You hear about it all the time. Trendies love chalking it up as the solution to all the world’s problems. The view is pretty good from stadium seating, but let’s take a little walk down to the arena floor and see what’s really involved with setting up a solar grid of your own.
I don’t even want to try tallying up the hours I’ve invested in battery research, inverter hook-ups, MPPTs, panel topology, and the like. I also don’t want to drag you down with a bunch of new concepts. I just want to deliver a clean, easy to follow batch of worries you will run into if you decide to add a little solar power to your own life.
Are Solar Panels Worth It?
In general, I’m going to say “no” here. You might be surprised to hear that, but hear me out. When thinking about a solar setup, most people want to convert the home they have now. They want to keep the same lifestyle, the same appliances, the same life, and somehow magically eliminate their energy bill.
Generally speaking, this isn’t going to happen unless you have a ton of cash to spend.
On the other hand, there are off-gridders who get by with a handful of scrap car batteries they picked up for free and a simple MPPT (which is basically a controller that charges your batteries from the solar array input). They use car-style inverters for 110v power when they need it, but design their power grid around 12v DC appliances. The setup is similar to that on an camper.
I fall in the middle. I want my house to have some modern luxuries, but there is a lot of stuff that I’ve done without. You don’t see it at a glance, but my little home here runs pretty darn efficiently where power is concerned. I don’t have central air and heating though.
The Biggest Energy Wasters
Your heater and air conditioning draw more power than anything else in your home. Appliances like a fridge or a big freezer come next, but they’re an order of magnitude lower in usage.
This means that setting up a solar grid to power your whole house gets much simpler if you dump these energy monsters. On the A/C side, I don’t know what to tell you. Most central air houses can’t be easily renovated for cooling, but there are alternatives to electricity.
I find that the best way around the energy use problem is using a diversity of different fuels and tools to accomplish your goals. This summer, for instance, I’ll be experimenting with a small root cellar if time permits. I heat my house with wood, and my water with LP gas (propane). Each appliance in my house is selected based on the best solution for me, not what some magazine says is best.
To put it shortly, if you don’t want to pay an electric bill, stop using so much electricity. Easier said than done, right? But that’s the ticket, and that is the response you will get from experts on forums and solar companies when you discover that your array is going to cost far more than you expected.
Solar is Cheaper than it’s Ever Been
Despite the above warnings, some panels can be had at nearly fifty cents per kilowatt, which is really cheap compared to when I started researching solar systems with serious intent. The charge controllers and inverters are coming down in price as well, as demand continues to increase, and the factories that make them can take advantage of economies of scale.
Shortly put, if you have reasonable goals for your solar setup, it’s a great time to get started.
Break Down Your Energy Use
Your average refrigerator uses around a kilowatt hour (kWh) per day. To figure up usage for any individual appliance, take the amp draw from it times 120 (amps times volts equals power), and multiply that by the hours per day that device runs. If you know the wattage, it’s even easier. A 100W light bulb which runs for 8 hours every day consumes about 800Wh, or 0.8kWh.
Here are some handy little formulas:
- Amps X Volts = Watts
- Watts X 1000 = Kilowatts
- Watts X Hours = Watt Hours
- Kilowatts X Hours = Kilowatt Hours (kWh)
An even easier way, if you are staying in your own house, is to simply check out your electric bill. If you are drawing 1000kWh per month, that’s around 33 kWh per day (1000/30). For all the cool gadgets out there to measure and meter energy flow to each appliance, on-grid systems can really benefit from simply checking the bill. This also makes a hell of a reference point for reducing your energy use overall. Replacing an old water heater can sometimes be much more cost effective than building out a solar array big enough to keep the old clunker running. Closing the shades on your windows can make your current A/C more effective, as can adding insulation in your attic. Making the most of a solar system really does including thinking outside of the box of solar panels.
Panels and Loads
Let’s say you’ve figured up around 20kWh per day for your system, just to toss a number out there. You don’t go straight to buying a 20kW system. This seems counter-intuitive, but solar panels are rated by output. That little ‘h’ makes a huge difference.
Check Google, and see what kind of peak sunlight hours you get at your location. Many things can affect this number, so it’s best to use a map. In predicting what kind of output you can get from your system, this is the number that everyone likes to reference, because it takes into account your latitude as well as average cloudiness and other factors.
Generally speaking, in a perfect world, if you lived in an area with five peak hours per day average, a 4kW array would meet your 20kWh energy demand. This is just a starting number.
The reason is that the solar panels are running at high output, around 4kW per hour, for five hours per day.
For a grid-tie system, this probably makes the most sense, and it’s easy to figure. Just don’t expect a miracle. Excess dirt, snow, or even a single leaf resting on a panel will drastically affect it’s output. Heat also affects the output. At the sunniest time of the year, panels often don’t perform as well due to running at temps much higher than specification.
For off-grid and battery backup systems, things get a bit more complex.
Batteries are great. They let you “charge up” once per day, and then meter out the energy at the rate you use it, instead of only giving you power while the sun is shining. They can quickly become a headache, however, as you struggle to worry how many of days you may need to depend on them if a storm rolls in or the sky clouds up on you.
For our 20kWh/day example, 3 days of power would be 60kWh, and that’s a whole lot of battery. My current bank is around 18kWh of lead-acid storage, and it cost me right at $2000.
The first murky culprit appears soon after you begin sourcing batteries. It’s never a good idea to cycle batteries below 50% of their capacity, so now you are looking at at least 120kWh bank to accomplish what you wanted, a 60kWh reserve. Again, conservation is the key. Often the A/C can be stepped down during cloudy weather, because it’s cooler outside. The opposite case goes for heaters. When it’s cold and gray, you need more power.
