Solar Panel Output by Sun Hours Chart for 12V Builds

A solar panel's nameplate rating is a laboratory measurement taken at 25C cell temperature with the sun perpendicular to clean glass, and a roof array in a vehicle meets none of those three conditions. The number that matters is delivered amp-hours per day, and it is roughly 75 percent of what the multiplication of watts by sun hours suggests. That single correction is the difference between a system that keeps up and one that quietly falls behind every day until the battery is flat on the fourth morning.

How much energy does a solar array actually deliver per day?

Start with the formula: real daily watt-hours = panel watts x peak sun hours x 0.75. Peak sun hours is not the number of hours of daylight. It is the number of hours of equivalent full-strength sun, defined as 1,000W per square metre, that a location receives in a day. Fourteen hours of daylight in the Pacific Northwest under overcast might be three peak sun hours. Ten hours of daylight in the Arizona desert in winter might be five.

The 0.75 is the derate: everything between the laboratory and your battery, stacked into one workable figure. It assumes a flat-mounted panel, an MPPT controller, a warm roof and a reasonably clean surface. Tilt the panel and aim it and you do better. Use a PWM controller, park under a tree and never rinse the glass and you do considerably worse.

Here is daily energy in watt-hours for every common array size across the useful range of peak sun hours.

Watt-hours are the honest unit for energy, but nobody sizes a 12V battery in watt-hours, so here is the same table in amp-hours at 12V nominal. Divide the watt-hours by 12. Against a LiFePO4 resting voltage of 12.8V the same energy is about 6 percent fewer amp-hours, so treat this table as the optimistic edge of the range rather than a promise.

Read across the 100W row and the seasonal problem appears immediately. The same panel that returns 44 amp-hours on a long clear day in the Southwest returns 13 on a short overcast one in the north, and 13 amp-hours will not run a fridge. Read down the five sun hour column and the linearity is obvious: solar output scales exactly with array size, so doubling the panels doubles the yield, right up until the roof runs out. A single 200W panel is the standard building block for exactly this reason, because it is twice the output for roughly the same mounting effort and half the roof penetrations per watt as two 100W panels.

What exactly does the 0.75 derate consist of?

It is not one loss, it is seven, and knowing which ones you can do something about is worth real amp-hours. Every figure below is a published typical range rather than a single number, because each depends on your specific roof, climate and hardware.

Those percentages do not simply add, they compound, which is why the combined figure lands around 0.75 rather than around 0.4. Multiply them: a panel losing 12 percent to heat, 15 percent to flat mounting, 3 percent to the controller, 4 percent to soiling and 2 percent to cable ends up at about 0.68 of nameplate, and a cleaner install with a tilted portable panel lands closer to 0.85. The 0.75 sits deliberately in the middle of that spread.

Three of the seven are worth money. Panel temperature is the cheapest to fix: a set of Z brackets costs a few dollars and creates the air gap that stops a panel bonded flat to a hot dark roof running 30C above ambient. Controller type is the largest single line: a Victron SmartSolar MPPT 100/30 against a bundled PWM unit is a 10 to 30 percent difference on the same panels, and the MPPT versus PWM comparison works through when the gap is at its widest. And cable loss disappears almost entirely if you wire panels in series and use a controller with a high input ceiling, such as a 150V input Rover Lite , because halving the current quarters the power lost in the roof run.

Partial shade is the one that ruins otherwise good installs, and it is the one with the widest range in the table. Cells within a panel are wired in series, so a shadow across a single corner can throttle the entire panel rather than the fraction of it that is dark. On a vehicle roof the shadows come from your own hardware: a roof fan, an antenna, a rack crossbar, a surfboard. Lay the array out on the roof and watch where the shadows fall through an afternoon before you drill anything. The solar panel mounting guide covers the layout and the sealing.

What will each array size actually run?

Turn watt-hours into a shopping decision. The table below uses the five peak sun hour column, which is a good clear-weather figure for much of the US in the warmer half of the year, and matches the array against a realistic daily load.

