"He Didn't Build That"...I Did (Part 6): Solar Inverters

Solar Inverter, 1,000 Watts, 12-volt input, 110-volt output

Solar Inverter Sizing

A solar inverter converts the direct current (DC) output of a photovoltaic (PV) solar panel into a utility frequency alternating current (AC) that can be fed into a commercial electrical grid or off-grid electrical network. 

Types of Solar Inverters:

There are three distinct types of solar energy inverters, each of which serve a different function and are used for a different type of solar energy system, although each inverter still converts DC into AC. For educational purposes, I’m going to give you a bird’s eye view of each type:

1. Stand Alone Inverters - used for off-grid solar arrays. In off-grid arrays, the solar panels generate direct current energy, which is then stored in rechargeable batteries. When energy is required, the direct current is sent from the battery to the stand alone inverter and then converted into alternating current which can then be used to power a home or for any other electricity needs.

2. Grid-tie Inverters - for use in any situation where your solar array is connected directly to your local power grid. These systems, known as grid tie solar systems, are more common in urban areas and are much cheaper than stand-alone or off-grid systems because of the lack of need for a battery.

3. Dual Inverters - also known as backup battery inverters and are used in a multi-function solar system. In this system, the direct current generated by the solar panels is first sent directly to a battery. The energy from the batteries is then sent to the inverter and converted to alternating current whenever electricity is required. However, when the batteries are fully charged, any excess direct current electricity that is generated is sent directly to the inverter and is then sent into the local power grid. This system combines the other two systems into one self-reliant and possibly money generating solar unit.

Based on our example, our inverter would be off-grid type, as we are not going to connect to a power utility electric meter. In short, it is stand-alone/portable type, which we can carry anywhere we need AC household current. 

There are two types of this kind of inverter: 

1) pure sine wave inverter – regulated, similar to AC household current

2) modified sine wave inverter – good for simple induction loads such as motors and light bulbs

Pure sine wave inverter is recommended because it is ideal for running loads with sensitive electronics, such as laptops, cell phones and other complex or state-of-the-art technologies. If on the other hand, your demand is simple power, and you don’t need to utilize today’s sensitive electronic devices either now, or in the future, a modified sine wave inverter is the more economical choice.

Inverter Sizing

From my previous blog: http://engineer2poet.blogspot.com/2012/09/he-didnt-build-that-i-didpart-3.html the total wattage consumed is only 475 watts. To compensate for future loads, I recommend a 1,000 watts or 1 KW inverter. Make sure you don’t plug in an appliance with a rating higher than the inverter rating. As a simple rule of thumb use only a 70% - 80% total load, that is 700 to 800 watts combined load to a 1,000-watt inverter. You either need to unplug the other loads if you want to use a single high wattage appliance, or buy the next higher capacity inverter to satisfy the loads.

Therefore the specifications for our inverter would be: Capacity = 1,000 watts, Input = 12 volts DC,

Output = 110 volts AC (US);  220 volts AC (Phils.)

Watch out for our last topic: Solar Cost and Payback Calculations…

He Didn't Build That" I Did...(Part 5)

Solar Charge Controller

Solar Charge Controllers

The solar charge controller provides a regulated DC output and stores excess energy in a battery. It also monitors the battery voltage to prevent under/over charging. It may be used to power 12-volt or 24-volt DC equipment with solar panels, as well as in charging cell phones and laptop computers.

Once we have sized our battery bank and solar panel array, determining which charge controller to use is comparatively straightforward. All we have to do is find the current through the controller by using:

  Power = voltage x current; P = V x I

Take the power produced by the solar panels and divide by the voltage of the batteries:

I = P/V = 400 watts/12 volts = 33.33 amps.

Now introduce a safety factor of 25%. Multiply the value we have found by 1.25 to account for variable power outputs: 33.33 x 1.25 = 41.6625 amps

In our example we would need at least a 45 amp. controller. The Flex Max 60 MPPT Charge Controller would fit our specifications, or we can have individual charge controllers for each 100-watt solar panel:

I = P/V = 100 watts /12 volts = 8.33 amps. x 1.25 = 10.41 amps

Therefore we can use one (1) - 60 amp. charge controller, or four (4) - 15 amp. charge controllers.  It’s your call; the choices you make will depend largely on the price of each controller. Usually charge controllers come free with individual solar panels, so check that out first before making any purchases. If you have to buy your own controller, I suggest that you choose one with a USB port(s) and with multiple DC voltage outputs: 3V, 6V, 9V and 12V for charging your cell phones and other electronic gadgets.

Next time, we are going to discuss solar inverters.

"He Didn't Build That" I Did...(Part 4)

Three (3) 12-volt batteries connected in parallel

Solar Battery Sizing

Days of autonomy

First of all decide how many days worth of energy we want to store in our battery bank. Generally this is anywhere from two to five. Let’s assume we want the battery bank to last three (3) days without recharging.

Battery sizing/bank capacity

Then we can calculate the minimum battery capacity in terms of amp-hours (AH). Use the value of 2,880 watt-hours per day from part 3 of this blog article: http://engineer2poet.blogspot.com/2012/09/he-didnt-build-that-i-didpart-3.html, and multiply them by the number of days you decided upon which is 3 days. This should represent a 50% depth of discharge on your batteries. Therefore multiply by 2 and convert the KWH result into amp hours (AH). This is done by dividing by the battery voltage.

Therefore, solving for the energy we need from the batteries:

Since there are 1,000 watts per kilowatt (KW), we divide 2,880 watt-hours per day by 1,000 to convert it to kilowatt-hour (KWH) = 2.8 KWH/day

E (batt.) = 2.8 KWH/day x 3 days x 2 = 16.8 KWH

Converting this to AH we have to divide by the voltage of the system. This can be 12V, 24Vor 48V for commercial application. In our project we will choose to use 12V which is common for household use. 

The minimum AH capacity is then 16,800/12 = 1400 AH

Now if we divide by our battery's rating, we find the number of batteries we must use. Using 250 AH (recommended size) battery rating:

Number of batteries needed = 1400/250 = 5.6 = 6

Therefore we need to use six (6) 12-volt batteries. If we prefer just one day of power storage instead of three days, then two (2) batteries would be just fine. 

Since solar grade type of batteries are still pretty much expensive nowadays, I recommend using the marine grade deep cycle lead-acid batteries usually used in boats and recreational vehicles.

The cost of batteries and other components of our solar project will be discussed in the last part of this series  of blogs, when I discuss about the rate of return of our investment.