Every van build forum has the same thread: “how much solar do I need for my lithium battery?” The answers are usually all over the place because nobody is doing the actual math, they are just repeating what worked for their rig. Let me walk you through the numbers the way I do it for every install.
Start With Your Daily Amp Hour Usage, Not Your Battery Size
The mistake I see constantly is people sizing solar off their battery capacity instead of their actual daily consumption. A 300Ah lithium bank tells you nothing about how much solar you need. What matters is how many amp hours you burn through in a typical day.
Add up your loads: a compressor fridge might pull 40-50Ah a day, a Starlink Mini around 30-40Ah, lights and phone charging maybe 10-15Ah, and a water pump a few more. A modest boondocking setup often lands somewhere between 80 and 150Ah of daily draw.
Tip: Run a battery monitor like a Victron SmartShunt or a Renogy BT-2 for a week before you buy panels. Watching your real numbers beats any spreadsheet estimate.
The Core Solar Sizing Formula
Once you know your daily amp hour usage, the formula is simple in concept even if the real world adds friction. Divide your daily Ah need by your expected peak sun hours, then convert to watts.
Peak sun hours are not daylight hours. In most of the continental US you get 4 to 6 peak sun hours a day depending on season and latitude, meaning the equivalent of 4-6 hours of full-intensity sun even though the sun is up for 10-14 hours.
Here is the math for a 120Ah daily draw at 12 volts:
- 120Ah x 12V = 1,440 watt hours needed per day
- 1,440Wh / 5 peak sun hours = 288 watts of solar, before losses
- Add 20-25% for wiring loss, panel angle, dust and temperature derating = roughly 350-360 watts
That’s why “just buy 400 watts” keeps showing up as informal advice. It is not far off for a mid-size daily load, but it is not universal either.
A Real Example: 200Ah Lithium Bank, Weekend Warrior Use
A customer I worked with had a 200Ah Battle Born bank in a Sprinter van, running a fridge, induction cooktop occasionally, and a laptop for remote work. Her daily draw averaged 140Ah once we logged it with a Victron monitor for ten days.
140Ah x 12V = 1,680Wh per day. Dividing by 4.5 peak sun hours (she camps a lot in the Pacific Northwest, where sun is inconsistent) gives roughly 373 watts of solar needed as a bare minimum. We installed 400 watts of rigid panels and she still runs the engine occasionally on multi-day cloudy stretches, which is expected and normal.
That’s the honest picture: solar covers most days, not every day, and that’s fine as long as you have a backup charge source.
Why Your Real-World Number Is Always Lower Than the Spec Sheet Math
Panel wattage ratings are measured under Standard Test Conditions in a lab: 25°C cell temperature and a perfectly perpendicular sun angle. None of that holds true on a van roof in July.
Hot panels lose output, roughly 0.3-0.5% per degree Celsius above 25°C, and a black roof in Arizona summer can push cell temps well past 60°C. A flat-mounted panel also loses a meaningful percentage of its rated output any time the sun isn’t directly overhead, which is most of the day.
Warning: Never size solar using the panel’s rated watts as your real-world expectation. Budget for 65-80% of nameplate wattage on a good day, and less in winter.
This is also where choosing an MPPT controller over PWM matters. MPPT recovers more of that lost potential, especially with colder mornings or partial shade, by hunting for the panel’s optimal voltage point instead of just clamping it down.
Accounting for Cloudy Days and Seasonal Swings
Peak sun hours are not a fixed number. A location that averages 5.5 hours in July might drop to 2.5-3 in December. If you travel seasonally or chase weather, size for your worst realistic month, not your best one.
Boondockers who winter in the Southwest desert still see meaningfully shorter days and a lower sun angle than summer camping further north. If most of your trips are spring through fall, size around those months and accept that deep winter will need a generator or shore power top-off occasionally.
I keep a simple mental model: whatever the math says, cloudy days cut output by 50-90%. Full overcast can leave you with only 10-20% of a clear-sky day’s harvest. That’s not a flaw in your system, that’s just physics.
Roof Space Is Often the Real Constraint
You can run the math and land on 500 watts, then discover your van roof, once you subtract the vent, the AC unit and the roof rack, only fits 300. This happens more often than people expect, especially on smaller vans like a Transit 148 or a ProMaster City.
When that happens, don’t force it. A hybrid approach works well: install what solar fits, then lean on a DC-DC charger running alongside your solar to recover the gap during driving days. Most people driving 1-2 hours a day can make up a genuine shortfall this way without ever plugging into shore power.
Portable ground panels are another option if roof space runs out, since you can angle them directly at the sun and get better performance per watt than a flat roof mount, though they mean extra setup each time you park.
Matching the Solar Controller to Your Battery, Not Just the Panels
Solar sizing doesn’t stop at wattage. Your charge controller needs enough amperage headroom for the array you’re installing, and it needs to actually talk to your BMS correctly for lithium’s charge profile.
A 400-watt 12V array can push close to 33 amps at peak, so a 30A controller is already undersized. Most installers round up to a 40A or 50A MPPT unit like a Victron SmartSolar 100/50 or a Renogy Rover, both of which have solid track records with LiFePO4 charge profiles.
Getting the absorption and float voltages dialed in correctly matters just as much as wattage. If you haven’t set that up yet, our guide to solar controller settings for LiFePO4 walks through exact voltage numbers for common brands.
A Quick Sizing Cheat Sheet
| Daily Usage (Ah at 12V) | Watt Hours/Day | Solar Needed (5hr sun, +25% buffer) |
|---|---|---|
| 60Ah | 720Wh | ~180W |
| 100Ah | 1,200Wh | ~300W |
| 150Ah | 1,800Wh | ~450W |
| 200Ah | 2,400Wh | ~600W |
These numbers assume decent sun and a properly configured MPPT controller. If you camp mostly in shaded forests or cloudy climates, shift up a row or two. If you’re mostly in the desert Southwest with unobstructed sky, you have a bit more margin than shown here.
The Battery Bank Side of the Equation
One underrated point: because lithium lets you discharge to a genuinely usable 20% state of charge without hurting the cells (see the Battle Born LiFePO4 resources or general chemistry background on Wikipedia’s LiFePO4 page), your usable capacity is larger for the same physical battery size compared to AGM. That means solar sized around your daily draw, not your total battery Ah, is doing the right job even if the bank never hits 100% every single day.
A lithium bank sitting at 70-80% most evenings and topping off fully once or twice a week is completely normal and does not shorten its life the way it would with lead-acid. Don’t chase 100% daily as some kind of requirement.
Putting It All Together
If you only remember one thing from all this: measure your actual daily amp hour draw first, then work backward to solar watts, then add margin for heat, angle and weather. Skipping straight to “how many watts should I buy” without that first step is how people end up either overspending on panels they don’t need or under-building and wondering why they’re always running a generator.
For most single-person or couple van setups running a fridge, some electronics and basic lighting, 300-500 watts paired with a properly configured MPPT controller and a 200-300Ah lithium bank covers the vast majority of boondocking scenarios. If you want to stress-test that assumption against real trip scenarios, our boondocking solar and lithium math breakdown walks through several real trip profiles side by side.
Get your daily number right, buy a bit more than the bare minimum, and you’ll spend a lot less time watching the battery monitor than most first-time builders do.
