How an RV Electrical System Works: 12V, 120V, and What Connects Them

The 12V DC side runs directly off the house battery bank, regardless of whether the rig is plugged into shore power. That includes interior and exterior lighting, the water pump, the furnace blower, the fridge control board on an absorption fridge, slide-out and awning motors on many rigs, USB charging ports, and the 12V compressor fridges found in smaller trailers and vans. None of this needs an inverter to run, and none of it cares whether shore power is connected, because it draws straight from the battery through a 12V fuse panel. Fuses on that panel follow the standard sizes in NEC 240.6(A), the same series used on the fuse size calculator on this site. When a 12V circuit fails, the fault is almost always local to that circuit: a blown fuse, a loose ground, or a battery that has dropped below the voltage the load needs to operate, not a shore power or generator problem at all. Checking battery voltage and the fuse panel first, before touching anything on the 120V side, is the fastest way to rule out half the system in under a minute.

The 120V AC side powers outlets, the air conditioner, a residential-style fridge if the rig has one, the microwave, and anything else plugged into a standard household-style receptacle inside the RV. That power comes from one of three sources: shore power at a campground pedestal, an onboard or portable generator, or an inverter drawing from the house battery and stepping it up to 120V. All three sources feed the same 120V panel, protected by breakers rather than the fuses used on the 12V side, and only one source powers that panel at a time. A rig plugged into shore power with the generator also running does not get power from both; the transfer switch, covered further down, picks one. This is also the side where an air conditioner startup surge matters, since a compressor pulls several times its running wattage for a moment on startup, which is what the AC startup load calculator on this site checks against whatever source is supplying that panel.

The 12V and 120V sides are not just different voltages, they are sized against different published standards on this site, and mixing them in one calculation produces a wrong answer even when the arithmetic is correct. DC wiring in the battery bay and to the inverter is commonly sized to ABYC E-11, the standard written for 12V systems on boats and adopted widely in RV DC work because the NEC does not really address low-voltage house wiring. AC wiring inside the RV, from the shore power inlet to the breaker panel, follows the NEC tables used in fixed residential and commercial wiring.

A converter is the component that takes 120V power, from shore power or a generator, and steps it down to charge the 12V house battery. It sits quietly doing this any time the rig is plugged in, which is exactly why it is the part most owners never think about until it fails or turns out to be wrong for their battery. A stock converter built for lead-acid will never fully charge a LiFePO4 bank, because it applies the wrong charge voltage and cuts off too early, a limitation covered in full in the converter replacement guide linked below. Without a working converter, the only way to recharge the house battery is solar or an alternator connection through a DC-DC charger, both covered in their own guides. If the battery never seems to reach full charge while plugged in, the converter's charge profile, not the battery itself, is the first thing to check.

An inverter runs in the opposite direction from a converter: it takes 12V DC power from the house battery and steps it up to 120V AC, so outlets and appliances can run off the battery when the rig is not plugged into shore power or a generator. That conversion is not free. A typical pure sine inverter runs at roughly 85 percent efficiency, a figure that varies by model, which means some of every amp-hour pulled from the battery is lost as heat rather than reaching the appliance. That loss is exactly why an AC load has to be converted to DC amp-hours, including the inverter's efficiency loss, before it can be added into a daily energy budget, which the boondocking energy budget guide below walks through in detail. A plain inverter has no charging function of its own; an inverter/charger combines both functions in one box, compared directly on the inverter versus inverter/charger page.

The transfer switch decides which single 120V source, shore power or generator, actually feeds the AC panel at any given moment. It does not combine sources or add their capacity together; it picks one and routes it through. On many rigs this switch is a separate manual or automatic component near the panel. On a rig built around an inverter/charger, the transfer switch is built into that same enclosure and switches automatically between shore power and the inverter's own output the instant shore power is removed, without the owner flipping anything. This is also why plugging into a pedestal while the generator is still running does not damage anything by itself in a rig with a working transfer switch: the switch simply routes whichever source it is set to accept, and the other source sits unused rather than fighting it for the panel.

Most RV electrical troubleshooting collapses into two questions: is this a 12V problem or a 120V problem, and is the fault in a load, a source, or one of the three bridges between them. Lights and the water pump dead but the air conditioner running fine points straight at the 12V side, most often a blown 12V fuse or a battery voltage low enough to trip connected electronics, not a shore power issue. Outlets dead while the lights and furnace blower work fine points at the 120V side instead, most often the shore power cord, the pedestal itself, or a tripped breaker in the panel. A battery that never reaches full charge while plugged into shore power for days is a converter problem specifically, not a battery problem or an inverter problem, since the inverter has nothing to do with charging. Working through which side a symptom sits on before opening a single panel is the fastest diagnostic step in RV electrical, and it costs nothing but a look at what is and is not working.

If you cannot yet see what your 12V side is actually doing, start with a shunt monitor before buying anything else. Everything past that is either closing the AC-to-DC loop with a converter, closing the DC-to-AC loop with an inverter, or protecting whichever side is exposed to an outside power source.