Published Saturday July 7, 2012: Updated 5/7/2014
Off Grid Solar Panels -- Is it worth it?
Grid-tied Solar panels are awesome and very cost effective, but when the power goes out, the inverter(s) shut off and you are left without any power. In our neighborhood, this happens about 20 minutes a year. Essentially never, which is why I went with an on-grid only system.
Still, it would be nice to have some way to be able to switch my setup in an emergency to work off grid. It can be done but will cost me about $3400 to retrofit my installation. In the mean time, keeping a few marine batteries charged and an inverter nearby will have to suffice for minimal backup power in an emergency.
Rule of thumb for Off-Grid solar installations:
In northern Utah, the average amount of full-time sun hours is 5 hours per day (to find your needs click here). This means if you want to run something purely off-grid using only solar power 24-hours a day, you need to divide 24 hours by 5 full sun hours. 24/5 = 4.36. So if your average consumption is 100 watts, you will need a 436 watt solar panel to equal the energy required to run something for 24-hours by only using 5 hours of full sunlight. In practice you would need an even larger solar panel since the losses involved in storing electricity in a battery are not zero.
You would also need a battery capable of storing all the energy produced in the day-time by the solar panel.
Off Grid Battery: To store 24-hours worth of energy sufficient to cover your average power consumption you need to multiply the average power consumption by 24-hours. For example: 100 watts x 24-hours = 2.4 kWh. For a 12V battery system, 2.4 kWh*1000/12 = 200 AHr battery.
A lead-acid battery should never be discharged all the way to zero or else its life is shortened significantly. A DOD (depth of discharge) of 50% will prolong the life of the battery system. So for the above example, you would need a 400 AHr battery.
A battery pack in my EV truck capable of storing 10.5 kWh @ 50% DOD.
Over the period of a year, you would actually need an even larger solar panel and a much larger battery to sustain your electric needs off-grid.
In the winter time, Our home's solar production is a small fraction of what we make the rest of the year. And it’s a small fraction for weeks and months at a time. For example, if I wanted to run our house completely off grid using the existing 6.2 kW solar array, I would need a battery large enough to store 1 MWh (megawatt-hour). That is somewhere on the order of 1500 large 12V marine batteries (with only allowing them to go to 50% DOD).
In the spring time, the panels make 3 times the amount of power that we need. After a couple of months, my super massive battery would actually be full and there would be no place to put the excess 500 kWh of energy. It would go to waste and not be available for the winter time when we need it. To have no wasted energy, we would actually need a battery large enough to store 1.5 MWh, (on the order of 2000 large marine batteries).
I know you really want to stick it to the "man" and go off grid but at what cost?
- The grid is essentially an infinite capacity battery that you don't have to pay to maintain. Actually, I still have to pay $5.00 a month for the privilege of having the utility's power meter on my house. But that is pennies in comparison to the cost of operating and maintaining a large battery backup system.
- Contrary to what the media portrays, power utilities actually love it when people install grid-tied solar panels. Solar panel installations distributed across multiple power substations make the grid much more stable and robust than it would be otherwise. This is why utility companies offer rebates to people who install grid-tied solar panels.
- When the grid is being burdened the most (hot sunny days), solar panels are making an abundance of electricity and feed it back to the grid. They cause the power meter to roll backwards. That excess energy then goes to supply neighboring customers. Grid-tied solar panels are the friendly neighbor of the power grid, willing to lend a hand to others in need.
Are you willing to make some major lifestyle changes, sacrificing convenience and luxury? When the grid is readily available, reliable and affordable, going off grid doesn't make any sense.
Remember Rule #1: Save energy and money without sacrificing convenience and luxury.
Off-grid does makes sense if you are in such a remote location that it would be prohibitively expensive to bring in power lines in order to tie into the grid.
Having a small emergency off-grid battery backup also makes sense if the grid is unreliable.