Solar Array Calculator: Panel Watts to Daily Amp-Hours
Daily amp-hours = panel watts x peak sun hours x 0.75, divided by 12.8. A 400W flat roof array with an MPPT controller in 4.5 peak sun hours delivers about 1,350 watt-hours, which is 105 amp-hours a day at 12V. The 0.75 factor is the panel temperature, soiling, cable and conversion losses between the sticker on the panel and the amp-hours a shunt actually counts.
A solar array is bought in watts and used in amp-hours, and the conversion between them loses about a quarter of the number on the sticker. Panel ratings are measured at 25C cell temperature under laboratory light at a perfect angle. A panel bolted flat to a dark vehicle roof in summer sees none of those conditions. This calculator applies the real derate stack so the figure you plan with is the figure you get.
Solar array calculator
Enter the total nameplate watts of the array and the peak sun hours for where and when you travel. If you do not know your sun hours, the sun hours by region chart has typical figures for winter, summer and the annual average.
What is a peak sun hour, and how many do you get?
A peak sun hour is one hour of sunlight at 1,000 watts per square metre, the standard intensity that panels are rated at. It is an energy total rather than a clock reading. A day that starts weak, peaks around noon and fades again might deliver the same total energy as five hours at full intensity, and that day is described as five peak sun hours even though the sun was up for fourteen.
Most of the continental United States sits between 4 and 6 peak sun hours in summer and between 1.5 and 3.5 in winter, and the swing between those two is the single most important thing to understand about vehicle solar. An array sized on an annual average will feel generous in summer and will not carry a fridge in the shortest months. Size for the worst month you genuinely travel in, not for the average. The sun hours by region chart gives seasonal figures by region so you can pick the right one.
Why does an array never produce its rated watts?
Because five separate losses sit between the panel rating and the battery, and they multiply rather than add. The 0.75 default in the calculator is the product of the first three.
| Loss | Typical cost | What is happening |
|---|---|---|
| Panel temperature | 10 to 15% | Output falls roughly 0.35 percent per degree C above 25C. A dark roof in summer runs a panel at 55 to 65C. |
| Soiling and mismatch | 3 to 7% | Dust, pollen, salt film and small differences between panels in the same string. |
| Controller conversion and cable | 5 to 8% | MPPT conversion loss plus voltage drop on the roof run. PWM is far worse, see below. |
| Flat mounting versus tilted | 10 to 20% | Only matters in winter and at high latitude, where the sun never gets high enough for a flat panel. |
| Partial shade | 20 to 100% | A single shaded cell can collapse a whole series string. This is not a percentage you can plan around, it is a parking decision. |
Panel temperature is the largest of the predictable losses and the least intuitive, because it gets worse exactly when the sun is strongest. Silicon output falls by roughly 0.35 percent for every degree Celsius above the 25C rating point. A panel bonded flat to a dark van roof on a summer afternoon reaches 55 to 65C, which is 10 to 14 percent of the rating gone before a single other loss applies. This is the reason a set of Z brackets that lift a panel an inch off the roof is worth real output rather than being a mounting convenience: the air gap lets the panel run cooler. The solar panel mounting guide covers how to do that without turning the roof into a leak.
Partial shade is the loss that is not really a percentage. In a series string, current is limited by the worst cell, so one branch shadow across the corner of one panel can cut the output of the whole string dramatically. Bypass diodes limit the damage but do not remove it. Parallel wiring isolates each panel from the others, which is why vehicles that park under trees are usually better served in parallel even though series wiring is more efficient on cable.
What does each array size actually deliver?
