Unless you are plugged into a landline at a marina, you have to generate all the electricity you consume on your boat yourself. You can have a huge great inverter to turn the 12v DC of the batteries into AC, but unless those batteries are kept charged, the inverter will suck them dry. And the way to charge them is to run the engine - a lot.
To give you an idea of how much energy you will need, complete a power audit.
Take each piece of equipment in turn and multiply it's current drawn by the time it is on during one day. You may need to use the following formula to work out amps.
WATTS = AMPS
VOLTS
So a 12v fridge running at a average of 28 watt / hour uses 2.3 amps / hour
| Amps Drawn | Hours used | Amp hour required | |
| Fridge | 2.3 | 24 | 56 |
| Internal lights | 10 x 1.5 = 15 | 3 | 45 |
| Water Pump | 4 | 2 | 8 |
| Bilige Pump | 4 | 1/2 | 2 |
| TV | 3 | 3 | 9 |
| Total | 118 Amps / day |
This shows I need 118 amps per day from my batteries, unfortunately the problems now start because of limitations within the batteries and the charging system.
Maximum battery discharge level
When a battery is discharged to 50% its voltage will become so low that its considered useless. Of more importance is the fact that when ever a battery is discharged to more than 50% it's life shortens.
Maximum charge level
The voltage needed to fully charge a battery would "boil" the electrolyte away and damage the battery, so basic alternator regulators are set to deliver a safe voltage that, unfortunately, will only give the battery about 80% charge.
So we now know that 118 amps capacity is in fact the capacity of only 30% of the battery, so to calculate how much battery capacity we really need the calculation is
118/30 x 100 = 393 amp hour capacity required
Thus we would need to install over three times the capacity we first calculated ( at least 2 x 220 amp/hr batteries or four 110 amp/hr batteries)
Battery care at this point becomes crucial as discharged plates destroy themselves if left unused or less than fully charged. Sulphation occurs making your batteries even less efficient.
To work out the charging time and alternator output we first must understand that the charge rate starts at the rated output then starts to drop. Eventally it will level off at a low level, where it stays, maintaining the batteries level of charge. The average charge is about half the alternators rated output. So as a rough guide we must take half the alternator rated output as the figure for calculations
Example
Alternater 150 amp unit will give you 75 amp output for calculation
This gives me 118 ÷ 75 = 1.57 hours of charging
so you would need to run your engine for 1 hour 35 mins per day to provide the power you need.
This time can be shortened by fitting more advanced alternators (such as Beta) and top quality batteries (such as Elecsols). This will increase the battery to about 90% of full charge.
Example
Beta Marine 43hp engine with 150 amp alternater and 2 x 220 amp Elecsol batteries
the charge time would be 118 ÷ 142.5 = 0.83 hours of charge or 50 mins running time
As we can see both batteries(30- 40%) and alternators( 50- 70%) do not run at full efficency, leading to most people under esimating their power requirements and literally being left in the dark.
A good boat builder will look at your required power needs and advise you on what you will need, unfortunately generators, battery chargers, travel power all come at a price.