6.9% (using local ambient temperature)
Estimated loss due to angular reflectance effects: 3.2%
Other losses (cables, inverter etc.): 14.0%
Combined PV system losses: 22.5%

Hot panels can mean a drop in efficiency. If your panels can’t be seen from the road or if the look is not of prime importance to you then you might want to avoid the all black look which can heat up more.