Collection, Storage and Capacity
So I work quite a bit with solar energy systems (panels, batteries, etc) for fun and for work. I also participate in a number of online groups on the subject. I find it interesting that some of the most basic amateurs know more than many tech-minded people but there always seems to be a large number of people somewhere in the middle who are trying to get a grasp on many of the concepts involted, most importantly is how much power they will need.
In another rambling "Battery Backups 101", I took the approach from wwhat the numbers really mean, but I did not dive too deep in sizing your system. Also because UPS sizing is quite different due to the nature of their systems, please oh please do NOT use this rambling to size your UPS system! Different UPS systems (based on brand, model, series, etc) indicate their output power differently from one another and they may not be stating the Watt-Hour power, but instead the output power based on other factors. Perfect example? The CyberPower CP1500PFCLCD is a 1kw (1,000 Watt) system. On half load (500 Watts) you SHOULD go for 2 hours and full load SHOULD give you 1 hour IF the system was based on a Watt-hour, but it's not so it won't. Half load gives you 10 minutes and full load gives you 2.5 minutes. While this specific UPS is PFC (a more reactive type of power for switching over to/from battery/line power more clean), there may be other factors of why the Power rating is 1kW and lasts a fraction of a Watt-hour. So again, this is NOT to size your UPS UNLESS you contact the manufacturer and get them to tell you the Watt-hour of the specific unit.
What-hour. WATT-hour! Energy is stored. Stored in batteries. Different chemistry of batteries have different characteristics. Some are extremely senstive to temperatures of varying degrees and some are quite tolerant. The perfect example, a AA battery. Are they all made the same? HELLS NO! It's a given that batteries such as AA, AAA, C, D and 9-volt manufacturers NEVER show any technical specifications about their batteries ON the batteries, it leaves a lot to be desired so we won't even talk about them. Also since we are really only concerned with batteries that can be recharged (no not the AA or AAA as I said we're not talking about them), we can talk about either 18650 Lithium, 18650 LiFePo4, 12V SLA, 12V Lithium and even 12V LiFePo4 batteries. Generally speaking these will be the more common batteries you may be working with when dealing with charging systems. The Tesla batteries are 18650 battery packs. Nissan Leaf is as well. Many power enthusiasts make their own power walls from 18650 batteries. Since LiFePo4 batteries are so dang expensive you'll find Lithium most common. For about 5 bucks USD you can have a 3.7VDC 2200mAh battery. If you want a 12V 100Ah Lithium power bank you will need 182 Lithium 18650 batteries rated at 2200mAh. No I am not giving a lesson on battery arrangements right now so forget that.
I call these ramblings for a reason. Get it yet?
The Voltage multiplied by the mAh (milli-Amp hours) will tell you how many total watts you can get from your batteries. My solar shed has 2 batteries that are 12V each. They are each 35 Amp-hours. They are tied in parallel which means the Amps (and Amp-hours) are doubled, but the voltage remains at 12 Volts. So that is 70 Amp-hours. 12 Volts times 70 Amp-hours is 840 Watt-hours. This means that my batteries have a total capacity of 840 Watts. If I connect a 30 Watt flood light to my battery, it will be drained in 28 hours. How? 840 Watt-hours divided by 30 Watts = 28 Hours.
So whatever I connect to my battery system should be less than the total capacity of my batteries. I mean I can over load it but that means that 2 things must be considered. First is the fact that I have 840 total watts. If the item I am powering requires 840 Watts per hour, that is 840 divided by 12 Volts, so that is 70 Amps. That is REALLY dangerous! I would need to ensure that the connection cables are correct otherwise you WILL see a bonfire! Oh yes that many electrons traveling that fast will heat those cables up really fast. Second, do I not love my batteries that I would force them to work that hard? Let me give you an example. 12 LED bulbs, 8 Watts each. That's a total of 96 Watts per hour. With 840 Watts total power in the batteries, that's 840 divided by 96 so I will get about 8 hours and 45 minutes of run time.
When considering a remote solar setup for a customer site, I may have 3 cameras, 1 Wifi AP and 1 LTE or Long-range radio that require power. The cameras are about 4 Watts each, the Wifi Ap is about 7 Watts and the LR radio is about 9 Watts. So we have a total of 28 Watts. We need to ensure that the system runs for 24 hours a day and has enough power to survive 3 days minimum without sun (overcast, rain, they will all produce electricity but not the full rated potential of the solar panel). So considering 28 Watts per hour, we need to calculate 28 Watts over 3 days (72 hours) and we get 2,016 Watts. So our battery need to be able to hold that much energy so 2,016 / 12VDC is 168Ah. That is the minimum size battery. Since finding a specific battery of that size would be difficult (and expensive) it would be best to oversize the battery to 200Ah (or just get 2 batteries at 100Ah each and connect them in parallel). This would increase the total available power to 2,400 Watts and actually provide up to 85 hours (3.5 days).
