One of the first things to understand is the difference between your AC system and your DC system.
AC power is what you have at your house. As you know, you shouldn’t stick your finger in the outlet (or the light socket, for that matter) because AC power is, well, powerful. It is the high voltage that will get you. We also refer to the AC system on the boat as the 120volt system (or the 110volt: the truth is that it is usually somewhere in between 110 and 130).
DC power is stored in batteries. It usually gets to the batteries from an AC source or from solar power. Boat systems tend to operate in multiples of 12 volts. 12 volts isn’t so bad, but I still don’t recommend trying to touch your tongue to the terminals like your older sibling used to suggest with a 9v (just me? sorry). But once you get up into 24v or 48v, which is used for larger systems, and especially for those fancy new electrical motors, things are getting more serious. Don’t underestimate any kind of electricity: even at its most mild it is far too good at starting fires.
Your house probably gets its AC power from either a solar system or the power lines. The power lines also carry AC power to your boat via the shore power cord. We often refer to this as plugging our boat in, but it has the fancy connections to allow you to pull a lot more energy than any individual household outlet can handle. You will notice that your Tesla and maybe even your dryer have fancy plugs for much the same reason. Or perhaps your only use of AC power for your boat is the trickle charger that you use to keep your batteries happy in the off season.
If your house has a solar system, there is a reason we refer to it as a system: it requires an inverter to take the DC power collected by the panels and change it into AC power that your house can use. Even if you have a Tesla wall battery, that is just storing DC energy to be inverted to AC when the sun isn’t shining. Your boat may also have a similar system. An inverter is used to take the energy stored in the batteries and make it available for AC devices, like standard vacuums, TVs, kitchen appliances, etc. If you have sophisticated electronics that require AC power, they will thank you for getting a True Sine Wave inverter. This creates a more clear electrical signal, where as older or cheaper systems kind of try to fake the wave and end up causing damage to systems that can tell the difference. This is definitely a place where it can be tempting to be penny wise (buying the cheaper modified sine wave inverter) and pound foolish (watching your laptop die).
If you live in the backcountry or have sensitive equipment or just like to be prepared, you may have a third source of AC power: a generator. Boats tend to have two different sources that take the place of a generator and are much more common on boats than in American houses, due to the fact that houses don’t often try to wander away from the power lines. Many boats have a generator, either installed somewhere or portable, that will allow them to have AC power wherever they are, for as long as they have fuel (remember to leave enough to get home again, please!). Batteries are not very good at producing power for heat, even with an inverter (or at least an inverter that will fit on your <100ft boat). As such, many power boats with electrical stoves will need to run the generator in order to cook.
You will notice that I have referred to inverting from DC to AC and converting from AC to DC. I will admit right now that the ins and outs of why that is requires one to really care about electronics a smidge more than I do. Nonetheless, it is important terminology to keep straight. One way we keep that straight onboard is to refer to a converter as a “battery charger.” That is much more straight forward. Why didn’t I start there? Because your alternator still manages to convert from the AC power generated by spinning things to the DC power that goes straight to your battery, do not pass Go, do not collect $200 at the battery charger.
It is with all of the above that you can see why one might want to purchase a battery charger/ pure sine wave inverter combo. You get one thing that does all the things, hook it up (well, if you have learned anything in this email, you might want to do some more research or hire someone else to hook it up), and forget about it. That is, until one or the other of the components burn out… Many of us still choose to get a battery charger and a separate inverter so that we know what the problem is and only have to replace one at a time down the road.
One reason it is important to know the difference is so that you can figure out how to best shut the power off in case of emergency. You may be able to shut down the AC main breaker and still operate off DC power to see your way around to fixing an issue. More simply, if something isn’t working, the first thing to check is usually going to be the corresponding circuit breakers and cut offs. You wouldn’t be the first person to call out an electrician to find out the problem is your boat isn’t plugged in. On our boat, we had a particularly frustrating situation with a light in the galley because when we were at dock and had plenty of time to trouble shoot, it was working. But when we were out sailing and just wanted to flip it on and see, it wouldn’t work… Someone had chosen to install a hard wired AC light. This is very rare on boats. Even boats that have plenty of power tend to use a combination of installed lights that run on DC and maybe a few lamps to plug into the AC system.
Keep safe, keep learning