8.6 Are the new T8 lamps with reduced power ratings more efficient?

A new series of T8 lamps was recently released that comes with reduced power ratings at the same sizes, e. g. just 51 W instead of 58 W for the 1.5 m lamp. The manufacturer claims these lamps could be used as a fully compatible replacement for the existing lamps without replacing a ballast, be there magnetic or electronic ones installed in an existing luminaire. Now customers start to wonder and to ask whether these lamps can actually save energy. Well, this depends on what you mean by that and what you expect.

In order to reduce the lamp input power with a given ballast you need to vary the lamp impedance. Assuming as a first approach the lamp were an ohmic load, which is far from true, it still remains acceptable to view it as a greatly active load, while the ballast is approximately inductive. The serial interconnection of these two elements theoretically yields two values of lamp impedance (here assumed to be resistive – Fig. 8.15). In the case of the traditional 58 W lamp the lower one is the one used. In order to shift to 51 W the lamp impedance needs to be further reduced. Unfortunately this yields a higher current, while the losses in a magnetic ballast increase with the square of the current. So a slight reduction in lamp power comes at the price of a steep increase in losses and hence gnaws on the system efficiency from both sides. While it would have been attractive to use the upper one of the two theoretically possible points of operation, this is practically impossible. A fluorescent lamp with a voltage drop of more than something around half of the line voltage will not start on same line voltage.

Fig. 8.15: In order to reduce the lamp input power with a given ballast the lamp impedance has to be reduced

It would have been fine if the lamp alone performed a significantly higher efficiency to offset the higher ballast losses, but another measurement (Table 8.9) revealed that this is unfortunately not the case. In fact the lamp efficacy is even poorer, and all the more is the system efficacy. Sure the absolute power intake is lower, but you receive a minor electrical energy saving at the price of a major loss of brightness.

Table 8.9: Measurements on a T8 lamp 51 W with a magnetic and an electronic ballast

With an electronic ballast the balance looks considerably better. The system efficiency is then at least the same. So when using this type of lamp the difference between magnetic and electronic mode of operation is considerably greater than with the ordinary type. Still, the morals is: If you can afford to sacrifice some light the best approach is to use good magnetic ballasts, regular T8 lamps and a voltage reduction unit. Then and only then you receive a major electrical energy saving at the price of a minor loss of light output.