DIY – Powerwall

DIY Powerwall Schematic

With solar panel technology being more mature in design, efficiency and a lot cheaper to make, I thought I would write about some of the things to consider before dishing-out a ton of money on a pre-built package like the Tesla Powerwall or many other similar packages that are on the market.

While some of the pre-built packages have some impressive specifications, like the Powerwalls’ 13.5 kWh of electricity storage, a continuous power output of 5 kW, and a 100% depth of discharge, it also cost $11,500 and then you have to add in the cost of the solar panels themselves and the installation, meaning you are going to spend about $25,000 or more by the time it’s all said-n-done.

Not a lot of people are going to have a spare $25,000 to $35,000 to spend, or are they going to be willing to take such a loan from a bank to finance such an endeavor.

In many parts of the world, they do not have a steady stream of electricity to start with, so their aim is going to be about how to keep a refrigerator and a couple of lights running.

For this article, we are going to concentrate of such an endeavor, not as a necessity, but more as a backup for when the power does go out. Either way, the principles and math that is in this article will serve as a foundation on how to accomplish building a solar power supply for your home.

You are going to want to start by making a list of the items you are going to need: Solar panels, Inverter, Charging controller, Good grade wire and a transfer switch.

As to be expected, what models and specifications of these items are all going to depend on your expected current draw. For example: TV, Refrigerator and two table lamps.

For that listed example, you will need to look at the back of your TV, Refrigerator and Table lamps for the amount of wattage that each of them require to run, then double it. If you are using a transfer switch, then you have to account for power loss that is going to occur from all of the cabling, connectors, etc. Hence why I stated to double the amount that you think you will need.

For our example, we are going to use a small to medium refrigerator, a 32″ TV and two 60 watt table lamps to do our math.

First, we are going to assume that our refrigerator requires 1000 watts, the TV requires 60 watts and each table lamp requires 60 watts for a total of 1,180 watts.

Keep in mind that when you turn on an appliance or in the case of a refrigerator that starts automatically when it needs to run, there is a large start-up current draw until the it’s running at normal operating wattage. So our refrigerator might want 1,500 watts for a few seconds. The same would hold true for the TV.

For this calculation, we are going to assume that we need 1,680 watts, meaning that you might be able to squeak by with an inverter like the one shown above, but it’s going to groan for a couple of seconds when the refrigerator starts. Something to note here is that not all inverters will run at the shown wattage on the label, meaning that your lessor expensive inverters will only have a duty cycle of about 80% of the total wattage shown on the box.

Next. What size battery are you going to need? If you are at 1,680 watts, you need to take 1,680 watts and divide it by 12 volts = 140 DC amps per hour from the battery, meaning you are going to need three of the batteries as shown above. You could get away with only having two, but your power supply is not going to last more than roughly 20 to 40 minutes before the battery is completely drained and that is something that is not recommended, so having extra battery left over is always advisable.

Last, but not least, what size of solar panel do you need. With our mock setup, you are going to need a panel that will produce about 30 amps, or a combination of panels that will produce 30 amps. Do keep in mind that if you are going to use multiple solar panels, you are going to want to add a combiner to your parts shopping list.

Now that we’ve got our parts list put together, how much is this going to cost? As noted above, depending on what your needs are, will greatly shape the cost of all of it. For our example above, I’ve estimated that the total cost to be $671, which is a lot! cheaper than $25,000 to $35,000.

Cost beak-down:

  • 30 Amp solar panel: $335
  • 2000 Watt inverter: $129
  • 63 Ah battery: $40 X 2 = $80
  • Battery charging controller: $15
  • Cable kit for battery to inverter: $12
  • Automatic transfer switch: $100

Hopefully this simple explanation of a solar backup system will help if you’ve ever thought about having a power backup system.