Another problem, which isn’t discussed as much, is the charge rate. Just because your panels are making 4kW, it doesn’t mean that all of that juice is going back into your batteries. Most charge controllers meter out power in steps. My inverter/charger will put out a maximum of 40Amps to my battery bank. My MPPT is rated to 100A. That means my generator (which powers me up through the inverter) can deliver 1920 Watts to my 48-volt battery bank. (40A X 48V) My MPPT running at full power could dump up to 4800 watts, but again, that isn’t the whole story.
Chargers will run peak current flow only to a certain point, and that point is around 80-90% of the battery’s full charge. At that point, the charging amps start to drop off to protect the battery. Basically, there’s a maximum voltage that can be applied to battery posts in order to charge them, and as the battery fills up, the charge time gets slower and slower. At 98%, the charger will go into another mode, called “float.” In this mode it drops to a lower voltage for hours to top off that last 2%. If your system is sized properly, this is where your batteries will live, between 80-98%, most of the time. That means slow charging most of the time.
Batteries themselves make sizing the whole system difficult, just by their nature. Awesome for starting your car, but a bit fickle if you want to use them for off-grid power storage.
It basically goes like this. You’re batteries start the night at 90%. You draw your current for the night and morning, and when it’s all over, they might be around 65%. A couple hours of peak sunlight bumps them straight up to 85%, and the charging falls off gradually over the next several hours. A float charge might last you till sundown, and you’re good to do it all over again.
If you’re going off-grid, oversize the battery bank a bit.
The other thing is the setup. Putting your batteries in series increases the voltage of the system, while wiring in parallel increases the amp hours. We could talk topology for hours, but there are essentially two systems worth considering. 12V and 48V for the final battery bank output. 12V systems have the advantage of running all manner of little gadgets designed for cars and RVs. 48V systems let you get the most “bang for your buck” from charge controllers, since the same amp output provides four times the power of a 12V system.
12V systems that run AC outputs (120V or 240V alternating current to a house) require big, fat wires and need to be monitored carefully so that you don’t draw too many amps and overload them. In general, these are great for tiny systems that won’t draw a ton of amps, light general lighting and electronics, maybe a little micro-fridge designed for a 12V system, etc. For big stuff, go 48V.
Converting amp-hours (battery talk) to watt-hours (inverter and solar panel units) is pretty easy. The amp-hour rating of the battery (specifically at the 20-hour discharge rate) times the voltage at the terminals will give you the watt-hour capacity for the battery. (This changes based on temperature and how fast you drain the batteries, as well as other considerations) Multiply this number times the number of batteries in your array, and you have a pretty good idea of the kWh that they will hold.
I’m using eight Trojan L16C batteries at the moment. Each is 6V and the 20-hr discharge rate gives me around 370amp-hours according to the little pamphlet. 370×6 is 2,220 watt-hours per battery, times eight is 17.7kWh for the whole bank. Remember, this only gives me a “usable” 8kWh, and actual use to keep my batteries from over discharging, and to keep from running my generator all night to top off the last 5%, is about 4-5kWh. This is pretty much my daily limit before they need to be topped off again, but for my current setup this time of year, that’s around 4-5 days worth of power, so it works out for me. Again, conservation beats a bigger system.
Inverters and Charge Controllers
Inverters convert battery power to house power. Charge controllers take power from the grid or generator or solar array, and convert it into battery power. That’s pretty much all you need to know.
If you aren’t good with electronics, I recommend getting help installing the electrical system, but the outset of it is pretty basic. You buy the inverter for what you need, and ensure that it will connect to your battery bank. I have a 4kW pure sine wave inverter that connects to a battery 48V system. The output is 240 single-phase, which can be split into two 120V A/C lines. This is a pretty common, general setup for powering a house panel. It also has a built-in charger that will run off 240V grid power (or my 240V generator).
Sizing your inverter is pretty easy, but don’t get too close on the math. Go with an inverter a bit larger than what you think you will need. It basically needs to be big enough to run everything that you want from it at the same time. Though my water pump doesn’t get used much through the day, when it’s on, it draws a lot of power. I sized mine to run the pump, a small A/C window unit, a fridge, and a few other things at the same time.
There is a drawback to going big. Most inverters will eat a percentage of their design output power to stay on. Some have a power-saver feature, but I have not found this particularly useful. My inverter uses more power to be on than all the LED lights currently installed in my little house. I’ve gotten into a habit of shutting if off when I don’t need power. Have to use my phone to find it again in the morning, but that’s a small price to pay, and it extends my batteries a long way. Remember, even a little 50W trickle adds up. Over 24 hours of operation, that 50W constant draw becomes 1.2kWh, about the same as a modern refrigerator uses in that same span of time. Let your noodle fry in that pan for a bit.
That’s about it for now
This article is getting much longer than I hoped, but I think I got some of the pitfalls out of the way. Here’s the short version. Reduce your energy needs as much as possible. Figure out how big of an inverter you need. Buy enough panels by sizing toward your peak sunlight hours and needed usage, and build out the rest of the system accordingly. You can always add another “row” of batteries later if you need them.
I built my battery bank and inverter first, and currently use a generator to top off the bank. I’m sizing my solar array for summer usage, enough to run a window a/c. Next winter, I expect to have plenty of spare power, and almost never to need the generator during a season it doesn’t like running in. This will give me plenty to use for indoor cooking and such.
Basically, my inverter and appliance needs dictated the rest of my system. Looking back, 4K was a bit small, but I’ll run with it till it needs replacing at the very least. Working around the available power has actually been a blessing, as it’s forced me to reconsider how I’ll handle tasks like washing clothes. As a result, I’m more efficient and require less on the input side (from a solar array, generator, or any other source of electricity). That means lower overall costs forever.