So to be clear, what I am saying is that in order to size your battery you need to know how many Watts you need per hour as well as per day. We know this setup requires 28 Watts every hour or 672 Watts every day. Then multiple that figure by the number of days to sustain the requirement. There are times I am asked for a 5 day system for this type of setup. That would be 3,360 Watts or 280 Ah. So again, 3 batteries that are 100Ah each in parallel would satisfy this.
One thing I touched on that cannot be ignored is charging the batteries. In this case we are using Solar. Everyplace on the planet has different ratings for how much optimal Sun (peak Sun irradiance or Solar Insolation) is received so that is a major consideration. There are plenty of maps out there that can tell you how much peak Sun you can receive based on your area but I have found that these maps are simply snapshots of time. In my area (the southern part of the NorthEast US) we are rated about 4 to 4.5 hours per day direct sunlight. In the summer, an extra hour is added and in the Winter, an hour is removed. So I am assuming these maps show the mean peak Sun hours and not directly related to any time of year. Different areas of the world may sway quite differently. For example, Alaska in the winter may receive less than 30 minutes while in the summer it may be 8 hours peak. Southern California may be consistent at about 6 to 7 hours per day. As I have not been to those areas year-round I cannot say for sure, but I can assume from the Earth's rotation around the Sun each year and the tilt of the Earth during the rotation this may be correct. Never gave it much thought beyond my region to be honest so I may be speaking out of my ass for all I really know (or care).
So my area is 4 hours so I need to desgin a panel system that will collect 80% off the required power within 4 hours. If I have a 672 Watt per day requirement and I can only get 4 hours of peak Sun per day, that means 672 / 4 hours is 168 Watts per hour collection. But wait! These panels are NOT 100% efficient so let's add a little buffer on there. Since panels are 5 Watts, 10 Watts, 25 Watts, 50 Watts, 100 Watts, 200 Watts, 300 Watts, etc, let's go with a generic of 200 Watts. Oh and we need to be sure that this is a 12V panel just to keep things simple. So over 4 hours I will collect 800 Watts (optimally, won't happen tho) but in the course of the day I should reach 800 Watts from peak and non-peak Sun. But as I said, it's not about the daily requirement, it's about holding out for a few days without peak Sun. So as I said earlier, I am looking at filling up 3 batteries at 100Ah each. That's 2,400 Watts in 4 hours. So 2,400 / 4 = 600 Watts per hour. So I am calculating that 2 panels at 300 Watts each in parallel would produce the required 600 Watts per hour, but these panels produce 24VDC. I would not be able to push 24 VDC into 12VDC batteries so either I would need to connect my batteries differently, which is not possible since there are 3 batteries or I would need to use a charge controller that converts the 24VDC to 12VDC and increases the output current. Ok so ...what? A charge controller. A device that is connect between the panels and the batteries. It controls the flow of the charge to ensure that the batteries are properly charged. There are 2 types, PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking). PWM is less expensive but cannot provide the conversion from the higher panel Voltage to the lower battery Voltage so MPPT it is. Any excess Voltage from the panels are converted to a higher current to create a steady stream of power into the batteries thus providing a more consistent charge.
Also let's remember one important thing. The devices are using power while the batteries are charging. During the charging period of 4 hours, the devices will use 112 Watts (28 * 4 = 112) so with this much power being used during the charging phase, we can assume the batteries are only receiving 488 Watts instead of 600 every hour. That means over 4 hours, the batteries are collecting only 1,952 Watts. This is about 2 hours shy of 3 days, which is acceptable.
So let's recap this mess but this time I'll use the universal language of Math.
Requirement: 28 Watts per hour (672 per day)
Run-time Required: 3 days (72 hours)
Run-time power required: 28 Watts * 72 Hours = 2,016 Watt-Hours
Peak Sun: 4 hours
2,016 Watts / 4 hours = 504 Watts per hour to be collected
Now the specifics on the technology to perform the collection and storage is important but less important for the purposes here. I used Solar as an example of sizing, you can use Wind but there are so many unknowns. Wind is less predictable than Sun in many areas and that alone creates an entirely new factor (like a mountain top may seem great but actually is not, whereas coastal is typically best for wind).
We now know what we need to do for sizing our system and storage of energy. We now understand that there are various technologies to help us achieve our goals. We also must understand that any technology that involves collection and storage of energy is not going to be cheap. Do not be fooled by low-priced marketing that seems too good to be true.
Just a quick side story related to this. We were recently affected by a storm that knocked out power for 3 days. My 840 Watt solar shed was helpful to provide power to our fridge for a few hours. But what if I needed power for the whole house? Let's do some quick math! If I were to solarize my home, this is what is involved:
Winter use requirement: 2.7kWh (65kWh/day)
Summer use requirement: 5kWh (120kWh/day)
So I need to calculate for my daily peak, which is summer (damn A/C).
