Watts, Volts, Amps...etc!
Understanding a Complete “Closed Loop” Electrical Circuit
Solar energy is a great source of electrical power, but it does not suit every application.
The purpose of this section is designed to provide the reader a basic understanding of a “closed” (not connected to the grid) or “portable” electrical circuit, and determine if you have an application suitable for a Solar Stik™ System.
A portable (“closed”) circuit is one that includes methods of producing, storing, managing, and utilizing power (in the form of a “load”).
Principles of “Stored Power” and “High Efficiency” Power Models
“Efficiency” can be expressed as a ratio:
When an appliance is operating from power stored in a battery, the circuit conforms to a “high-efficiency” power model, because power is only taken from the battery as required for operation.
An example of a “low-efficiency” power model is a traditional fuel-driven generator being used to directly power appliances. Once connected, the generator must be running to provide ANY power, even if the appliances are only drawing power intermittently or in small amounts.
“Portable” power = “Limited” power
All portable power generators (whether traditional fuel, sunlight, wind, etc.) are limited by varying factors, so using portable power often requires the operator to establish a “power budget”. Selecting technologies to be used in a portable power circuit are often made in accordance with what resources are, or are not, available.
The greatest benefit that can be realized in ANY portable power model is by using a “high-efficiency” (battery-based) electrical circuit.
Harnessing Renewable Power from the Environment
Solar & wind power can usually provide a significant portion, if not all of the daily power requirements for a given application. Although they are cost-effective methods of generating power, they do rely on sources that are occasionally interrupted (cloudy and no-wind conditions).
Harnessing Traditional Power from Fuel-driven Generators
Generators can usually operate without regard for the surrounding environmental conditions. The limiting factors for generator operation is usually the availability of fuel & the need for ongoing maintenance. Restricting the generator to a single function (such as Battery-Charging) will reduce run-time and cost, ensuring that any energy created from burning fuel is stored as POTENTIAL ENERGY in the battery, and then delegated to the loads only as required.
It is critical to have a balance between all the working components of a closed-loop electrical system. Striking a “balance” means knowing the total power consumption, generation, and storage requirements for successful appliance operation.
Proper balance between power generation and power consumption also yields an electrical circuit that operates at very high levels of efficiency, reducing costs & logistical burdens, especially in situations where portable power generators are typically used (where the Solar Stik™ System is designed to operate).
The Learning Curve of “Independence”
The more one understands how to effectively design & use an electrical circuit that is based on “stored power” principles, the more autonomy one can achieve.
Determining the Characteristics of a DC Electrical Circuit
In the following sections, the following principles are briefly covered:
How to find the power requirements (Watts) for each individual appliance (Load) while operating.
How to determine the power consumed by an appliance over a 24 hour period.
How to properly size a battery bank to meet the load requirements.
How to determine the amount of power needed to replace energy used by the loads.
How to choose a solar/wind or hybrid power system that meets or exceeds the total load requirements.
There are many other sections on this site that will cover certain sections in greater detail. Feel free to click on the links and then return to this page at any time.
The Basics of Electricity
AC & DC Electrical Circuits
The first thing we will cover is a little about basic electrical systems. There are 2 commonly used types of electricity:
Both of these types of electricity are measured in voltage. Most small DC systems use 12 volts (12V DC), and most AC systems (U.S.) use 120 volts (120V AC).
If your energy is supplied from the utility grid or a gasoline generator, it is probably an AC circuit. If your power is derived from a battery, it is a DC circuit.
AC systems are commonly found in buildings or in urban areas where there is easy access to “Grid” power.
DC (battery) systems are commonly found in motorhomes, sailboats, powerboats, cars, campers and in rural areas where traditional “Grid” power may not reach.
Next, let’s dive a little deeper into the specifics of a DC system. A “load” is anything that consumes energy in your electrical system. All electrical systems are constructed based on the amount of energy that the loads will require. There are three factors that combine to successfully supply energy to the load:
Volts, Amps, & Watts
- “Voltage”or “Volts” is the force that “pushes” the electricity through the wire… the higher the voltage, then the higher the force of the “push”.
