Frequently Asked Questions

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svetz

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Links to commonly asked questions with long explanations:
How do I figure out how much Power I need?
You can look on the devices label, usually it'll say how many watts it consumes. Let's say your fridge says it draws 200 watts. But you want watt-hours. Not all devices run 24×7, they turn on when they need too. Power draw can also be seasonal. The best way to get an idea is to measure it (e.g., a watt meter) in actual conditions. Link to more information.
Example: you leave a watt-meter connected to your fridge for a week and it says you use an average of 107w/hr and a peak of 130w/hr.

How Big should my inverter be?
The inverter will have a continuous rating and a peak rating. The continuous should be greater than the sum of all simultaneous devices at their peak rate. What watt meters won't tell you is the startup draw of devices. This can frequently be 3 or 4 times the maximum power listed on the devices label.
Example: For the fridge above you want at least a continuous of 130 watts, and probably 3×200=600 for the peak rating. To be safe add 120%, so a 150W continuous inverter with 600 watt peak.

Will a modified wave inverter work for me?
Probably not. Any device that uses magnetics (e.g., transformers, motors, induction stoves, computers, TVs, medical equipment) will either not work, work poorly, draw excessive power, or have unwanted side effects (e.g., a TV might have hum). More...

How many panels do I need?
Panels are rated in watts at a very specific set of lab conditions you'll probably never see. Without getting into really complicate equations, folks use an insolation map. This will give you the amount of "usable" sunlight hours per day. So, you can multiply the panel wattage by the number of usable hours and then multiply by .8 for other losses to get roughly the average amount of watt-hours your panel will generate by day (this also depends heavily on the panel's tilt angle, the time of year, etc.). SAM is a program that can give you seasonal data with various tilt angles.
Example: From your bills or power budget you decide you need to generate 35 kWh each day. From the insolation map you see you get 5 hours of usable sunlight. 35,000 Wh/d / 5 h/d = 7000W. Due to conversion and other losses you'll need to divide by .80 for 8750W. You pick the type of panels your want e.g., 340W LG Neon2, since they're 340W panels you need 8750/340=25.7 panels, round up to 26 panels. Note that the actual power per day generated will vary throughout the season and weather conditions.

Do I need Batteries?
If you want power when the sun isn't shining, have devices with a startup draw greater than the panels outputs, or will need power when the occasional cloud passes over head, then you'll want batteries.

But if you wish to power a small device without a battery, you can use a dc-dc converter: Click here to learn more

How many batteries do I need?
Lead: number of watts per hour /.5 x number of hours of backup / .8. Example: 107W/h / .5 x 24 hrs / .8 = 6420 Watts , AH = w/v, so 535 AH @ 12V
LiFePO4: number of watts per hour / .9 x number of hours of backup / .8. Example: 107W/h / .9 x 24 hrs / .8 = 2854 Watts, 238 AH @ 12V

What angle should I tilt my panels at?
You can best determine this with SAM (NREL's free modeling tool). The Earth's tilt is currently about 23.26 degrees, so the seasonal variation in the height of the sun is about 46.5 degrees with the sun being the highest in summer and at it's lowest in winter. If you lived on the equator (0 longitude) the sun would vary from +23 to -23 degrees, so by placing your panels at 0 degrees you'd maximize the power all year long. But it can get complicated. Suppose you lived on the equator, but it rained through the entire summer? In that case you might want to tilt your panels to optimize them for the winter. Similarly, some appliances like Air conditioning consumer more power summer (or heat in winter); so you should tilt them to optimize summer production; right? Not necessarily, if you have net metering you'd want to maximize total production to minimize your energy costs. You might have to disregard efficiency for the sake of expediency; for example the possibility of hurricane force winds might produce so much force that you should have no tilt. Check out https://www.diysolarforum.com/index.php?forums/solar-panel-angle-and-location.33/ to see what others are doing.

Where can I learn about local Incentives?
Try this database, start by filtering by "state".
 
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svetz

Works in theory! Practice? That's something else
Joined
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Messages
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Location
Key Largo
AC & DC Voltages & Currents are Dangerous!

Here's a chart showing it only takes a few milliamps to go from one hand
to the other to have really bad things happen.

Your skin is a pretty good insulator. The problem is your blood is a pretty
good conductor. Your skin can be breached as low as 30V; less if it's humid
or you're wet. A string of panels in the U.S. can be up to 600V, and even in
twilight or moonlight can have hundreds of volts. You don't get "power"
from the panels in those conditions as the low amps aren't enough to
power the inverter. But as you can see from the table, it doesn't take
much current traveling hand to hand to kill you.

Amps can kill you, voltage can kill you.
Capture.PNG
Be Smart, be Safe! Please be careful around any sort of wiring and use the appropriate safety gear!​
 

svetz

Works in theory! Practice? That's something else
Joined
Sep 20, 2019
Messages
5,630
Location
Key Largo
How much solar power can I expect where I live?


Complex & Accurate

Download free software like SAM or the online PVWatts. These programs take into account many factors such as elevation and weather. They are also fairly hard to use, so be sure to watch their video tutorials.


Quick & Dirty
Use an insolation map or calculator to get a number that represents the number of hours of sunshine at 100% strength for your location. The insolation number changes throughout the year based on the length of the day and the angle of the sun, so be sure to check the insolation values against your power seasonal power needs to ensure you're covered.

Let’s say you have four 250W panels and the insolation number is 3, then the calculation is 4 x 250 x 3 = 3000 watt-hours per day.

But keep inefficiencies in mind! 3 kW of power at the panel has to go through the MPPT (say 98% efficient), then stored into the batteries (say 90% round trip efficiency), and through the inverter (say 92% efficient) so 3000 Wh x .98 x .9 x .92 = 2432 Wh.
 
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