The power consumption of our test system ranged from 130 watts to 343 watts. Asus's passively cooled GeForce 6600 GT held the record for lowest power draw in 2D mode under Windows, while our SLI-configuration of two 7800 GTX cards marked the other extreme, guzzling the most power in our F.E.A.R. benchmark.
Let's run some numbers: a GeForce 7800 GTX SLI setup consumes 100 watts more per hour than a single GTX card. Based on an average operation of only two hours per day, that comes to about $15 per year. At eight hours per day, the extra cost rises to around $60 per year. In the worst case scenario - constant operation under 3D load - the power bill will be an extra $180 per year.
Two GeForce 6600 GTs in an SLI setup draw the same amount of power as one much faster GeForce 7800 GTX; due to its extreme clock speeds, eVGA's water cooled card draws 287 watts in 3D mode, which is roughly what a dual 7800 GT configuration requires. Then again, the two 7800 GTs would draw 13 watts more in 2D mode. Overclocked components draw an additional one to three percent more power.
Conclusion: SLI requires a brawny power supply.