- “Current” is the rate of electricity flow through a wire, and is measured in “Amps”.
- The total power that is generated or consumed within the circuit is measured “Watts”.
Determining the Electrical Characteristics of a DC Electrical System - “The Formula for Determining Power”
Imagine that you own a camper and you need to build a DC electrical system for it. Your camper will have two loads in its electrical system, a 12 volt DC refrigerator, and a 12 volt DC light, with each one consuming different amounts of energy. (As we discussed, “consumed energy” is measured in watts. You will see that a “watt” is represented by the letter “P” [for “power”] in our electrical formulas).
There is a formula to determine the power characteristics of an electrical system, and it is as follows: VOLTAGE, multiplied by AMPS, equals POWER (V x A = P). To the seasoned electrician, this formula is known as “Ohm’s Law”.
Variations of this formula can be used to determine all three values:
V x A = P
V = P / A
A = P / V
Let’s say that your 12 volt DC light bulb consumes 25 watts of power, and you need to know how much current (AMPS) it will draw.
We need to use the following formula:
AMPS equals WATTS divided by VOLTAGE (A = P / V). Inserting the values that we know, we then have A = 25 /12, and solving the equation we have A = 2.1 amps consumed by a 25 watt light bulb in a 12 volt system.
Pretty easy, right? Good!
Now let’s look at the refrigerator. Every electrical appliance has a data plate or tag on it that tells you what type and how much power is necessary to operate the appliance. If no data plate is present, then the manufacturer’s literature will have it in its pages. Depending on the manufacturer, you may be reading the operating energy requirement in watts or amps.
If you know the voltage (V) you can use either value (watts or amps) to determine the other.
For example, if the Power consumption rating is given on the data plate is “60 watts”, then plug in the system voltage and to determine the current (AMPS) value.
P/ V = A, or in this case, 60 / 12 = 5… so the refrigerator draws 5 amps.
Now let’s figure out the total of amperage draw for the camper’s DC system. Simply add the values of the amp ratings of all of the loads together. The total amp draw for the light and the refrigerator is 7.1 amps!
The “Heart” of Any DC System…The Battery
Now that we have the total load for the system, we need to learn how it relates to your battery or batteries (battery bank). Batteries store energy, and often have ratings that correspond the amount of energy they can store, known as the “Battery Capacity”. This is usually expressed as “amp-hours”.
It important to have the proper battery capacity for many reasons:
- Proper capacity should be able to supply TOTAL power required by the loads in your particular system
- Proper capacity should be able to fully recharge from the chosen power generation sources.
- Proper battery capacity ensures that there is enough energy to power your loads between charges.
Amp-Hours - If you were to look at a deep-cycle battery, you will probably find a label that rates the battery capacity in “amp-hours”. This relates to how many amps can flow from the battery to the load, in the amount of time it takes to consider the battery as “discharged”.
For example, if the battery label reads “125 amp-hour reserve”, then it means that it can provide 25 amps continuously for 5 hours (25 amps x 5 hours) and at the end of 5 hours, the battery would be considered “discharged” and out-of-service until recharged.
Watt-Hours - Like “amp-hours”, a “watt hour” is power consumed over a certain amount of time. If you have the battery ratings for voltage and amp-hours (AH), you can also determine the watt hours (WH) that are stored in the battery. (V x AH = WH). For example, a 100 amp Hour battery at 12 V DC can supply up to 1200 watt hours of energy.
Daily Power Requirements: Amp-Hours & Kilowatt-Hours
Let’s back up for a minute. We figured that the camper refrigerator and light total current draw was 7.1 amps when they were both operating, but you may not use the light all the time, or the refrigerator may only actually operate for a total of 20 minutes during an hour.
To get an accurate picture of your system’s daily requirements, you will need to get the sum total of ‘hours per day’ of refrigeration and light operation time.
