I just did a little math. Correct me if I'm wrong.
Local electricity prices averages at 0.18 Euro / kWh.
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1 MS0006 at 20/4 would consume 0.180*20=3.6kWh a day (according to GN's website MS0006 consume about 180 Watts)
To grow 1 hell of a plant (say AutoX) under 2 MS0006s I would use 3.6*2*77=554.4 kWh or 554.4*0.18=99.792 Euro in electricity cost.
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Taking TaNg's words, that is where your reasoning and math are completely off.
2 Ms0006 = 590€ = 380w a.p.d.
1 Rebel = 250 + 6x 117€= 952€ = 178w a.p.d
Price difference = 952 - 590€ = 362€
Wattage diff = 380w - 178w = 202w
(202w * 20h)/1000 = 4.04kwh savings
4.04 * 30 days * 12 months = 1454.4 kw per year * 0.18€ = -261.8€ year you
save using the Rebel. Pays for itself in 3 and half years (952€ / 261.8) compared to buying 2x Ms006
But thats only half the story.
The Rebel itself costs to run, per year, (178*20)/1000*30*12*0.18 = 230.68€
2x Ms0006 cost per year = 492.48
In 3.5 years, you spend in electricity
Rebel = 230.68*3.5 = 807.38€
2xMs006 = 492.48*3.5 = 1723.68€
Difference = 1723.68 - 807.38 = 916.3€
So what this means is that the Rebel pays for itself in savings in 3.5 years. BUT if you buy 2x Ms006, during that same that period you spend an extra 916€ in electricity.
SO in 3.5 years
Rebel = 952€ cost + 807.30€ to run = 1759.3€ spent in total
2xMs06 = 590€ cost + 1723.68 = 2313.68€
So the real world life difference, in 3.5 years, including initial investment is 1759.3 - 2313.68 = -554.38 in favor of the Rebel.
554.38 / 3.5 = 158.4€ a year savings