Like was said... wow, 4 months ago. Time flies. The difference is mostly in frequency, not voltage. A 4khz inverter running on 9v will be as bright or brighter than a 2khz running on 12v.

Note: Tim no longer sells the 2khz inverters. Stopped selling them just after I ordered my stuff [!]. He now sells the 4khz, so you don't really need to worry about that. Run it on 9v if you'd like, 18v would shorten the EL's lifespan.

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