I'd steer away from using a car amp for "non-car use" - car amplifiers aren't especially good quality (sonically), and they're quite inefficient (use a lot of power, and generate wasted heat)
If u insist on using a car amplifier -
-If your amp draws ~ 15A, get a 18-20A power supply. The current drawn isn't continuous, it spikes and dips according to the music you play, and the volume. e.g. Low volume = less current, bass transients and techno music = a lot of current drawn. You need to overrate the PSU substantially, to allow for these spikes - not doing so will cause the power supply to burn out with extended use, or may constantly trip its overload protection mechanisms. (If you amp draws less, than over-rate proportionally - for 5A draw, 8A PSU, 10A draw, 14A PSU)
-To ensure you reach the amps max power and efficiency, use a 13.8-14.5V supply voltage. Do NOT exceed 14.5V. As rexter said, the actual battery voltage is higher than 12V. ~14.1V is ideal.
-Use a "low noise" power supply, or a transformer based alternative. Some of the cheaper switchmode power supplies give out really dirty voltage - this comes through the speakers as hissing, pops, high pitched squealing, clicks etc.
-Make sure you use big thick power cables from the PSU to your amp
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To calculate approximately how much current an amplifier draws -
Total Power / Supply Voltage = Current; then increase by 20% to allow for inefficencies.
So for 4x45W:
180W / 13.8V = 13.85A
13.85 X 1.2 = 16.6A MAX
It does seem like a lot, but it isn't continuous, and most car amps will never reach their "labeled power" without heavy distortion, so in effect, the max current draw will be ~13-14.5A, as you said, so use a 16-18A PSU.
Hope thats clear enough