In the Pokedex entries for Black, Y, and Alpha Sapphire, Volcarona is stated to have provided a replacement for the sun When volcanic ash clouded the atmosphere.
To find out how powerful this is, we need to find out how much energy from the actual sun hits the Earth.
The sun releases 3.8e+26 Joules of energy every second.
Energy that hits the Earth = 3.8e26* half of the surface area of Earth / Surface area of a sphere with radius equal to 1 AU
E = 3.8e26* 2πr^2 / 4πR^2 = 3.8e26* 1/2* (r/R)^2 = 1.9e26* (6371000/149600000000)^2
E = 1.9e26*0.0000000018
E = 3.42e17 Joules per second, 81.73 Megatons, or City level+