Hi,
15W T8s overdriven to 4x levels...shoud do about 30W to 40W of light. Its really hard to say since the loss start to go up quite quickly at anything above 2x. A quick measurement with a multimeter can determine how much power you will actually get. But definitely...at least 25W. The overdrive gain is much higher with T8s than with T12s.
WH5 and F32T8s are the same ballast. I prefer WH5 since they have a better waranty and they are easier for me to explain to other reefers.
For your case, if you truely want a dawn/dusk operation, then I would experiment with F32T8 ballasts that are dimable. I haven't done it before, but they are a safe way to do the dawn/dusk. However, these ballasts are expensive. The other way is the Icecap.
If you think the dawn/dusk think is too much work, then I would just hook up a WH5 with two lamps in series and be done with it. When lamps are hooked up in series, two 18" tubes looks like one 36" tube. You can chose between 2x or 4x overdrive....its up to you. 2x gives you most the bang with good tube life and low heat. 4x give you the most light, but shorter tube life (around 6 months) and more heat.
Hope that helps.
- Victor.
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