A reader wrote in asking if she could create an emergency backup power source for her apartment with some solar panels.
This is an interesting concept, and has been promoted with some portable solar panel setups, implying their suitability for that sort of purpose.
A key challenge with solar panels is they need sunlight to work, raising the question of what to do at night. In fact, there are a number of questions, so it seemed like a great additional to our series of solving power cut problems.
As you can see, designing (and installing) a solar power system is complex. We don’t recommend you do it all yourself, but hopefully this article will help you understand the issues and parameters.
A Quick Introduction to Solar Cells
Solar cells are amazing devices, that almost literally create electricity from nothing – well, from nothing other than sunlight. Solar cell panels have dropped in price, and also have increased in efficiency, meaning that a certain area of solar cells now provides more watts of power than was formerly the case, and costs less. This is of course a good thing. With solar energy systems dropping in cost, and regular utility/grid/mains power slowly edging up in price, solar power systems are increasingly cost-appealing and viable.
Regular normal solar cell panels tend to measure either about 39″ x 65″ (containing 60 actual cell units) or about 39″ x 77″ (with 72 cells). There are other sizes too, but these tend to be the most common.
A single solar panel will generate between about 250 – 350 watts of power in direct bright sunlight. This means you get about 15 – 18 watts of power per square foot of panel. Panels typically have a working life well in excess of 20 years (some have still been going well after 40+ years); indeed, it is common for the panels to come with 20 – 25 year warranties on them. They seldom completely fail, but the amount of power they generate slowly declines, year by year (maybe by 0.25% – 0.5% a year). A panel weighs about 35 – 45 lbs, and probably costs a bit less than $1 per watt of power it produces – higher wattage panels will cost over $1/watt, lower wattage panels will cost less than $1/watt, because the most efficient (high wattage) panels involve more costly technology.
The power the panels generate is DC power, not AC power (such as you get from your mains power). It might be slightly above 12V or 24V or 36V, maybe some other voltage.
It is probable that a single solar panel with 250 – 350 watts of power is not enough for your needs, so the chances are you’ll want to have two or three or more panels, if you have the space for them. Most people of course place their panels permanently on their roof, but if you’re in a condo/apartment, maybe you don’t have a roof. Let’s talk some more about placement.
Solar Panel Placement
You want your panels to face as close to directly south as possible. This averages out the angle the sun is at – at solar midday, the sun will be straight in front, and of course, during the morning it will be to the east and during the afternoon to the west. If you can’t face your panels due south, that is okay too, but they’ll be less efficient and generate less total power during a typical day.
You also want to angle the panels, at an angle that is the same as your latitude. So if you are in Los Angeles, for example, your latitude is around 34° N and you’ll want your panels to be inclined 34°. If you’re up in Seattle, at 48° N, your panels will be on more of an angle, and so on. Again, this doesn’t need to be exact, but the closer to that angle you can get, the more power per day you’ll get from your panel or panels.
If you have temporary/portable panels, try to align them as best you can. The good news is if you’re keen to get every last drop of power, you could point your portable panels to lower on the horizon and east in the early morning, then to optimum for a few hours around midday, then lower and to the west for the balance of the afternoon. You can also get automatic tracking devices for permanent panels that change the panels’ tilt angle, but these are costly and usually can’t be cost-justified. It is less expensive and better to simply buy another panel or two than to spend substantial sums on tracking units.
Converting the Panel Voltage to Usable Voltage
The chances are that whatever DC voltage the cells provide is not convenient for anything in your home. The voltage will also vary depending on how bright the sun is and how heavy the loads you are running from the panel or panels.
So you need to convert the voltage from the output DC volts of the panel or panels, either to a standard DC voltage (for example, maybe you have 12V or 5V systems) or, more typically, to regular 110V AC.
Note that when connecting two or more panels together, you have a choice of connecting them in series or parallel. There are reasons in favor of each method of connecting, and it depends also on how many panels in total you have and the device you use to take the panel voltage and convert it to usable voltage.