To me it's obvious that I need a battery bank (a group of batteries interconnected to provide greater storage). Since DC voltage drops as it travels over distance it would be best to use a higher voltage rating for both the panels and batteries. So 48 Volt batteries (or a 48 Volt battery bank) is needed. 120kW total per day / 48 Volts means I would need 2,500 Ah of capacity in my battery bank. We can do the math 2 different ways.
5kWh / 48 VDC = 104.167 Ah then 104.167 * 24 = 2,500 Ah
120kWh/Day / 48 VDC = 2,500 Ah
Either way, we need a total of 2,500Ah to store energy for each day. Now I need to charge these big nasty buggers. I would want to use the largest-size panel I can get, which right now is 400 Watt Monocrystaline. So now I am thinking, 120kWh/Day over 4 hours means I need to collect 30kW each hour. Given that the panels are 400 Watts each, I would need:
30000W/400W = 75 Panels
My rooftop is about 15 feet by 40 feet so that gives me a total of 600 sq ft (or about 86,400 sq in) to install. Each panel consumes about 80x40 inches (or about 3,200 sq in) so I would need to maximize the space so instead of individual mounting bracks, I would need to put then as close together as possible, so a rack is required. This means that in a perfect world..
86,400 / 3,200 = 27 Panels
This is about 1/3 the power I need (want really, I have a lot of computer equipment) but still not enough for my 120kWh summer time requirement. In fact 27 panels rated at 400 Watts each over 4 hours would only give me about 10,800 Watts which meets my hourly requirement however since I can only collect for 4 hours, I am only collecting about 2/3 the amount I need every day. So then I need to ask myself, WTF? Does this mean that I can never truly go off-grid? Well with my usage and requirement, yes. I would need to get more efficient. My dishwasher, clothes washer and dryer, central A/C, major appliances, they all are a part of the problem. Then I have my networking. Cisco gear are damn hogs! The consume a ton of energy. Especially PoE gear. I am reducing my computer footprint by using Micro-sized PC's which consume from 65 to 90 Watts each instead of masssive 500 to 1,000 Watt PSU systems. I will eventually have 5 computers and I am dead set on them each being 65 watts. There will be a few external drives plus laptops so total computing power will be under 1kW as opposed to the previous 3kW (my server was a hog).
Still not enough power so what is the point? Well if I put my head deep into this, I cna figure that my office/lab may be able to get solarized. So I have 5 LED displays, total about 150 Watts (30 Watts each). Then the computers, once reduced to 65W each will be 325W. So 575 Watts. I have 3 internal drives and 2 external drives. The internal are 3.5" so that's 9 Watts each totalling 27 Watts and the external are 2.5" @ 3 Watts each, totalling 6 Watts. So 575+27+6=608 Watts required. Now since all HDD's will not be always active, they may actually consume less, but we need to calculate for max power. So the computers alone, 608 Watts per hour, 24 hours is 14.5kWh/Day. Now divide that by 4 hours Sun, I would need to collect 3,648 Watts in that time. That would require 9 panels rated at 400 Watts each. This is a lot of power to collect. Now I would need a battery system. 14.5kWh/day divided by 12VDC is 304Ah batteries. So 3 x 100Ah would suffice. Again, we found the crutch in my house, my computers.
I mentioned I have the roof space for 27 panels, 1/3 of that would be for the office lab alone. This does NOT take into consideration any network gear. That will probably take another 9 panels, maybe 10. Between the PoE devices, the switches, printers, cooling/ventilation? is it worth it? Maybe but right now I'm not moving forward on this idea.
So let's roll back the clock about an hour into reading this and you'll see that I did mention UPS battery backups. How are they rated? VA's are useless to us. The Power output is useless because of some stupid method they use to determine the Wattage. It's not rated Wh, it's Watts. The Wattage noted on these systems are the maximum output power available. Not the capacity of the unit. So then what we would need to do is when searching for a UPS system, we would need to find a unit that has the correct battery size to produce the Ah or Wh required. The CyberPower unit mentioned has 9Ah @ 12VDC which is 108Wh. That's not much really. The big question is, is this 1000 Watts distributed over all 12 outlets or only the 6 battery outlets? Well it does not say! I assume it's all 6 since 1000 Watts / 12 VDC = 83.89 Amps. Over 6 outlets is 13.8 Amps per outlet. So that is rating is based upon the battery and the electronic components of the UPS and not necessarily the surge protection side which has no battery protection. So if the unit was 1450 Watts with 8 outputs, that would still be about 15 Amps per outlet. Most outlets are rated 15 Amps as in NEMA 5-15P. There are NEMA 5-15/20R outlets which can go up to 20 Amps then there are L5-30R which are twist-locking 30 Amp outlets for higher output. This means that the total power is distributed across all outlets and no one outlet can receive all the power even if you plug in only 1 device.
The point here is people that we need to understand the capacity of the storage device (in this case, the batteries) in order to know how much Power you will get and how long it will last.