Let’s say that you added it all up and estimated that the total amp draw from the batteries was 7.1 amps for 8 hours in a 24 hour period. This means that you are consuming about 57 amp-hours per day (7.1 amps x 8 hours = 56.8 amp-hours).
In the same manner, the watt-hours can also be calculated. If the refrigerator and light consume a total of 85 watts and operate for a total of 8 hours daily, then the total watt-hours consumed in a 24-hour period would be 680 watt-hours.
While labeled differently, the appliance energy requirements are exactly the same:
12 volts multiplied by 56.8 amp-hours = 681 watt-hours (12 x 56.8 = 681)
Finally, it is common to refer to power in terms of “kilowatts”. A “kilowatt” of power equals 1000 watts (kilo = 1000). Using this term, 680 watt-hours can be expressed as 0.68 kilowatt-hours.
Now that we know how much energy we need to use on a daily basis, we need to determine how to replace the 57 amp-hours or 0.68 kilowatt-hours of power that you consumed from the battery during one day.
Let’s take a look at energy production of a 100W Solar Stik™. According to the data plate on one 50 watt solar panel, it produces about 3 amps at 18 volts DC. Two panels, therefore, produce 6 amps. The panels will need to produce 6 amps for 10 hours (60 amp-hours) to replace the energy used by your DC loads. This amount of energy will keep the battery (or Power Pak) fully charged and allow uninterrupted daily use of your refrigerator and light!
The Benefits of “MPPT” Charge Controls
A standard Solar Stik™ has two 50 watt solar panels, giving us 100 total watts of solar power. The solar panel’s rated “Output Voltage” is about 18 volts (which is about 6 volts MORE than the average 12 volt battery).
Using our standard formula A = P / V (amps = 100 / 18), we find that the Solar Stik™ panels produce 5.8 amps, or about 3 amps per solar panel. On average, one can expect to be able to get 10 hours per day of direct sunlight, providing the user with about 60 amp-hours of energy.
In order for charging of a battery to occur, a charging source must have a voltage GREATER than that of the battery voltage. The higher voltage provides the “push” that enables current to flow into the battery. Charging voltage must also be limited to prevent a battery from over-charging or over-heating. This is where “charge controllers” play a role in a solar power system.
Also, it should be noted that anytime power is converted from one form to another, it requires power to perform the conversion. This is known as “loss of efficiency” in a circuit.
Since the Solar Stik™ Panels operate at 18 volts, you may be thinking “…but my batteries are rated for 12 volts”. This is why we recommend (and provide in every package) the use of an MPPT (Maximum Power Point Tracking) charge control. This charge control converts 18 volts DC to about 14 volts DC by using a small internal computer.
Allowing the solar panels to operate at their “Maximum Rated Voltage” instead of the “Battery Voltage” results in quicker recharge of your batteries. Let’s perform the math to see the difference. First the MPPT math:
A = P / V
A = 100 / 14
A = 7.2
Using an MPPT charge control, the available current from the panels to the batteries is 7.2 amps. This means that 86 watts of power are being produced.
Non-MPPT types of charge controllers force the solar panels to operate at the battery’s voltage as below:
A = P / V
A = 100 / 18
A = 5.5
In other words, the 5.5 amps from the solar panels yields only 66 watts of power to charge the battery.
In conclusion, an MPPT charge controller, increases the available current from the panels to the batteries from 5.5 amps to 7.2 amps… an increase of about 25%. This means faster and more complete charging of the battery.
There is a always some energy loss in the transfer of the power generated at the panel to the batteries. This loss is greater for older systems that do not have MPPT technology. It is always better to use a MPPT charge controller with any solar or wind generator system.
Using a DC to AC Power Inverter
“What about using an inverter in my system to run a 120 volt appliance? How much power will it draw?”