The grid below is delivered amp-hours per day at 12V, flat mounted with an MPPT controller and the 0.75 derate applied. These are the numbers to plan a build around.
| Array | 2 sun hours | 3 sun hours | 4 sun hours | 5 sun hours | 6 sun hours | 7 sun hours |
|---|---|---|---|---|---|---|
| 100W | 12 Ah | 18 Ah | 23 Ah | 29 Ah | 35 Ah | 41 Ah |
| 200W | 23 Ah | 35 Ah | 47 Ah | 59 Ah | 70 Ah | 82 Ah |
| 300W | 35 Ah | 53 Ah | 70 Ah | 88 Ah | 105 Ah | 123 Ah |
| 400W | 47 Ah | 70 Ah | 94 Ah | 117 Ah | 141 Ah | 164 Ah |
| 600W | 70 Ah | 105 Ah | 141 Ah | 176 Ah | 211 Ah | 246 Ah |
| 800W | 94 Ah | 141 Ah | 188 Ah | 234 Ah | 281 Ah | 328 Ah |
Read the 400W row across and the picture becomes clear. In midsummer at 6 peak sun hours a 400W array delivers 141 amp-hours a day, which covers a comfortable two person build with room to spare. In midwinter at 2 sun hours the same array delivers 47 amp-hours, which does not cover the fridge and the heater alone. Nothing about the hardware changed. The solar output by sun hours chart holds the same grid in a printable form with the intermediate array sizes filled in.
Set that against your own consumption figure from the power consumption calculator. The honest planning question is not whether the array covers the load on a good day. It is whether the array covers the load on the worst plausible day of the trip, and if it does not, whether the battery bank is large enough to bridge the gap until it does.
How much array can the controller take?
A charge controller is rated by its output current in amps, not by panel watts, and the conversion is the array watts divided by the battery voltage. A 400W array charging a 12V bank delivers up to roughly 30A at the battery, which is why a Victron SmartSolar MPPT 100/30 Solar Charge Controller (Bluetooth) at 30A is the default pairing for that size. Push the array to 600W and you need 45 to 50A, which means stepping up to a Renogy Rover Lite 60A MPPT Solar Charge Controller (150V PV) or its equivalent.
| Array | Peak charge current at 12V | Controller size | Controller output fuse | Minimum cable, 10 ft run |
|---|---|---|---|---|
| 100W | 8 A | 10 A | 15 A | 12 AWG |
| 200W | 16 A | 20 A | 25 A | 10 AWG |
| 300W | 23 A | 30 A | 30 A | 8 AWG |
| 400W | 31 A | 30 to 40 A | 40 A | 8 AWG |
| 600W | 47 A | 50 A | 60 A | 6 AWG |
| 800W | 63 A | 60 to 70 A | 80 A | 4 AWG |
Two things about that table matter more than the numbers. First, the controller output fuse protects the cable between the controller and the battery, and it lives at the battery end, because that is where the energy is. Second, the cable column assumes a 10 ft one-way run and a 3 percent voltage drop target. A longer run needs a larger conductor, and the wire gauge calculator will tell you which. Never assume the cable that came with a panel kit is correct for your run.
The panel side of the controller has its own limit, the maximum PV voltage, and it is the one that destroys controllers. Panel open circuit voltage rises as temperature falls, by roughly 0.3 percent per degree C below 25C, so a series string that measures 90V on a warm afternoon can exceed 100V on a cold clear morning. Check the string voltage against the controller ceiling using the coldest temperature you will ever park in, not the temperature on the day you wire it.
Flat, tilted or portable?
A flat roof panel is the baseline: it produces whenever the vehicle is anywhere, it needs no setup, and it cannot be stolen while you are away from camp. Its weakness is angle. In midwinter at northern latitudes the sun never climbs high enough for a flat panel to see much of it, and output can fall to a third of the summer figure.
A tilted panel recovers most of that, typically 10 to 20 percent over the year and considerably more in winter, at the cost of a tilt mechanism you have to remember to fold down before driving. A portable panel such as the EcoFlow 220W Bifacial Portable Solar Panel (IP68) that you aim and move through the day does better still, often 30 percent or more above a flat panel of the same rating, and it has a second advantage that no roof panel has: it works when you park in shade, because you can put the panel in the sun and the vehicle under a tree.
The sensible answer for most builds is a fixed roof array as the baseline plus one portable panel as winter and shade insurance. The portable versus roof mounted comparison works through the trade properly, and how to install solar on a van covers the fixed install end to end.
What this calculator cannot tell you
It cannot tell you the weather. Peak sun hours are long run averages and a run of four overcast days will deliver a fraction of the figure the calculator prints. That gap is exactly what the battery bank is for, which is why solar sizing and bank sizing are one decision made twice rather than two separate purchases.
