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.
Read More in the Rest of our Series on Solving Power Cut Problems.
- Identifying Your Power Needs
- The $10 Emergency Light that Lasts a Month
- Solar Power
- Small Portable Generators
Articles still to be published
- Larger Generators
- UPS units
- External Batteries
- Your Car Can Help
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.
6 thoughts on “Solar Panels for Emergency Power”
Typical roof solar panels generate between 210 and 300 watts of power. Using DC power, there are two ways to convert this power to AC.
1. Central Inverter – all panels connect to a central inverter.
2. Micro Inverter – each panel has its own little inverter.
The advantage of Central inverters is there are inverters with battery backup, and generator options. (e.g. outback radian)
The disadvantage of Central inverters is since all panels are set up in a chain, if one panel performs poorly, then all of them do. Since you are also dealing with DC wiring, you have thicker, more dangerous wires, and a more complex install.
Most tax credits are for grid tied systems. In a grid tied system, unless you have a top of the line central inverter system, (which there are no additional credits for in most states) your inverters shut down if the grid goes down.
Why? The answer is simple, there is no mechanism to prevent the appliances in your house from pulling more power than the panels will provide. As such, they disconnect as a safety measure to protect the inverters and panels.
Thanks for these comments.
It is a while since I last saw a (full size) panel with as little as 210 watts of power. Even 300W is the low end these days, and some of the 72 panels go up to 370 watts. See, for example, this site
I agree, there are solutions to the “you lose your power if the grid goes down” with a grid-tied system, but I felt that a bit complex for an overview article that was mainly focused on standby rather than online systems. And there are other concerns for why they do shut down as well, the utility doesn’t want your system still energizing the power lines when their power has failed. Makes it dangerous for their repair workers.
David, another issue with using solar as a backup power plan is that some non-battery supplied systems (all?) are grid-tied – there has to be power available from the grid for the inverters to function. So when there is a regional power outage – when you want the free solar power the most – your inverters shutdown. These days I only rarely see outages other than from hurricanes which are also mercifully rare, so it’s not a big issue. But it would be nice…
I totally agree – it is a colossal logic-fail to design a system that has, at its heart, the concept of being a standby system for power cuts, only to have the system itself require mains power in order to work!
Most “grid-tied” systems are, by definition (the word “tied” subtly implies this without making it obvious) dependent on the main utility power being available. But it is possible to get dual purpose systems.
David I agree. Mine were installed in 2012 and were 255w/panel back then. Also, I believe central inverters need(ed?) a minimum amount of generated power to begin providing power, like 1kw or something. Whereas micro-inverters just produce, all the way down to zero. But my info may be dated.
Ah, I remember those days. If I’m matching the years and the watts correctly, what you bought in 2012 were high-end at the time, but now of course, improvements in cell efficiency have seen stunning 40% increases in power.
The really exciting thing? Even the most efficient cells are still only about 20% efficient. There are new technologies promising >30% efficiency. I’ve no idea what the practical/theoretical limits are on efficiencies, but we’re still moving up the curve.
As for micro-inverters, their main advantage is that they avoid the effect of a series-connected array of panels whereby if one panel becomes less efficient, it drags down the entire set of panels. You get slightly better net “real/usable” power from micro-inverters, albeit at a higher up-front cost in the first place.
If you’re off-grid needing every last watt of power, and/or if you only have a very limited area to deploy panels, then of course you’ll chase after every watt at almost any cost. But for most of us, there comes a point of “vanishing returns” where it no longer makes sense to keep enhancing the net power received.