WATT HOURS VS PEAK WATTS – WHAT’S THE DIFFERENCE?

As you learned previously, watt hours are the total amount of electrical demand in one day. It is the summation of every drop of energy you plan on using for the day – much like planning how much gas to pump, when going on a small road trip. Peak watts, is a different concept.

Peak watts is the highest energy demand you’ll be using while off grid. In terms of our gas comparison, ask yourself: “During my road trip, what is the longest stretch of road between gas stations?”. The amount of gas you would need to drive between the two gas stations would be considered your “peak watts”. In simple terms, “What is my single largest energy demand in a given amount of time?”.

Here is a real-life example. If you power a laptop, and a cell phone at the same time, each at 75 and 10 watts respectively, your peak watt usage would be 85 watts. Simple right? Let’s add more circumstance to our example.Let’s say between the hours of 8 am to 12 noon, you power your phone and laptop. Then around 2 pm, the hottest part of the day, you power your air conditioner, using 1100 watts of power. Would your peak watts be 1185? Good try, but not exactly. Since peak watts is about calculating maximum demand, and the air conditioner was powered outside the hours of the laptop and phone, your peak watts would be 1100 watts.

PUTTING IT ALL TOGETHER

So how does peak watt and watt hour usage affect your solar choices? In an idealistic scenario, we would like to build a system that is 100% sustainable to your energy usage/life.This means we need to size your battery bank to run the appliances you need at night.It also means we need to build a solar array that will collect enough energy to consume your day time energy usage, and replace your night time battery usage.Let’s break it down by component.

PICKING SOLAR PANELS

Is your appliance used during the day time or night time? Because the sun is nonexistent during the night, watt hours used during the night come from your battery bank. Watt hours used during the day, can be offset by your solar panels. Consider the exercise below: You building an off grid solar system for your cabin retreat. During the day, you work remotely on a laptop, and make coffee for the morning. At night you relax, watching your TV, read something, and fall asleep listening to a radio on sleep. You’ve figured out your watt hours, and bought a 12 volt 200 amp hour battery. But how do you size your solar panels? First, lets convert the 200 amp hour battery to watt hours. We do this by multiplying the amp hours by the voltage:12 volts x 200 amp hours = 2400 Watt Hours

Next we add up the energy usage we’ve used for the day.

Test your knowledge. Read about Teragy solar panels here.

PICKING A BATTERY

Remember in the Math of Solar tutorial when we added your daily watt hour usage? Picking a battery is simple. How sustainable would you like your system to be? Would you like to power your life style for 1 day without sun? Or 3 days? The calculations are simple.

Next we multiply your total watt hours by the number of days we’d like to power without recharging. Lets do 2 days.1269 Total Watt Hours x 2 Days = 2538 Watt Hours

Next we’ll divide this number by the voltage you are deciding to run your solar system in. For this example, we’ll use a 12 volt system.2538 Watt Hours / 12 Volts = 211.5 Amp Hour Battery

Now if you’re a stickler about absolutely being able to power your appliances for 2 days without sun, a 300 amp hour battery would go above and beyond what you need. But if your budget conscious, a 200 amp hour battery could be a close fit to your energy demands. Now keep in mind, if your energy demands go up, you’ll most likely need multiple batteries that total your daily consumption.

Exercise: Go ahead and try the exercises below yourself! Pick a list of appliances you’d like to power throughout the day, then use the calculation chart below. See if you can pick the right battery for your needs, in the multiple choice below. Feel free to generate as many exercises as you need by hitting the refresh button, and refilling your appliances.

PICKING AN INVERTER

We hope you did your homework to understand what peak watts are! At Teragy, our basic special formula for picking inverters is the average watt rating of all your appliances, added to your highest watt rating of all your appliances. Examine the calculation chart below to see this principle in action.

SO WHAT ABOUT INVERTER / CHARGERS?

Inverter / chargers do the same thing as an inverter, with the added benefit of charging your battery on grid. Having an on board charger opens up your charging diversity, allowing you to plug your off grid system directly into a generator or 120 / 240 AC outlet.

Make sure your batteries can charge at the given amps your charger will put out. And don’t for get to ask about charging profiles – charging profiles allow you to safely charge AGM, Wet Cell, Lithium or all battery types while prolonging life span.

PICKING A SOLAR CHARGER

Solar chargers (controllers) translate the inconsistent energy of the sun, to a smooth consistent energy for your battery and DC system. Picking the right solar charger comes to calculating the total amps from your solar panel(s), and the volt rating of your battery, DC system, and solar panels. But before we dig into this, lets go over some vocabulary.

In / Out – Similar to an old VCR, DVD player, or a stereo receiver, solar controllers direct energy coming “in” from the solar panels (sometimes referenced as the solar panel side), and translates them “out” to your battery or DC power system.

Step Up / Step Down – Your solar panels can have a different voltage than your existing DC or battery system. Step down and step up controllers help bridge the voltage difference between your solar panels and your native DC system. When you put together a 48 volt solar panel system with a 24 volt DC system, you are effectively “stepping down” the voltage via the solar controller. Same applies for “stepping up”. Most MPPT chargers are built with a “step down” ability to match your higher output solar panels to your lower voltage batteries or DC eco system.

MPPT – This stands for Maximum Power Point Tracking. This is the latest technology in solar controllers, and for our purposes, we will only be covering MPPT charge controllers. MPPT charge controllers deliver more power, and adapt to a much wider range of solar applications than PWM controllers.

The first step to choosing a controller is understanding how much amperage and voltage you need to manage. Behind your solar panel is a silver sticker that will list your panel’s voltage, and amperage. If you have multiple panels, multiple your single panel amperage by how many solar panels you have. See the example below to illustrate this principle.

Exercise: Do you understand the concept above? Write down the voltage, watts, and amps of the solar panel you are trying to buy. Add up your total amps. Did you get it right? Use the calculation chart above to check your work. Looking for extra credit? Play around with the Wiring Strategy feature and see how it affects your solar controller specs!

LARGER OFF GRID SYSTEMS

There are multiple skill sets involved in choosing the proper set of solar controller(s) for larger off grid solar systems. In the next installment tutorial, we will cover picking solar controllers for large solar arrays. The tutorial will go over series and parallel wiring, voltages, wiring thicknesses, and all-in-one off grid solar controllers.

Do you need help picking out the right solar controller for your system? Call our solar specialists today, and let us find the right fit for you.