It also cannot tell you whether the roof will hold the array. Panels are light, roughly 14 lb for a 100W rigid panel, but brackets, rails, cable, a gland box and the hardware to mount them add up, and many vehicles have a lower dynamic roof load limit than owners expect. Run the payload calculator with the roof figure from your own owner's manual before committing to an 800W array.
Finally, everything here is arithmetic drawn from published manufacturer specifications and standards documents. It is researched guidance rather than an electrical certification, it does not replace ABYC E-11 or the installation manual supplied with your controller and panels, and any lithium installation should be inspected by a qualified installer before it carries load. Every circuit in the array, on both the panel side and the battery side, must be fused at the source of power at or below the ampacity of the smallest conductor it protects.
Where to go next
- Battery bank calculator, to size the storage this array feeds
- Best solar panels for a van roof
- Best solar charge controllers
- MPPT versus PWM, and what the difference is worth
- Solar panel mounting guide
Frequently asked questions
How many amp-hours will a 100W solar panel produce in a day?
Roughly 25 to 35 amp-hours a day flat-mounted on a roof in good summer sun, and 8 to 15 in winter. The calculation is 100W times peak sun hours times a 0.75 real-world derate, divided by 12.8V. At 5 peak sun hours that is 375Wh or about 29 amp-hours. One panel is approximately one compressor fridge, which is a useful thing to hold in your head while planning.
What is a peak sun hour?
One peak sun hour is one hour of sunlight at 1,000 watts per square metre, which is the standard intensity panels are rated at. A location that receives 5 peak sun hours does not get 5 hours of daylight, it gets a full day whose total energy equals 5 hours at full intensity. That is why the figure is roughly 4 to 6 in most of the United States in summer and 1.5 to 3.5 in winter.
Why does my array never hit its rated output?
Because the rating is measured at 25C cell temperature under laboratory light at a perfect angle. A panel on a dark vehicle roof in summer runs at 55 to 65C, and output falls about 0.35 percent for every degree above 25C, which is 10 to 15 percent gone before anything else. Add soiling, cable loss, controller conversion and a flat mounting angle and 75 percent of nameplate is a realistic ceiling.
Should I wire panels in series or in parallel?
Series raises voltage and keeps current low, which lets you use thinner cable from the roof and lets an MPPT controller start working earlier in the morning. Parallel keeps each panel independent, so shade on one does not collapse the others. Series suits large clean arrays on long cable runs, parallel suits vehicles that park under trees. Check the controller maximum PV voltage before wiring anything in series, allowing for cold weather voltage rise.
Is more solar always better than more battery?
Usually, per dollar. A watt of panel typically costs less than the battery capacity needed to store what it produces, and panel output arrives every day while battery capacity only buffers. The limits are physical rather than financial: roof area runs out, and an array that outruns the controller or the battery charge acceptance rate is wasted. Once the roof is full, more battery is the only remaining lever.
Does an MPPT controller really produce 30 percent more than PWM?
In the right conditions, close to it. A PWM controller pulls the panel down to battery voltage, so a panel with a maximum power point of 18V delivering into a 13.2V battery loses roughly a quarter of its potential. MPPT converts that excess voltage into extra current instead. The gap is largest in cold weather, when panel voltage rises, and smallest on a hot day with a nearly full battery, where it can fall to under 10 percent.
Electrical safety: 12V is low voltage but not low energy. A lithium house battery can push several hundred amps into a short circuit, and undersized or unfused cable is one of the most common causes of vehicle fires in DIY builds. Every circuit must be fused at the source of power, within a few inches of the battery terminal, at or below the ampacity of the smallest conductor it protects. Size cable for voltage drop as well as ampacity, torque terminals to specification, and have a lithium installation inspected by a qualified installer before it carries load. The figures on this page are researched guidance, not a substitute for ABYC E-11 or your component manufacturer's installation manual.
How we choose: we compare published manufacturer specifications, standards documents including ABYC E-11, and verified owner reviews. We do not test gear in person. Vehicle payload, roof load and charging limits vary by model and year, so confirm yours against the door jamb sticker and the owner's manual rather than any number published here.
Totalling your own build weight against your payload? The Overland Build & Power Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.