Solar Generators vs. Portable Solar Power Banks
When it comes to small, off grid portable solar power applications, there are several products on the market that manufacturers claim to be “solar generators.” So, what are they and how do you choose the best one?
First things first - THERE IS NO SUCH THING AS A SOLAR GENERATOR.
By definition, a generator is a machine that converts one form of stable energy into another, e.g., a dynamo that converts mechanical energy into electrical energy. Solar energy is hardly stable. All those “solar generators” on the market are nothing more than varied size batteries with built-in inverters and displays with usable outputs. Further, most of them are not sold with solar panels for charging, and barely give you enough power to turn on a few lights for a couple hours and charge a cell phone.
We prefer to use the term Portable Solar Power Bank as a more accurate product description. And, defining your preferred size in terms of battery capacity and desired outputs is going to depend on your intentions and requirements for off grid use. We already addressed how much power you need for small portable devices in our earlier blog post, Choosing a Portable Power Bank.
In this discussion, we are going to address the larger, more stable lithium-iron phosphate (LiFePO4) battery with usable outputs as the most ideal solution for temporary off grid power, and save the conversation on deep cycle gel and lead acid batteries for another day.
Why LiFePO4? That’s easy. It’s clean. It’s green. And it’s safe. A LiFePO4 battery is comprised of nontoxic materials, has superior chemical and thermal stability over other lithium ion batteries, enabling it to store larger quantities of energy, and remains cool at room temperature.
Before selecting your off grid portable battery power bank, there are a few things you need to decide upon:
- Battery wattage and capacity
- Built-in inverter and outputs
- Charge controller technology
- Using solar panels for charging
Point 1 – Battery Wattage and Capacity
Round up all the electrical appliances that you intend to use with your off grid system: lights, toaster, coffee makers, microwaves, portable electronics, etc. Check the power supply for each item and write down the number of watts each one will draw. If only volts and amps are listed on your appliance, watts = volts x amps.
Next, write down the number of hours in the day you intend to use the appliance. Note that we are measuring watt hours because electronic devices and household appliances consume power in various voltages. In order to calculate how much stored energy you will need to charge small portable devices such as cell phones and tablets, please review Choosing a Portable Power Bank.
You will be surprised to discover how power hungry your electric appliances really are.
Let’s do a quick example.
- Microwave: 1000 watts x .25 hours (15 minutes/day) = 250 watt hours
- Lights: 4 light bulbs at 60W each = 240 watts x 2 hours = 480 watt hours
- MS Surface Pro (Input): 100V x 1.16A = 116 watts x 3.4 hours = 394 watt hours min
- Mobile Device Charge: 5V x 2A = 10 watts x 1.33 hours = 13.3 watt hours (be sure to multiple this if you need to charge more than one mobile phone).
Baseline Battery Power Need: 1137.3 Watt Hours
Ideally, you should aim to choose a battery that will provide at least 25% more watt hours than your baseline, to avoid draining your battery down to zero. In this example, watt hour requirements are:
1137.3Wh x 1.25 = 1421.6Wh
Note that you can opt to stretch available wattage further by using LED lights at significantly lower wattage requirements, and expand the use of your power supply for other appliances.
Point 2 – Inverter Options
Next, before you run out and buy a large portable battery power bank, consider the inverter capability. The inverter’s job is to regulate the incoming energy load (varied amounts of solar power) down to stored energy in the battery, and distribute the energy at levels usable for home electronics via built-in AC, USB or other outputs. Commercial choices are pure sine wave or modified sine wave. Pure sine wave delivers a cleaner, smoother electrical current, typically required for larger appliances. Modified sine wave will suffice for smaller electronics.
You want to have a built-in inverter that provides a variety of electrical input/output options to maximize flexibility in how you can both charge the battery and use the power supply.
Typical power charge inputs are:
- DC charging port (connected to standard AC outlets)
- Power pole
- 12V auxiliary power outlet (aka car cigarette lighter)
- Solar panels
Typical power outputs are:
- USB 5V with amp outs ranging from 1A-3.3A (note that laptops and most tablets require more than 1 amp out; newer laptops may require higher, regulated voltage)
- Auxiliary outlets ranging from 11V – 14.8V up to 15A
- 12V power poles also with 11V – 14.8V, but higher amperage up to 25A
- “Standard” outlets with 110-120V and varied amps out, depending on the battery’s wattage capacity.
Be sure to read the product description of the battery/inverter combination before you buy, and remember that W = A x V. So, if a power outlet’s max amperage is 10A, and can use up to 120V, you would be using a maximum of 1,200 watts with this particular outlet, depending on the appliance being plugged in. If the battery capacity is higher than 1,200 watts, then you have extra wattage available to run other devices, such as low wattage lights, or cell phone charging, simultaneously.
