Hi. I'm Amy Beaudet from the altE Store. Thank you for watching the last of our video series
on Designing an off-grid system. For this final video, we'll select the inverter for
our off-grid system. Our previous videos helped us determine how much power we needed to make,
what size battery bank was needed to store it, and what solar panels were needed to generate
that power, and what charge controller is needed to manage that power. Now we'll choose
the inverter. We're getting closer to completing the system, we've figured out the house loads,
done the batteries and panels, the charge controller, the final component is the inverter.
I did another video discussing the different types of inverters. We are just going to do
a brief reminder of an off-grid inverter's role. The primary job of an inverter is to
convert the DC (or Direct Current) power from the battery bank, as seen in the red line,
to AC (or Alternating Current) power needed for most appliances. To do that, it must take
the constant DC voltage and change it to a sine wave curve that goes above and below
0 volts. When inverters first came out, the most common way to do this was to make the
voltage go straight up and down, creating a blocky signal. This is called Modified Sinewave,
seen in light blue. More advanced Modified Sinewaves make multiple steps, trying to come
close to a pure sinewave. You can see an output of a modified sinewave on an oscilloscope
at the bottom left. It is an approximation of a Pure Sine wave shown on the right. So
other than how the signal looks, what's the difference between the 2 outputs? A modified
sinewave inverter can be used for simple systems that don't have any delicate electronics or
audio equipment that may pick up the choppy wave and produce a hum. Tube TVs and motors
with brushes are usually fine with modified sinewave. Your digital clock will likely act
funky, and battery rechargers quite often just plain won't work. Some equipment may
seem to be working fine, but may run hotter than with a pure sinewave and reduce the life
of it. It's very difficult to say what will and won't work with a modified sinewave inverter.
Pure sine wave is always needed for a grid tied system. It is generally needed for newer
LED TVs, CFL light bulbs, and inductive loads like brushless motors. Generally, Modified
Sinewave inverters are less expensive than a pure sinewave inverter, so they are still
commonly used in simple systems. But as technology advances, the cost of pure sinewave inverters
is coming down, making them much more affordable and the favorite option.
Once the Charge controller has charged up the battery bank, the off grid inverter converts
the 12, 24, or 48VDC battery bank to AC voltage. The AC output depends on your requirements,
in North America you can use 120V single phase, 240V split phase, 208 or 480V 3-phase, etc.
Depending on how you wire the output of the inverter, and which inverter you get, you
could have both 120V and 240V as an output. You need to determine what your loads require,
and select and configure the inverter accordingly. An off-grid inverter can not sell extra power
back to the grid. However, an inverter/charger can connect to the grid (if available) to
act as a battery charger. For instance, if you have a boat or RV with an inverter/charger,
when you connect to shore power, you can use the grid to charge your battery bank when
the solar doesn't provide enough power. But that AC connection is one-directional, it
will only take from the grid, not send back. Likewise, you can often connect a generator
to the AC input of an inverter charger to top off the batteries when needed. This is
a common configuration for off-grid homes that need more power in the winter than the
sun can provide.
Battery based inverters have a lot of options to choice from. Not all of the inverters have
all of the features, so you need to decide which features are required, and select the
inverter based on which has them. Some of the features are: The ability to charge the
battery bank from an AC source like the grid or a generator, even automatically starting
the generator when the batteries are low, and turning it off when they are charged.
Some can automatically use the generator to assist with high loads. Since the inverter
is often installed in an out of the way location near the batteries, a remote control or display
in the living area is useful to keep an eye on the system. Some inverters even have the
ability to monitor the system remotely via the web. This is very useful for part time
locations that you are not always there to keep an eye on. Many inverters can be stacked
to increase either the voltage or the current, or both. This allows you to use multiple inverters
in a master/slave configuration, automatically turning on only the inverters as needed, conserving
battery power. To size the inverter, let's add up all of the devices that may be on at
once, and add up the surges. Inverter should handle 13+30+375+1400+120 = 1938Watts continuously.
Note that we don't include the DC lights, as they will be running directly off the batteries,
not through the inverter. When we add up the surges that may come on at once, 13W+30+1500+4200+1100=6843W.
That's 3 times higher than the continuous wattage. So we need to find an inverter that
can handle that big of a surge. As you can see, sizing an off-grid system is a delicate
balance. When selecting an inverter, there are several key factors to look for. I've
discussed many of them in previous videos. Looking at the datasheet of a Magnum Energy
PAE series inverter, we can compare the MS4024PAE and the MS4448PAE. The first thing to look
for is input DC voltage. The 4024 connects to a 24V battery bank, you can see the input
voltage range is 18V x 34V. Whereas the 4448 is a 48V battery bank, its voltage range is
36V x 64V. Both of these models are able to output split phase 120V and 240V, 60Hz. So
these can be used in North America with regular household appliances, and with larger appliances
that use 240V, like a well pump. You can see here that the 4024 can handle a continuous
load of 4000W, and the 4448 can do 4400W, so both of them are able to handle our continuous
loads of 1938W. But now lets look at the surge rating we were talking about. We calculated
we need to be able to handle a surge of up to 6843W. The 4024 can handle 5800W for up
to 5 seconds, but the 4448 can handle up to 8500W. That will allow us to handle our 6843W
surge, so I'm picking the 48V MS4448PAE for my inverter. We've now done all of the major
components of an off-grid system except 1. The over current protection, or breakers.
I did another video series where I went into more detail of the breakers. For this one,
I'm just going to do a quick sizing of them. There are 4 main segments that will require
breakers, between the panels and the charge controller, both in the combiner box and in
the DC Load center, between the charge controller and the battery, between the battery and the
inverter, and between the inverter and the AC loads. Between the Panels and charge controller,
each string uses the math: Isc x 1.25 for NECs 3-hour continuous x 1.25 over-irradiance.
The Combined Strings use Isc x 1.25 3-hour x 1.25 over-irradiance x # strings. Between
charge controller and battery use the Charge controller rating x 1.25 3-hour continuous,
Between the Inverter and battery, use the Rated wattage of inverter / Lowest operating
battery voltage x 1.25 3-hour. And finally, on the AC side, Between the Inverter and the
AC loads, use the Rated wattage of inverter / AC voltage x 1.25 3-hour. There you go!
Wow, that about does it. Check out our website for a selection of all
of the pieces you need for your system, including the inverters. Also watch more of our Video
Series on our web site. We've got a team of highly trained Technical Sales Reps available
to help you plan your system, give us a call. And don't forget to check out the rest of
our site at altestore.com, where we are making renewable, doable.