Let’s say that you have a microwave oven that is rated at 600 cooking watts. The manufacturer’s plate indicates that it operates at 900 watts. If we know the microwave uses 120V AC, then we can figure out the amperage it requires to operate from an inverter. First, let us calculate the AC amps required:
A = P / V
A = 900 / 120 = 7.5 amps
The microwave uses 7.5 amps of AC (alternating current) from a 120V source.
An inverter powered from a DC system (battery) allows you to run AC appliances by converting 12V DC to 120V AC. Since an inverter consumes some power in the act of converting power, we must also take that into consideration.
To find out the current (amps) from the battery (12V), use the following quick calculation:
AC watts (inverter output) divided by 10 equals the DC input current (amps).
900 / 10 = 90 amps.
Using the example 900 watt microwave oven powered by an inverter, then about 90 DC amps must be supplied by the battery.
Using DC Appliances in a DC Electrical System
When possible, it is recommended that you use a DC appliance in any application where the energy source is a battery. You will use less energy when compared to that used by an AC appliance powered by an inverter to perform the same function. Remember, there is approximately a 10 percent loss while using an inverter to produce the AC.
For example, a DC refrigerator may consume less power than an AC refrigerator will consume, but the result is the same… cold food storage. Dedicating some extra effort in finding DC appliances in place of traditional AC appliances when building your DC based system will prove extremely beneficial. The internet allows for easy product sourcing and comparison regarding DC appliances.
Many of these items are used every day by people who have no idea that they are using a DC powered product. When purchasing an electrical appliance, remember to ask if a 12 volt DC adapter is available for the product.
A Brief Overview of Solar Panels
There are two commonly used types of rigid solar panels. These are Multi-crystalline and Mono-crystalline panel technologies. Both are excellent and efficient means of DC power production. The only differences are that the mono-crystalline usually have a slightly higher efficiency and the multi-crystalline usually are slightly cheaper to manufacture.
There are also Amorphous silicon solar panels and newer technologies emerging like CIGS (copper-indium-gallium-selenide). These types of panels are available in either rigid or flexible (rollable and/or foldable) configurations. The advantage of these technologies is that they are less expensive to produce, but less efficient and they require more surface area to achieve that same power production as their Mono/Multi-crystalline counterparts. Also, Nanosolar technology is under development which could also allow irregular surfaces to produce power.
All solar panels work at their rated output in direct sunlight. So as the sun drops lower to the horizon in the late afternoon, most flat-mounted fixed solar panels gradually lose their power output. This means that a fixed mounted panel system works at rated capacity for about 3 to 4 hours per day (during “peak sunlight”), and summarily, are an inefficient means of power production.
To counter the inefficiencies of flat-mounted panels, it is usually necessary to install a larger solar panel array that maximizes the amount of power that can be produced during the daily period of “peak direct sunlight”. Solar Stik™ panels are articulated and they are able to face the sun from dawn to dusk, maximizing solar panel power output. This ability enables a smaller array to produce the same amount of power as a larger fixed position array. More on solar panel technologies and the Solar Stik™ designs can be found in the section “Choosing the Right Solar Panel”.
For a detailed introduction on how solar panels generate electricity from sunlight, please visit the following: science.howstuffworks.com
The Solar Stik™ System is so versatile that it could be used as an “everyday” power system for a wide spectrum of applications.
If one’s daily power consumption is greater than what a single Solar Stik™ can generate in one day, it is possible to add power generating capability in a multitude of ways:
- Multiple Solar Stiks™ can be “daisy-chained” or “tethered” together to create a larger power generating system. In the same manner, multiple Power Paks can be connected together to store the power.
- A boat could have a wind generator or other solar panels added to it’s deck, or an RV could have additional flat mounted solar panels placed on its roof. The methods and equipment for power production are numerous and usually easy to obtain.
- A Solar Stik System can be hybridized with other technologies, including traditional fuel-driven generators.
There are almost limitless variations in how the system is used… many requiring greater understanding of electrical systems, but hopefully we have given you a basic understanding of portable power and its use as a “closed-loop” power system.