PV Gap

Harvey Said:

Solar power, number of panels needed?

We Answered:

You seem to be confusing volts and watts, so let's review some definitions:

A 'volt' is a unit electrical "pressure". It has its uses but it's a minor player here. In this context, it should be used to see that the parts all match up. And a 26.6 volt panel doesn't sound like a match to a 12 volt inverter! As you probably well know, you don't try to power a 6 volt anything with a 12v battery. You can damage it.

Make sure the inverter can handle the voltage of the panels. Buy the panels first. See the wiki link on inverters.

A 'watt' is a unit of _power_. A computer might take 70 watts. A hair dryer typically consumes 1500 watts. If you run a hair dryer for 2 hours straight, you've used a certain amount of _energy_ and, for our purposes, we can use the term 'kilo watt hours'; the hair dryer would have used 3 kWh (2 hrs x 1500 watts / 1000).

The starting point of your calculation should be your electric bill which'll define how much energy you use per month; the value will be in kWh. I'm starting you here because 16 kW (kilowatts, not kWh) is a lot and I'm suspicious that there's been an error along the way.

Suppose your electric bill says your household used 720 kWh in a peak month. Divide that by (roughly) 30 days and 24 hours/day (720 hours) to find that your average power is 1000 watts (you can see why I chose 720 kWh). So you have to add in some fudge factors to account for overcast days and solar collection inefficiencies. Don't go crazy with the overcast days thing (unless you live on the Oregon coast); I'll return to this topic later. The 'solar collection inefficiencies' result, usually, from the panels not being pointed at the sun all the time - and the sunlight gets weaker as the sun nears the horizon (extinction coefficients...). The solar industry says that, for my latitude of 38 degrees, a stationary solar panel will produce energy as though it got direct sunlight for 4.3 hours. So our 1st fudge factor is 24/4.3 or 5.5; the panels would have to produce 5.5 kW. But wait, there's more...

Everytime you store or retrieve energy from batteries, you lose about 10%. It's a corollary of the "no free lunch" rule. So multiply the 5.5 by 1 / .8 = 6.9 kW. And your inverter might dump 10% of its efforts into heat. 6.9 kW / .9 = 7.7 kW.

7700 watts / 210 w = 37 panels. That's a bunch but this midpoint calculation is just to get an idea of 'near worst case'. So, still ignoring the overcast day issue, we got to about 1/2 of the 16kW. But my starting point of 720 kWh wasn't wimpy. The house that came from has a hot tub, 3 computers, air conditioning in a hot climate etc.

You have options:
1. If you have the land etc, why not have the panels track the sun? You can improve greatly on the 4.3 hour value.
2. Consider only doing part of your house's appliances, the really necessary ones: refrigerator etc. If air conditioning is a major energy sink, look into alternatives (smart vents that let hot air out, for instance).
3. Ebay frequently sells panels for about $2/watt. Pay an expert to make sure you're getting a high quality set of panels. $2/watt x 7700 watts is $15400 and you still need mounting, an inverter, etc. Still way shy of $125000. You could pay the expert $800 and still make out like a bandit.
4. The amount of money considered here is substantial enough for you to consider other sources of energy savings, such as double paned windows, thicker insulation, modern and more efficient air conditioner, refrigerator etc.

Regarding the overcast days, you don't have to just multiply by 2 or whatever. You can make your own "energy plan" such that you gleefully use all solar energy when it's available but use the grid to supplant it when it's overcast. Much of the time, you'll likely have an excess of energy which you can sell to the power company.

In the above, I _presume_ you're not building a house in Timbuktu where there is no power company. Obviously some of your options change for such a locale (but you'd sure have lotsa sun!).

I've also assumed in the above that the solar panels are being kept clean. Another reason to keep them off your roof.