How to do solar/wind
I've always looked at ways to reduce our cargo when going out to the land. A big issue is needing a generator with enough fuel. The noise is also a problem during hunting seasons. For that reason, I learned about solar and wind. I will explain the concept of building a system to give you an idea of what's involved and cost. You should consult with a professional about installing a solar/wind system.
Solar? Wind? Battery? How much power do we need? This requires some knowledge of electricity and basic math. I would need to power the RV that is on our hunting land. My average usage is about 2A (amp) with spikes up to 5A while running the air conditioner.
It's best to know your actual power needs which can be measured in many ways. My generator displays the usage but it could also be done with hand held meters. You can also look at the specifications on the appliances you need to run. Everything lists the power requirements from the manufacturer. Your power consumption is the single most important information you need when designing a system. If you are not familiar with electricity then you should consult with a professional.
To make the calculations we have to put everything in the same terms. It is best to use watts as the common term. Watts (w) can be calculated by multiplying the volts (v) with the amps (a). When the RV is connected to a generator or utility it uses 120V AC with a max of 30A. For this example, I will use the numbers for the max of the RV which is 30A.
The RV has a maximum of 3,600w. (120 x 30) Kilo-watt hours (kwh) is just the amount of power used over time. If I used 3,600w every hour then my consumption would be 3.6 kwh. This is probably more what you use in your home.
Solar/wind come in many voltage configurations 12v, 24v, and 48v. (There could be others but there aren't common and I haven't see such kits available.) How do you know which you should use? It's mainly a question of the size of wire you use. Wire must be of a certain size to handle the amps. Electricity traveling through a wire generates heat. An undersized wire will melt or catch fire if the rating is exceeded. The reason why fuses and breakers are used in circuits is to prevent wires from exceeding their limits.
A 12V system would have 300A. This is calculated by the Watts divided by the volts. (3600 / 12 = 300) In addition to wire size, your batteries and charge controller must be rated for the amps. 300 is too big. Increasing the volts will reduce the amps. In this case, I would select a 48V system. (3600w / 48v = 75a).
At 75a, I will need 3 gauge wire to connect the system. Check the specifications on batteries and controller for their max. We can also put in fuses/circuit breakers for 75A. The higher voltage systems are usually more expensive.
Batteries
The sun doesn't always shine nor does the wind always blow. We need batteries to keep the lights on during times when power can't be generated. First, we must decide how capacity we need. Batteries are rated by voltage and amp hours (ah). If you had a single 100ah 48V battery then you have 4,800W (100 * 48) This means that one battery would discharge in l hour 20 minutes from the 3600w consumption.
Keep in mind, that I'm describing the capacity and not a real world application. First, the battery would need to be able to handle a constant 75a draw and batteries can't be drained completely. For example, some lithium batteries can only use 50% of their capacity. It depends on the type of battery you're using. You must pay close attention to the specifications of the batteries. The best type of battery is beyond the scope of what I want to cover here.
If you had 10 batteries (48v 100ah) then they will provide about 13 hours running capacity.
48 x 100 = 48,000
48,000 / 3,600 = 13.33
If your batteries can be drained by 50% then your usable capacity is 24,000w or 6.67 hours at maximum load. If you can't find batteries that can handle a 50% drain then you will adjust the number of batteries to compensate.
If you can't find a 48v battery then you can buy two 24v batteries and wire in series. This means the negative post of one battery is connected to the positive of the next battery (then connected to the controller). This increases the voltage of the battery but keeps the capacity the same. Two 24v 100ah batteries wired in series will produce 48v at 100ah.
You can increase the capacity by wiring batteries in parallel. This means the positive post is wired to the positive of the next battery. Two 48v 100ah batteries wired in parallel will produce 48v at 200ah.
A more detailed explanation can be found here. Series Vs Parallel https://lithiumhub.com/series-vs-parallel/
Other Considerations
Everything connected to your circuit will consume power. All wires have power loss due to resistance (resulting in heat). All inverters, transformers, and controllers will consume power. All the usage should be considered in your calculations. There are no freebies.
Recharge
We must recharge once the batteries are drained. If we have a total capacity of 24,000 watts then the watts of the power source will determine how long it takes to recharge the battery bank. The voltage should match your system. We would need a source that outputs 48v.
