# How does the efficacy of solar cells on Mars compare with Earth?

Mars gets less than half the light that we get on Earth and there are dust storms, but the atmosphere is much thinner and there are no clouds. After all factors have been considered, how effective are solar cells on Mars (compared with those on Earth)?

Or, in other words (if you prefer a more precisely-defined question), how much harvestable solar power is available (per unit of area) on average on the respective equators of Earth and Mars?

I'm asking in order to understand possible power solutions for hypothetical future colonies. So assume expensive panels with all the bells and whistles like heat regulation, solar tracking (if that's a thing), etc.

Since cloud coverage varies around the Earth, I'd like to get best and worst case scenarios (no clouds and 100% cloudy all the time, respectively) to put the average solar power output potential of Mars into perspective.

• This is an interesting question. One problem is that on Earth, annual cloud cover ranges between almost zero to almost 100% depending on where you look. Africa, Australia, Western regions in the Americas all have large areas of very low average cloudy skies. How would you like the comparison done? Also, temperature matters - can we afford to add temperature regulation systems, or do we have to plop them down on the ground and live with whatever temperature they happen to find? – uhoh Feb 13 '17 at 1:27
• Yes add temperature regulation. Looking with regards to an optimized system for a long-term colony. As for the cloud coverage, we'll have to do best and worst case scenarios. The reason I'm asking is to put the estimated Mars power performance value into perspective. Thanks. – JSideris Feb 13 '17 at 4:46
• OK great! Can you edit your question to make it clear there? Comments are not permanent. Any important clarification or change should be made in the question itself, where it will continue to be seen. – uhoh Feb 13 '17 at 5:23
• This NASA paper goes into detail of what they had to consider in terms of solar power for the rovers. – Adwaenyth Feb 13 '17 at 13:07
• Possible duplicate of Solar panels on Mars? – david_c Aug 15 '18 at 16:18

## 1 Answer

You might want to check out solar potential maps.

This gives you your "on average" potential power harvested for the Earth, by summing up the daily solar potentials - which builds in accounting for weather, night etc. According to this map, the best areas are near the equator in Chile and slightly north of the equator in Chad/Libya/Sudan, which produce around $2800 kWh/m^2$ per year - roughly $320 W/m^2$.

On Mars, the calculations are slightly more difficult. Mars' irradiance is roughly $590 W/m^2$ - half that of Earth. Mars barely has an atmosphere, however - it's around 0.5% of Earth's so we can essentially ignore atmospheric effects. Temperatures on the Martian equator vary significantly - around $25 ^oC$ at noon, and around $-60 ^oC$ at night (I have no idea how this varies with Mars' orbit, so take that as you wish). Given that the panels won't be operating at night, and Mars has an Earth-ish temperature at day, I don't think temperature is too big of a factor - Mars is colder than Earth, but Solar panels don't mind the cold. NASA is running solar powered rovers on Mars, and they seem to be doing fine, until they got caught in dust storms.

Dust storms are a big problem. According to this research paper from 1993, there is a 1 in 3 chance of a planet-wide dust storm occurring on Mars every year. These last a few weeks - a few months, on average. There are also "smaller" continent sized dust storms which last a few weeks as well. For the sake of argument, lets say that there is a 10% chance that our base is in a dust storm at any given point in time. Martian dust storms vary in intensity - lets say that, on average, 2/3 of sunlight is blocked. Accounting for the day-night cycle, we have a solar potential of $275 W/m^2$ - this seems like an estimate on the high end, and a low end estimate is probably around $200 W/m^2$.

Again, these figures are just potentials. I haven't accounted for efficiency (around 20% - feel free to calculate that if you want). I also haven't accounted for the panel lifespan - on Earth, this is 15-20 years. On Mars, I expect this to be significantly lower due to dust storms.

EDIT: Martian Irradiance

EDIT2: According to folks who did detailed calculations on Reddit, the average value for irradiance is right around 100-120 W/m^2. This figure better accounts for dust storms, variations in orbital distance, Martian tilt, etc. This means that Martian solar panels, at max, would be around 1/3 as efficient as their most efficient counterparts on Earth.

• Can you add some sources/calculation to back up the estimate of Mars irradiance being 590W/m2? In this reddit thread for example, it seems that Mars irradiance might be a lot lower (in the range of 100-150W/m2). reddit.com/r/Colonizemars/comments/79k3z4/… – BlueCoder Sep 4 '18 at 15:39
• @BlueCoder Found several sources on the 590 W/m^2 so I think that is the right value. That being said, the Redditors look like they know what they are doing - I think they did similar estimates as to what I did, taking day/night and dust-storms into account. The question then becomes as to why my estimates are 1.5-2x their estimates, which I'll look into. – Nikhil Murali Sep 5 '18 at 11:22
• Yes. At the top of the atmosphere , it should be around 590 W/m^2 given that Mars is at an orbital distance of 228 Gm versus Earth's 150 Gm, and then using the inverse square law.. (150/228)^2 * (1362 W/m^2) ~ 590 W/m^2. The trick is the atmospheric filtering. – The_Sympathizer Sep 6 '18 at 4:43
• Yes, I agree that one of the tricks that reconcile the data is that 590 W/m^2 does not account for atmospheric filtering. Beyond that, however, 590W/m^2 is also if you just face the sun constantly. A more complex calculation shown in the following link, shows that even at the top of the atmosphere irradiance is a lot less if you account for your position latitude on Mars and the day/seasonal effects (i.e. you do not always face the sun the same way). web.archive.org/web/20180221172014/http://ccar.colorado.edu/… – BlueCoder Sep 6 '18 at 10:56
• Just search this site! Solar Flux on Mars Surface FYI 590 W/m^2 may be the solar constant in space at 1 AU of 1361 W/m^2 multiplied by the ratio of Mars's semimajor axis to Earth's, squared (which is roughly 0.43). See this calculation. So numbers for a fixed panel for example will need both seasonal adjusting and deal with atmosphere and planet rotation. – uhoh Sep 7 '18 at 2:36