This device will regulate the panel voltage and possibly change it from DC to AC. If it changes it from DC to AC, it is an “inverter”; if it is instead used to charge batteries, it is a charge controller.
A Problem with Solar Power and its Solution
Most of your electrical appliances expect and need stable power to run correctly. If the voltage drops too much, that can damage the device, or make it run slowly/weakly and not work properly.
If the sun goes behind a cloud, the power generated by the solar cells goes down to match the reduced energy that shines on it. The astonishing thing is that even though the day still seems “bright” to us, that cloud can reduce the energy from your solar panels down to 25% of normal; and if they are dark storm clouds, maybe even down to only 10% of full power. So your 300W solar panel was happily powering your computer (150W) and a couple of 15W light bulbs with power to spare, then all of a sudden, it drops to somewhere between 30 and 75W – the light bulbs dim and go out, and your computer does the same (and possibly damaging the data on your hard drive as the voltage slowly drops and dies).
Most people are surprised at how impactful clouds can be. There are other issues too, particularly if your panels are wired in series, where a partial occlusion on one panel then disproportionately “drags down” the performance of all the panels.
Not so surprising of course is that solar panels do not work at all at night, which is also the time you’re most likely to want lights in particular to be working.
One other issue – not so much a problem as an opportunity. Maybe some of the time, you’re not using all the power the solar panels are generating. The unused power is just wasted. But at other times, of course, you would ideally like to be able to use more power than the panels are generating.
The solution to all these issues is to add a bank of batteries to your solar panels. Have the solar panels power the batteries through a charge controller. Then have the batteries drive an inverter. That way, if you are generating excess power, it will be captured and stored in the batteries for later – when it gets cloudy or for night time. And when it does get cloudy, the system will switch from the solar energy to the stored battery energy, keeping the voltage and power flowing steadily.
Of course, this means that your total costs have now greatly increased from that suggested $1/watt for the panels alone. You need to add extra for installation, for an inverter, and possibly more again for a charge controller and some batteries too.
Are You in a Good Location?
Some parts of the country get more solar energy each day than other parts. In simple terms, the further north you go, the less power there is, as is shown on this map.
But notice the scale to the color coding. Even the lightest colored parts of the country are still generating power, and almost two thirds as much as in the darkest/reddest parts of the country. There’s still power to be had, everywhere in the country.
You’ll probably end up spending thousand dollars to create a fairly low powered basic stand-by solar power system, and if it is purely to be used as a standby emergency power source, it is something you mightn’t use more than once or twice a year. Compare that to a simple portable gas (petrol) powered generator which would cost $400-600 and give you 2000 watts of power for as long as you wished, day and night, and burn less than a dollar of gas per hour. So, with solar, while you’re getting “free energy” and limitless energy, the up-front fixed costs of capturing and converting that energy are enormous, and are unlikely to ever be repaid if you’re only using the system for a few hours, a few times a year.
But there is a way you can get better value from an investment in solar. However, it requires you to spend more rather than less money!
If you increased the capacity of your solar system, you could use it all the time to provide the basic power you need, and supplement it with grid power as and when needed. This is termed a “grid-tie” system.
You might even be able to sell any surplus power you generate back to your utility company – what is sometimes termed “running the meter backwards”. Even better than simply running your meter backwards, in some cases there are opportunities to sell the power you generate to your utility for more money than what you pay for the electricity you buy and consume from them.
In such a case, you’d want to be careful to ensure your system also has some battery backup for energy storage, so you can operate either together with the grid power or, if the grid power goes down, locally on your own without grid power.
Note also there are federal tax credits available to reduce the cost of purchasing solar panels and associated hardware. These might expire at the end of 2020, and of course, may change in terms of what is eligible and how generous the credits are. There may also be state/local credits, and sometimes your utility company might contribute to the cost as well.
The company you choose to buy your equipment from will be able to advise you on the current credits.
Depending on how your numbers and usage calculates out, you might end up getting your up-front costs back to you in 5 – 10 years, and then be enjoying ongoing benefits for decades into the future.