The charging is handled by a controller. The controller must match your voltage and amps. This is usually expressed as watts with ranges for the volts and amps. The controller also must be compatible with the type of batteries you are using. The type of output is also important. I need 120v to power my RV. However, if I want to provide power to the water well then I need two phase 240v.
Wind
Wind power is probably the most expensive and least reliable. It requires a turbine and compatible controller. They will be rated in volts and watts. An 800W 48v turbine will take 30 hours to completely recharge your battery bank. (24000 / 800 = 30) Wind speeds vary based on altitude with the slower speeds being at the surface.
Turbines have wind speed operating ranges. There's a minimum, max generation,
and safe operating speeds. The 800w unit needs 6mph to generate some power but 9mph to make the full 800w. High wind
could damage the turbine above 12mph. Most turbines have an automatic shutdown when the wind is not with an acceptable range. Your local conditions must be taken into account. A turbine will be useless if you average is 3 mph.
The minimum height needed is at least above any trees. At my location, the minimum is 30 feet tall above the tree canopy. You can find the average wind in your area on weather maps (eg: https://www.ncei.noaa.gov) or hand held measuring devices. I would need the turbine to be at least 50 ft above the ground get the minimum (6 mph) in my area. I would need to go a high as 100ft to regularly get the full 9 mph needed. If you live in a high altitude then you may need to go higher for faster wind because air density is a factor.
The higher the tower is, the harder it will be to perform regular maintenance like greasing and replacing worn parts. A cheap turbine may require more maintenance and may not produce enough power for your needs. If freezing rain is an issue in your area the a turbine maybe unusable for days until it thaws.
The wind will not be constant even at the optimal height for a location. You may only get 2 hours a day of usable wind. For most people, wind power impractical.
Solar
Solar panels are cheaper and easier to maintain when compared to wind. The only requirement is a clear sky view. The amount of power they generate is dependent on the angle to the sun which first means pointing in the direction of the equator. (northern hemisphere would point south)
The panels should be at the optimum angle to the horizon. Sites like https://footprinthero.com/solar-panel-tilt-angle-calculator provide a calculator based on your location. The angle changes with the season but you can set your panels at a median. Mounts that automatically adjust the angel are available. They follow the sun to maintain the best angel. They are expensive and consume power. In my opinion, they aren't worth any extra power you might get.
Panels don't generate every minute the sun is out. Cloud cover, sun setting, and sun rising will change the power generation. The usable hours are easy to determine. There are many websites that will show the peak sun hours. The amount of hours will be lower the farther away from the equator you are. This represents the average amount of usable sun per day. This is just a guide. Lets assume that it's 5 for the example.
This means a 200w panel (48v) will generate a maximum of 1000w per day. (200 * 5 = 1000) It would take 24 days to charge your batteries with one panel. 24000 / 1000 = 24 1,000 is the total watts generated per panel per day. If you wanted to recharge the battery bank in a single day then you would need 24 panels. This doesn't include any power consumption while your batteries are charging.
The watts vary by panel. Panels are no different than batteries and can be wired in series or parallel to achieve the voltage of your system. The point to remember is that if you wire in series to increase the voltage then you need more panels for amps. It's possible to buy panels with the correct voltage. They achieve it my internal wiring.
Panels need to be regularly cleaned. Dirt or leaves will reduce the efficiency. Panels are limited to how much sun they can convert to electricity. This is due to the materials use described by the Shockley–Queisser limit. The efficiency limit is about 33.7% for most panels currently available. This means only 33% of the sun's energy is converted to electricity. The best panels (and most expensive) available are about 21% efficient. There are some panels available that claim higher levels. They achieve this by multiple layers not improvements on the cell.
Panel will lose efficiency each year (about .5%) which quickly adds up since the efficiency is low to start with. You can expect the panels to last at least 15 years before needing to be replaced. This will vary depending on the quality of the panels.
In the real world
In the example, I used the max 3,600w for the calculations. This might be typical usage for a house connected to the utility grid. I expect the usage to be less in an off gird application. The RV can use propane for most of the appliances and can operate on 12v. 120v would be nice to charge the phone or satellite.
In the beginning, I measured my average need as 2a. I would need 240w per hour which means the battery bank in the example will last a little over 4 days. 8 panels could charge the battery bank in about 5 days while still using 240w during 12 hours in day.
The good news is that at 240w system has a max of 20a at 12v which means you could use a 12v system and save money.
