See my comment about paying off higher interest loans first even if the loan is "too small to matter". You pay it off first, period, or you have no financial sense at all.
A veg lamp gets twice as much utility as a flowering lamp, thus for every watt you buy, you get double the value for your initial investment. Obviously you should upgrade this first.
I'm glad you're not my financial adviser. Let me deconstruct it for you:
1st Assumption - everyone vegs under CFLs or fluoros, and not under HIDs (metal halide etc)
2nd Assumption - everyone vegs 24 hours on - which, if you know anything about photosynthesis, is a waste of energy
3rd Assumption - everyone has a separate veg chamber and doesn't veg and flower under the same light
4th Assumption - the cost of LED and drivers is linear. It is not. The best value is in buying the biggest lights and drivers (hint: flowering lights)
But I'll humour your argument anyway.
By your own admission, you save 1344Wh in veg vs 2160Wh in flower - that's a 61% advantage to upgrading your flowering light each and every day for the next however many years you use it.
Add to that, the flowering light is producing BUDs - bigger flowering lights nearly always = bigger bud yields. Veg lights and time are always limited to flowering space and light.
It doesn't matter how big or efficient your veg light is - if you don't have a flowering light (and space) to support it, that light and energy are wasted.
Now let's get back to the real world . . .
Most of us run 18/6 or 20/4 in veg (the fastest veg cycle I have used is 11/1). Veg times are nearly always a percentage of flowering times. For example, an eight-week flowering plant usually needs only three-four weeks of veg (less in hydro) - depending on how many plants you flower, in what space and under whatever light.
So your maths is already skewed.
Finally, it never ceases to amaze me that some people agonise over saving a few $ on lights.
How much does an ounce of weed cost?
Just spend the money on flowering lights: the increase in yield will likely pay for the lights in the first grow.
Then on the next grow you can put the money towards a better veg light.
I'm sorry, but if what you said was true, we would all be doing it. But we're not - for good reason.
And yes, I was being facetious about painting your drag car instead of upgrading your engine. But only just.
After all, you drag car isn't going anywhere without an engine, and your plants aren't going to produce buds unless they are flowered.
Flowering lights pay for themselves in yield - veg lights, not so much (to put it charitably).
