A quick preliminary check can be done as follows:
After a good run on engine power to fully charge battery banks, turn off all 12vDC electrical switches while at
anchor. This can also be done while plugged in at dock - but switch off shore power. Check your battery voltage using the helm-mounted voltmeter - fully charged battery banks should read just over 13 volts.
If less, perhaps your battery cells are at fault, or there's an open circuit somewhere. As said above, it's doubtful a single bulb of the size of an
anchor lamp will draw more than 1-3 amps.
Now, with all accessories still off, see if voltage drops significantly when switching on the
anchor light. If so, you may have a short circuit.