8.7 Are T5 lamps more efficient?

Reports about new lighting systems and renovations of lighting installations regularly quote the »new more efficient T5 lamps«, as if it went without saying that the efficiency of a T5 lamp is by default higher than that of a T8 lamp. Albeit, a look at the catalogue data already reveals that this, if at all, only applies to the so-called T5HE lamps optimized for High Efficiency. Those classified T5HO, optimized for High Output, perform significantly poorer than a commonplace triphosphor T8 lamp (Table 8.10).

In the cases of electronic ballasts the input power and light output remains stable independently of the input voltage, while the input power to a magnetic ballast system of course varies greatly with input voltage. So a point can be found (at 217 V) where the measured lamp power in a 58 W T8 lamp driven by a magnetic ballast is exactly 49 W and thereby matches the rating of an existing T5 lamp with a light output of 4300 lm. But at this point, namely of equal power inputs to the T5 and T8 lamps, the light output of the T8 lamp is already ≈4600 lm – even though it was operated at mains frequency here and the T5 lamp, of course, at high frequency, as specified. This casts serious doubts over the practical effect of the theoretical efficiency improvement at high frequency operation. Or over the »more efficient T5 lamps«. Or both.

Table 8.10: Catalogue data of T5 HE and T5 HO lamps with electronic ballasts compared to the measured data of T8 lamps with magnetic ballasts described in detail in Section 8.4

Due to the curious fact mentioned earlier that the Directive allows higher losses in an electronic ballast than in a magnetic one, e. g. a 54 W T5 lamp with a class A3 ballast may have a systems power of 63 W (Table 8.11), yielding a ballast loss share of 14.3%, while the magnetic B1 system with a 58 W lamp – formally and officially – must not exceed 64 W and is thereby limited to a loss share of 9.4% (Table 8.1). But it was also mentioned there that in practice the lamp power with a magnetic ballast is found to be only between 53.5 W and 54.5 W, and that in the end of a day the systems power is crucial and not its split across lamp and ballast. Howsoever, through the theoretical or the practical approach, the T5 lamp hits a tough challenge to match the expectation to provide a better efficiency than a good T8 magnetic system has. On top of this, the unfortunate fact that in one system the rated light output is reached more or less around the rated power intake and in the other one even far below, both catalogue data and the Directive yield unrealistic payback times. Unfortunately this will never ever be discovered, since the electricity consumption of the lighting installation is not registered separately and because during a renovation a new system will always replace an over-aged one which is insufficient in all respects. Never ever will e. g. an optimized modern magnetic system be replaced with an optimized modern electronic system. So the energy savings remain a matter of belief and trust in what the specifier specifies.

Table 8.11: Values and classes of linear fluorescent T5 lamps with ballasts (values for B, C, D classes missing because these lamps are specified for use with electronic ballasts only)