Aluminum foil is a great reflector of solar radiance. White paint is also pretty good, but reflects 'softer'(highly diffuse) light. Reflective Mylar or mirrors create the 'hardest' light(least diffuse, also called specular). A 400 watt HID would be sufficient. Otherwise there are fluoro and LED(pricey, but usually last a decade and use very low power, create very little heat, and are similar in efficiency to fluoros) alternatives.
There currently are LEDs that surpass HPS in efficiency(over 150 lm/watt), but they're still 'perfecting' the technology. So unless you can smuggle LEDS out of a laboratory.... Nvm. The major problem is dealing with the heat generated by HO LEDs.