Finally, please keep in mind that amps are how much electricity an appliance is using, so if your small appliance tries to use more amps than the inverter output is providing, the appliance won’t run.
Point 3 – Charge Controller Technology
Solar charge controllers are required for any solar panels that provide output options at or above 12V, and are typically built into portable solar power banks.
There are two types of charge controllers: Pulse width modulation (PWM) and Maximum Power Point Tracking (MPPT).
Portable battery power banks (aka “solar generators”) will have one or the other technology built-in. PWM capability is fairly simply in that it reduces the incoming solar power voltage down to that of the battery to which it is connected, and lowers the amount of power applied as the battery gets closer to a full charge.
Portable solar power banks with built-in MPPT technology are more sophisticated (and expensive). Solar panels with high wattage ratings often deliver far more voltage than needed. MPPT technology takes advantage of extra voltage to effectively reduce the amount of time needed to recharge batteries. Where you have multiple solar panels with wattage over 100 intended to charge one or more large batteries, MPPT technology is your best bet for efficient and timely solar charge.
When shopping for portable solar power banks, if the product description does not indicate MPPT technology, it is highly unlikely to be a feature as it is considerably more expensive to manufacture.
Point 4 – Opt Off Grid: Time to Charge Using Solar Panels
Charging your batteries using solar panels requires substantial solar panel wattage ratings for successful charging in a relatively reasonable amount of time. A 6W panel will do nothing more than trickle charge a 12V battery, if you are clever enough to properly angle that panel toward the sun during peek sun hours. A 100W panel is a good starting point, and additional solar wattage combined with an MPPT charge controller will greatly enhance your charge speed.
Though not a perfect calculation by any means, to roughly estimate time to charge with a set of given solar panels, look at 3 factors:
- Number of watts your portable solar power bank can deliver out
- Solar panel wattage rating
- The solar irradiance figure in your geographic location, as expressed in kWh/m2/day.
To determine the solar irradiance figure of your geographic location, visit the interactive photovoltaic map provided by National Renewable Energy Laboratory, US Department of Energy. On the tab “Data Layer” check the box for “solar photovoltaic”. Click the tab “Query” and zoom in to your exact geographic location in the lower 48 states, or click on “region query” and highlight a section of the world map for which you want to see the data. You will see results expressed in Wh/m2/day, or watt hours per square meter of solar panel surface area, per day. Convert this figure to kWh/m2/day.
To determine approximately how many total watts of energy per day that you can get from your panels, multiple your solar irradiance figure (kWh/m2/day) by 75% to account for inefficiencies during the charge process, and then multiply that figure by the total wattage of your solar panel.
Let’s look at one example.
- 1500W LiFePO4 Portable Solar Power Bank
- 180W Solar Panels
- 5.3kWh/m2/day in Miami, FL
RESULT: 5.3kWh/m2/day x .75 x 180W = 715.5W per day.
APPROXIMATE TIME TO CHARGE: 1500W = 1500W/715.5W = 2.1 days of ideal Miami sunshine.
Of course, if your portable solar power bank allows simultaneous charge and discharge, you can use a bit of power during the day while keeping it charged for power reserve after dark.
For temporary off grid power needs, Solar Sporting Goods highly recommends the Aspect Solar 1500W + 1500WH Pure Sine Wave Portable Solar Power Kit with Maximum Power Point Tracking (MPPT) for efficient solar charging. Double your output power by daisy chaining 2 - 1500W PoweRacks together. With 2 PoweRacks combined, this all-in-one solution is enough off grid power to handle a small cabin or tiny home for a few hours each day, if used conservatively. It is robust enough to handle large appliances for short periods of time, and can still be used to keep the lights on each evening.
Looking for small, portable refrigeration? Check out our assortment of Dometic CoolFreeze Portable Powered Cooling Boxes in our camping section. A great solution for temporary off grid refrigeration when connected to the Aspect Solar 1500W PoweRack portable solar power bank, or use can even keep this cooler going with an auxiliary output.
 To calculate hours needed to charge mobile devices, you need an extra math step. The formula is:
- Ah device battery x 33.3% (conversion loss %) = Ah required
- Ah required / charging amp current = time to charge, in hours.
The MS Surface Pro battery size is 5087mAh, or 5.09Ah
- 5.09Ah x .33 = 1.7Ah; therefore 5.09Ah + 1.7Ah = 6.8 AH (5.09 + losses)
- 6.8Ah / 2A port = 3.4 hours to charge
An average mobile device battery size is 2000mAh, or 2Ah.
- 2Ah x 33.3% = .66Ah; therefore 2Ah + .66Ah = 2.66Ah
- 2.66Ah / 2A port = 1.33 hours to charge
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WE'LL SEE YOU ON THE SOLAR SIDE!