Halacha requires that the Hanukah candles burn for a half-hour after they are lit, and one must therefore ensure that the candles are long enough, or that there is enough oil, to sustain the flame for a half-hour. Of course, the commonly sold Hanukah candles are tall enough to burn for this amount of time, and the specially-prepared oil lamps which many people use likewise contain enough oil to burn a half-hour. But if somebody prepares his own oil lamps, he must ensure to provide enough oil to sustain the flame for at least a half-hour.
If somebody has a limited amount of oil, and when he lights the candles he is unsure whether or not they will burn for a half-hour, should he light the candles, and, if so, does he recite the Berachot?
Hacham Ovadia Yosef ruled that one should certainly light the candles in such a case, and he may even recite the Berachot over the lighting. Although we normally follow the rule of "Safek Berachot Le’hakel" – that a Beracha is not recited in situations of uncertainty – the Berachot are nevertheless recited in this instance, because two separate uncertainties exist. Besides the obvious uncertainty as to whether the candles will burn for a half-hour, there is also a different question, namely, whether this even matters. Two Rishonim – the Samag and the Hagahot Maimonoyot – were of the opinion that the Hanukah candles do not have to burn for a half-hour, and one fulfills the Misva even if the candles burn for just a moment. Although Halacha does not follow this view, it may nevertheless be taken into account and be considered a "Safek" (uncertainty). As such, we have here a situation of "Sefek Sefeka," where there are two possible reasons why one fulfills the obligation with this lighting: 1) the oil may suffice to sustain the flame for a half-hour; 2) even if the oil will not sustain the flame for a half-hour, it is nevertheless sufficient according to one view. Therefore, the Berachot may be recited.
Rabbi Bitan (Hilchot Hanukah, p. 101) writes that he asked Hacham Ovadia to reconcile this ruling with his ruling in other contexts that a Beracha is not recited in situations of "Sefek Sefeka." Elsewhere, Hacham Ovadia establishes that even if there are two factors that allow for the possibility of reciting a Beracha, the Beracha is nevertheless not recited, since its validity remains uncertain. Why, then, does Hacham Ovadia allow reciting the Berachot over the Hanukah candle lighting in this situation on the basis of "Sefek Sefeka"? Hacham Ovadia explained that this situation differs because we deal with a Beracha over a Misva. When a "Sefek Sefeka" establishes that one is obligated to perform the Misva, because there are two possible reasons why it is required, then the Beracha is recited. Since the Sages required reciting a Beracha over certain Misvot, one must recite the Beracha when he performs it in situations of "Sefek Sefeka," because the Misva is considered obligatory. When Hacham Ovadia ruled that a Beracha may not be recited in situations of "Sefek Sefeka," he spoke not of Berachot recited over Misvot, but rather of other kinds of Berachot, such as Berachot over food and the like.
Summary: When lighting Hanukah candles, one must ensure that the candles are tall enough, or that there is enough oil, to sustain the flame for a half-hour. If one has only a limited supply of oil and he is unsure whether it can sustain the flame for a half-hour, he nevertheless lights the candles and even recites the Berachot.
