People usually come to us with a battery in mind. "I want 400 Ah of lithium." That is a fine starting point, but it is the answer to a question we have not asked yet. A system that holds up off-grid is sized from the loads outward, in a chain where each link depends on the one before it. Get the order right and the numbers fall into place. Get it backwards and you end up with a bank that never fills or an inverter that trips on a kettle.
Here is the chain we walk, every time. We will run one illustrative example with round numbers alongside it so the logic stays concrete. Real numbers depend on your rig and how you actually use it, so treat the example as a worked method, not a spec for your van.
Start with the loads, in watt-hours per day
Everything begins with energy, measured in watt-hours per day. We build a list of every load, estimate how long each one actually runs, and add it up. The trick is honesty about runtime, not just wattage.
A typical van load list looks something like this:
- 12V compressor fridge: draws maybe 50W but cycles roughly a third of the time, so call it ~400 Wh/day. This is almost always the biggest single number.
- LED lighting: a few hours of evening use, ~50-100 Wh/day.
- A Maxxair-style roof fan: a few watts on low up to ~35W flat out, so anywhere from 50 to 300 Wh/day depending on the season.
- Water pump: draws roughly 5-7A (~60-90W) but runs only seconds at a time, so it is often under 50 Wh/day.
- Device charging (phones, laptop, lights): ~150-250 Wh/day.
Add an induction cooktop or air conditioning and the picture changes entirely. A single induction burner pulls ~1,500-1,800W at high power; ten minutes of cooking is ~300 Wh. Roof-top air conditioning is in a different league and usually reshapes the whole design.
For our example, say a modest van pulls ~1,500 Wh/day. That is our anchor number. Every link downstream is sized to serve it.
Days of autonomy and the reserve
Next: how many days do you need to run with no meaningful recharge? That is your autonomy. A weekend in sunny Arizona might want one to two days. Someone parked in the trees, or chasing winter weather, wants more.
Battery banks are not 100% usable. Chemistry sets the floor. LiFePO4 can be cycled deep — the cells will tolerate near-full discharge — but we design to roughly 80% usable depth of discharge, keeping ~20% in reserve to protect cycle life and leave a buffer. Lead-acid is the opposite story: you only get about 50% before you start shortening its life, which is one of the main reasons we rarely spec it anymore.
So the math for our example, at one day of autonomy:
- 1,500 Wh needed per day
- divide by 0.8 usable for LiFePO4 = ~1,875 Wh of nameplate capacity
- at 12V that is ~156 Ah
Round up and you are at a 200 Ah, 12V LiFePO4 bank (~2.4 kWh nameplate at 12V) for a single day with margin. Want two days of true autonomy with no sun? Double it. This is exactly why the "400 Ah" request is sometimes right and sometimes overkill, until we know the loads and the autonomy behind it.
One caveat that matters in the cold half of the year: LiFePO4 should not be charged below about 0°C (32°F). Pushing charge current into a freezing cell plates metallic lithium on the anode, and that damage is permanent. The cells still deliver power fine in the cold — it is charging that is the problem. If you chase winter, we plan for it with a battery that has a built-in low-temperature cutoff, internal heating, or both, so the bank protects itself.
Recharge sources: solar, alternator, shore
A battery bank is a bucket. Now we size the taps that refill it. There are three, and most builds use two or all three.
Rooftop array. Panels never make their nameplate. Between heat, wiring losses, panel angle, soiling, and an MPPT controller's real efficiency, we plan on a derate of around 0.75. We also use a realistic number of good-sun hours for the region and season, not the optimistic peak. In strong Southwest sun, figure 4-5 equivalent good hours; in winter or under tree cover, far less. For our 1,500 Wh/day van, a 400W array at 0.75 derate across ~5 good hours is roughly 1,500 Wh on a clean day. That covers the daily load in summer but leaves no cushion, so we would often spec 600W to recover from a cloudy stretch.
Alternator via DC-DC. Driving charges the bank, but you cannot wire lithium straight to the alternator. A depleted LiFePO4 bank has very low internal resistance and will pull every amp the alternator can produce, holding it at full output long enough to overheat its windings and diodes. A DC-DC charger like a Victron Orion-Tr sits between the start battery and the house bank to regulate that. A 30A unit at 12V is ~360W, so about 360 Wh into the bank per hour of driving. We also keep the charger's draw at or below roughly half the alternator's rating — the common rule of thumb is 50%, and we often stay more conservative than that — so we are not running the alternator flat out on a long climb.
Shore or generator. The inverter-charger handles this. Plugged in, a unit like a Victron MultiPlus-II refills the bank fast and is the fallback when sun and driving both come up short.
The goal is matching daily harvest to daily draw with margin for bad days, not maxing out any single source.
Inverter: size to the largest simultaneous AC load
The inverter only matters for AC (120V) loads. We size it two ways at once: continuous and surge.
Continuous is the largest set of AC loads running at the same time. Surge is the brief spike when a motor or compressor starts, which can be two to three times its running draw. A low-frequency inverter-charger like the MultiPlus-II handles surge better than a high-frequency design; the 12V 3kVA model runs ~2,400W continuous (at 25°C, and it derates as it gets hotter) and peaks around 5,500W for a second or two, long enough to start a compressor or pump.
For our example van with an induction burner near 1,800W and not much else on AC at once, a 2,000-3,000VA unit is the right neighborhood. Stacking an induction cooktop, a microwave, and an air fryer simultaneously is what pushes people into larger inverters, and the bank and wiring have to grow to feed it.
Wire, fuse, and the monitor
The last link is the one that quietly decides whether the system is safe and whether you can trust it.
- Wire is sized to the current and the run length so voltage drop and heat stay in spec. The inverter and battery cables are the heaviest in the rig.
- Fuses, including a Class-T or similar right at the battery, protect every conductor. The fuse protects the wire, not the device. A LiFePO4 bank can dump enormous fault current, so this is not optional — and on a boat it is exactly the kind of overcurrent protection ABYC E-11 (AC and DC Electrical Systems on Boats) is built around.
- A shunt monitor like the Victron SmartShunt measures current through a shunt and counts every amp-hour in and out, then reports true state of charge. Voltage alone lies on lithium — its discharge curve is flat, so the pack reads about the same near full as it does near empty. Without a shunt you are guessing, and guessing is how people end up stranded at a flat bank or chronically undercharged.
That is the whole chain: loads to autonomy to bank to sources to inverter to the wiring and monitor that tie it together. Change one number near the top and everything downstream shifts, which is exactly why we size in this order rather than starting from a battery on a shelf.
If you want to put your actual loads and travel pattern through this, that is what our system consultation is for. Or just tell us about your rig and we will work the numbers with you.