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Van Life Solar Panel & Battery Sizing Calculator: The Off Grid Guide

There's one question that stops almost every campervan build in its tracks: how much solar power do you actually need? Get it wrong on the low end and you're stranded with a dead battery two days into the wilderness.

Get it wrong on the high end and you've burned through thousands of dollars on equipment collecting dust on your roof.

This solar sizing calculator was built to cut through that uncertainty. Rather than spitting out a rough estimate based on oversimplified rules, it works through the actual physics of your system accounting for how inverters bleed energy, how battery chemistry limits what you can use and how heat quietly eats away at your panels' rated output.

What you get at the end is a specific trustworthy number you can actually buy equipment around.

HOW TO USE THE CALCULATOR

Getting accurate results means putting accurate data in. Here's how to move through each section of the tool:

Step 1 — List every device you plan to run. Think through your whole day: the 12V compressor fridge that runs constantly, the roof fan, LED strip lights, a water pump, your phone and your laptop. Every watt counts.

Step 2 — Identify whether each device runs on AC or DC power. This distinction matters more than most people realize. Anything you plug into a standard 120V wall outlet requires an inverter to convert your battery's DC power first.

That conversion burns roughly 10% of the energy before it ever reaches your device. The calculator applies that penalty automatically for any appliance you flag as AC-powered.

Step 3 — Choose your system voltage. The vast majority of van builds run on 12V. If you're outfitting something larger a school bus conversion for example or running a power-hungry appliance like a mini-split air conditioner, a 24V or 48V system may be more appropriate.

Step 4 — Select your battery chemistry. The type of battery you use determines how deeply you can drain it before causing damage which directly affects how much usable capacity you actually have.

Step 5 — Fill in your location variables. Enter how many days you want to run independently without any sun, your region's peak sun hours and what type of charge controller you'll be using.

STEP 1: CALCULATING DAILY WATT-HOURS

Your entire solar system design flows from one foundational number: how many watt-hours you consume in a typical day.

The math itself is simple multiply a device's wattage by the number of hours you use it daily. A 12V refrigerator drawing 45 watts and cycling for 10 hours burns through 450 watt-hours. A tablet charger pulling 30 watts for 3 hours adds another 90 watt-hours to your total.

Where things get more complex is with AC appliances. Your inverter doesn't convert power for free. When it transforms 12V battery power into 120V household current, heat is produced as a byproduct and that heat represents electricity you paid for but can't use.

Correctly categorizing your devices in the calculator ensures that energy loss gets baked into your system size from the start rather than showing up as a nasty surprise later.

STEP 2: SIZING YOUR BATTERY BANK

Knowing your daily consumption in watt-hours is step one. Step two is building a battery bank capable of storing and delivering that energy safely.

Battery banks are rated in amp hours but the rated number on the label isn't the same as the usable number not by a long shot. This comes down to depth of discharge, or DoD: the percentage of a battery's capacity you can actually tap before risking permanent degradation.

For lead acid and AGM batteries, that ceiling sits at 50%. A 200Ah AGM bank sounds substantial, but you only have access to 100Ah of it. Push past that regularly and you'll be replacing batteries far sooner than expected.

Lithium iron phosphate (LiFePO4) batteries operate by a different set of rules. Their usable capacity extends to 85–90% of the rated amount they weigh significantly less and they handle faster charging without complaint.

A 200Ah lithium bank gives you 170Ah or more to work with. For long term van life use they're the preferred choice for good reason.

The calculator handles the DoD math automatically once you select your battery type. It also multiplies your base capacity by your desired days of autonomy the number of consecutive sunless days your system should survive without falling short.

STEP 3: SIZING YOUR SOLAR ARRAY

With your battery capacity established, the final piece is figuring out how much solar panel wattage can reliably recharge it.

The key variable here isn't how many hours the sun is up it's peak sun hours, a measurement of solar intensity rather than duration.

During summer you might have 14 hours of daylight but only 4 or 5 of those hours deliver enough solar radiation to drive your panels anywhere near their rated output. In cloudier months or northern latitudes that figure can shrink to 2 or 3 hours.

On top of that, panels installed on a van roof rarely hit their rated wattage in practice. Road dust, non-optimal tilt angles, resistance in the wiring and panel temperature on hot days all chip away at output.

To account for this the calculator adds a 20% buffer to your required panel wattage so your real-world results still meet your real world needs.

Your charge controller selection also shifts the final number considerably. A PWM controller the older, cheaper option operates at roughly 75% efficiency, converting a meaningful portion of the voltage coming off your panels into heat rather than charging current.

An MPPT controller the current industry standard runs at around 95% efficiency by actively adjusting the electrical conversion to extract maximum usable power from your array. Changing your controller type in the calculator immediately updates the required panel wattage to reflect the difference.

COMMON MISTAKES TO AVOID

Even with a well designed calculator a few recurring errors can throw off an otherwise solid build:

Sizing only for summer: A system that works flawlessly in June can leave you stranded in November. If you plan to travel or camp year round, run your numbers using the worst peak sun hours you expect to encounter not the best.

Overlooking always-on devices: Propane leak detectors, inverters in standby mode, and charge controller displays all draw a small but constant trickle of power. Over several days of off-grid camping, those tiny loads accumulate into something worth accounting for.

Cutting corners on wiring: No matter how capable your panels and batteries are, undersized wire gauges create voltage drop that robs you of power and introduces a genuine fire risk. Electrical sizing doesn't stop at the components.

CONCLUSION

Designing a van electrical system becomes far less overwhelming once you break it into three straightforward questions: how much energy do you use daily, how much storage can hold it safely and how much solar can refill that storage reliably.

Work through those three answers carefully with real numbers and accurate assumptions and you'll have a system that performs exactly as expected long after the build is finished.

Use this calculator as the foundation before you spend a dollar on components.