You're about to drop $3,000 on van power. Here's how to make sure you get it right the first time.
If you're mid-build and staring at a shopping cart full of LiFePO4 batteries, a Victron inverter, and a stack of solar panels, you already know the terror: what if it's not enough? Or worse — what if you overbuild by $1,500 and haul dead weight around for years? Every van and RV converter hits this exact wall. Sizing your electrical system is the single most expensive decision in the build, and it's the one people get wrong most often.
Let's walk through how to actually size a DIY van solar and battery system the right way — the numbers that matter and the mistakes that quietly wreck builds.
Step 1: Build a real load list, not a guess
Everything starts with your daily energy consumption in watt-hours (Wh). List every device: 12V fridge, roof fan, water pump, laptop charging, lights, Starlink, induction cooktop, and that Nespresso machine you swear you'll use every morning. For each, multiply watts by hours used per day.
The fridge is the sneaky one. A compressor fridge doesn't run 24 hours — it cycles, so it might only pull power 30–50% of the time. People who assume constant draw wildly oversize their bank. People who forget the fridge runs all night undersize it. Both cost you.
Step 2: Size your battery bank in usable Ah
Here's where the LiFePO4 vs AGM decision bites. With AGM you can only safely use ~50% of rated capacity. With LiFePO4 you get ~80–90%. So a "100Ah" AGM battery gives you 50Ah usable, while a 100Ah LiFePO4 gives you 80–90Ah. If you built your budget around AGM Ah numbers and then bought lithium, you either overpaid or got confused halfway through.
Rule of thumb: figure out your daily Wh, divide by your system voltage (usually 12V), then add a buffer for cloudy days and depth-of-discharge limits. That gives you the Ah you actually need — not the marketing number on the box.
Step 3: Size solar for YOUR climate, not the datasheet
A 200W panel does not make 200W. In real conditions you'll see 60–80% of rated output, and only for peak sun hours — which might be 5 hours in Arizona summer and 2.5 hours in Pacific Northwest winter. Sizing solar without factoring in your actual travel region and season is how people end up running the engine or shore power constantly.
Step 4: Don't skip wire gauge — this is the safety one
This is the part that keeps people up at night, and rightly so. Undersized wire on a high-current inverter run isn't just inefficient — it's a fire risk in a vehicle you sleep in. Wire gauge depends on the amperage, the wire length (round trip), and acceptable voltage drop. A 2000W inverter can pull 160+ amps at 12V. Run that through wire that's too thin over a few feet and you've got dangerous heat and voltage sag. Get your battery-to-inverter cable, your MPPT wiring, and your fuse sizing right before you buy — swapping cable after the fact means tearing apart finished cabinetry.
Do the math before you spend the money
Doing all of this by hand across spreadsheets is exhausting and error-prone, and a single wrong assumption cascades into hundreds of dollars of the wrong components. That's exactly why I point people to RigWatt — the RV & Van Electrical Load & Battery Bank Calculator. You plug in your appliances, your climate, and your usage, and it tells you the battery Ah, solar watts, and correct wire gauge you actually need — before you drop thousands on the wrong gear.
Measure twice, buy once. Your future self, sitting in a warm van with a fully charged bank on a rainy day, will thank you. Run your build through RigWatt here and get certainty before you check out.
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