Compare the two numeric columns before you buy. At five peak sun hours a 400W array covers a typical full-time load of about 105 amp-hours a day. At three peak sun hours, which is a perfectly ordinary figure for a northern winter or an overcast week anywhere, the same array delivers 75 amp-hours and the system is losing 30 a day. That is why solar alone is not a charging strategy for anyone travelling outside summer, and why a DC-DC charger that harvests two hours of driving is the correct partner to a roof array rather than a competitor to it. The solar versus alternator comparison works through the split.

Roof area figures are approximate because panel dimensions differ between makers, and they exclude the space you need around each panel for brackets and for a hand to reach a bolt. Size the controller by charge current rather than by array watts: at 12V, array watts divided by about 14V of charging voltage gives the current, so a 400W array can push around 28A and wants a 30A controller with a little headroom. Oversizing the controller costs money and nothing else. Undersizing it means clipping output on the best days of the year.

What is the difference between nameplate watts and delivered amp-hours?

Here is the sentence worth remembering: nameplate watts describe the panel under laboratory conditions for one instant, while delivered amp-hours describe your roof over a real day, and the second number is roughly the first multiplied by peak sun hours and by 0.75 and divided by 12.

That distinction resolves most solar arguments before they start. A 200W panel is not a 200W power source, it is a device that on a good day in the right place will put roughly 63 amp-hours into a battery. On a two peak sun hour day it will put in 25. It never puts in 200 of anything. When somebody says their 400W array runs their whole van, they mean it does so in the season and the latitude they are in, and the same array in a wet northern winter would not run their fridge.

The other thing the distinction fixes is expectation about instantaneous power. A 400W array on a perfect day at solar noon really does briefly produce close to 400W, which is around 28A into the battery. That is a satisfying number to watch on a shunt monitor and it is not what you get for the rest of the day. Peak sun hours exists precisely to compress that whole curve into one honest daily figure, and it is the only figure worth designing from.

One more practical note. A portable panel you aim at the sun genuinely out-produces a flat roof panel of the same rating, often by 20 to 40 percent, because it defeats both the tilt loss and the shade problem: you park in the shade and put the panel in the sun. A folding 100W panel connected through an SB50 Anderson connector is a genuinely useful supplement to a roof array rather than a compromise, and the portable versus roof-mounted comparison covers where each wins.

How do I size the array for my own build?

Work backwards from the load, then design to the worst month you will actually travel in rather than to the annual average. Four steps.

One, find the daily load in amp-hours. The appliance power draw chart lists twenty-nine real loads with realistic hours, and the power consumption calculator adds up your own. Most builds land between 50 and 210 amp-hours a day.

Two, find the peak sun hours for your worst travelling month. The sun hours by region chart gives winter, summer and annual figures for two dozen regions. If you travel the Pacific Northwest in winter, the honest number is closer to 1.5 than to 5, and no array size fixes that.

Three, divide. Required array watts equals daily amp-hours times 12, divided by peak sun hours, divided by 0.75. A 105 amp-hour daily load at 5 peak sun hours needs 105 times 12, which is 1,260Wh, divided by 5, which is 252W, divided by 0.75, which is 336W. Round up to 400W. The same load at 3 peak sun hours needs 560W.

Four, add the battery to cover the gap. Solar and battery are not interchangeable: the array replaces energy and the battery buffers the days when it cannot. A Litime 12V 100Ah LiFePO4 Battery (Group 24) holds 85 usable amp-hours, which is most of one day for a full-time build, so two or three of them is the normal answer alongside a 400W array. The solar array calculator runs all four steps at once, and the solar installation guide covers getting it onto the roof.

Everything on this page is calculated from published panel specifications and standard derate conventions rather than measured by us. It is researched guidance, not an electrical certification, and it does not replace ABYC E-11 or the manuals for your panels and controller. A lithium installation should be inspected by a qualified installer before it carries load, and the controller output must be fused at the battery end within a few inches of the terminal.

Where to go next

• Sun hours by region chart, the input this whole page depends on.
• Solar array calculator, your load and your region in one place.
• Appliance power draw chart, to find the daily load first.
• MPPT versus PWM charge controllers, the largest single line in the derate stack.
• Best solar panels for a van, the panels themselves compared.