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Our Houston area HOA does not allow roof top solar panels and I doubt they would survive our hurricanes too. I discovered NRG Energy sells these portable solar panels under their goal zero brand.

If we used these solar panel kits to simply charge our phones everyday, how much energy would we take off the grid?

Our monthly utility bill is about $40 while gas is about $90. How much money would buying one kit or several kits save?

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  • Why would they not allow rooftop solar panels?
    – gerrit
    Commented Feb 24, 2015 at 16:30
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    @gerrit because it's a master planned community and they want every house to look similar. Also Houston is the oil and gas capital of America so they discourage renewable energy in general.
    – Danger14
    Commented Feb 24, 2015 at 16:32
  • @gerrit I just found out we can install rooftop solar panels but have to pay $25 to submit an application to HOA design committee.
    – Danger14
    Commented Feb 25, 2015 at 7:02

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I'm afraid these little mini solar kits are much smaller in scale than rooftop panels. They can be great for charging gadgets when away from mains electricity, but they're not an economic substitute.

Let's make some assumptions:

  1. You have three mobile phones in your household.
  2. Each phone has a battery with a capacity of 2Ah (2000mAh) at 5V
  3. One of these solar chargers allows you to keep all of those phones fully charged without having to use the mains.
  4. If you were charging from the mains, the efficiency of that charge would be 80% (I've plucked this figure out of thin air; it may well be wrong).

The total amount of energy that the three batteries can hold is 2Ah x 5V x 3 phones = 30Wh. The total amount of electricity that you would use to charge these from the mains is 30/0.8 = 37.5Wh.

In most countries, electricity is paid for by the kilowatt-hour (kWh). The amount of electricity that you'd be saving is 0.0375kWh/day. Assuming that the cost of electricity where you are is 20 cents/kWh (I have no idea whether this is realistic), then you would save 0.75 cents/day, or about $2.70/year.

Some of the assumptions above may be way out but I think it's safe to assume that, since the solar charger costs $100, it would never pay for itself.

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The Goal Zero PV panel is rated at 20 Watts. Specifications here

Here is the average insolation figures for Houston
The line in the 4th chart titled "Insolation, kWh/m²/day" gives the kWh per square meter per day ~= hours of equivalent full power sun. You'll see that Houston average about 4 kWh/m^2/day. That means that an eg 20 W panel will produce about 4 x 20 = 80 Watt hours per day maximum.

If you average 4 hours of equivalent sun per day annually and if you use 100% of the panels output (which you won't) then at $0.12 unit:

Watt-hours/day = 20 x 4 = 80 Wh = 0.08 kWh/day

kWh = units per year =  365 x 0.08 = 29.2 units.

Value of energy in mains electricity equivalent at $0.12/unit = 
29.2 x $0.12 = $3.50

That may sound wrong.
It is.
Actual amount will be less due to matching panel to load, not steering panel toward ssun etc.

Goal Zero has come close to achieving its goal - at least as far as the amount of energy provided goes. Such systems are valuable when grid power is not available but utterly uneconomic when grid power is present.

[FWIW: I design small solar lighting systems as part of my 'day job' so I'm confident that the results are 'in the order of correct']

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