The horrid power of the meltagun (40K) (Please Error Check)

[NecronLord]

His voice was drowned out by the abrupt hiss of the melta as Jurgen fired into the wall, instantly flashing a dozen cubic meters of ice into steam...

Ice, Density: 917 kilogram/cubic meter
The heat of vaporization of water is about 2260 kJ/kg

Now, 917*12 = 11,004 Kg
2260*11004=24,869,040 KJ ~= 25 GJ

This doesn't account for the energy used to move the ice from 0˚C to 100˚C, nor the energy used to move it from its initial temperature to 0˚, now the latent heat of fusion (melting) which will be pretty high also.

Latent heat of fusion of Ice is about 334 kJ/kg (IIRC)
334*11004=3,675,336 kJ
3,675,336+24,869,040= 28,544,376 kJ ~= 29 GJ

Fear the melta!
Note however, that it took an unknown amount of time to do this. Though "instantly" is kinda suggestive of a short time.

[Sr.mal]

Just be glad it wasn't a multi-melta.

[wautd]

His voice was drowned out by the abrupt hiss of the melta as Jurgen fired into the wall, instantly flashing a dozen cubic meters of ice into steam...

Though "instantly" is kinda suggestive of a short time.

As fast as instant death when getting shot in the head I'd say.

Scary shit anyway

[Imperial Overlord]

The math confirms what we knew already: fear the melta. Even the humble lasgun is deadly when you work out the numbers. Double digit GJ was a little higher than I expected.

[NecronLord]

It also helps us in discussing necrons. This same melta... melted them. Whereas they've been able to get through a 15 GW laser mesh. Backeting their upper energy tolerance nicely that.

[Raxmei]

For the temperature change add
11,004 kg * 4190 J/kgK * 100K = 4,610,676,000 Joules, or 4.6 GJ for short, not counting the unknown and probably insignificant excess coldness of the ice and hotness of the steam.