Wanting to be Off Grid

Re: Wanting to be Off Grid

Let me try answering a different way... First, this is just simply a rough number that will help you choose a direction. Your actual costs could be 50% of what I said or even 2x as much--But from an engineering point of view, it is a "good enough" number to understand the problem and figure out what to do next.

Very roughly, an off grid system is about 10x the cost ($/kWH) of utility power for the "average" home. So, your $400 per month electric bill would cost you around $4,000 per month in actual money out of your pocket (total cost over 20 years divide by total kWH used/generated over 20 years).

And the cost to build such a system... Around 1/2 of that 20 year cost has to be paid for "up front" to buy the components and install the system:

• $400 per month * 10 off grid cost * 12 months per year * 10 years = $480,000 up front cost

Just to do some very rough math--Assume you pay around $0.10 per kWH, that is 4,000 kWH per month. Designing a nominal off grid system (the numbers I would be recommending as a starting point) would look like this:

• 4,000 kHW per month * 1/30 days per month = 133 kHW per day
• 133,000 WH per day * 1/0.85 inverter eff * 1/48 volt battery bank * 2 days of storage * 1/0.50 max discharge = 13,039 AH @ 48 volt battery bank
• 13,039 AH * 48 volts * 1/(6 volt * 220 AH golf cart battery) = 474 "golf cart" batteries

Obviously, I would not tell you to buy GC batteries--Just to give you a scale of something that you would be familiar with. If this was done with ForkLift batteries, it would look something like:

Crown Industrial Battery - 24 Volts, 1875 Amp-hoursPrice: $7,180.66

• 13,039 AH * 1/1,875 AH batteries * 2 (two batteries in series for 48 volt) = 13.9 = 14 fork lift batteries
• 14 * $7,180.66 per battery = $100,529 battery bank

Next, solar array... If you have a minimum of 4 hours per day of sun (typical US average minimum sun for ~9 months a year):

• 133,000 WH per day * 1/0.52 over all system efficiency * 1/4 hours of sun per day = 63,942 Watt Array minimum

And a fork lift battery typically needs a minimum of 10% rate of charge:

• 13,039 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 99,909 Watt array minimum (based on battery needs)

So--The "nice" sized array would be around 100,000 Watt... To purchase and install such an array would be in the range of $2 per watt (panels+racking+Wiring)

• 100,000 watt array * $2 per Watt = $200,000 for the solar array

Need charge controllers--The typical large MPPT controller is ~80 amps of output current:

• 99,909 watt array * 1/59 volts charging * 0.77 panel+controller eff * 1/80 amps per charge controller = 16.3 = 17 charge controllers
• 17 charge controllers * $610 each = 10,370 each

By the way, the batteries will last ~15 years (if you take good care of them, and most people around here have "killed" the first battery bank by accident, so it may not even last 5 years)--So you will need to purchase at least one more battery bank in the next 5-15 years. Your electronics (charge controllers, AC inverters, backup battery chargers, etc.) will last around 10+ years--So there is another $100,000 expense for batteries (plus inflation which is getting really bad right now). Plus another $20,000 or more for electronics at ~10 years.

Plus a genset (call it a 100 kVA genset) and maybe 10 gallons of diesel per hour every time you fire it up. Plus a few thousand gallons of diesel on property for long term storage/emergency use (another $50-$100,000 expense).

As I said, we can "fine tune" the requirements--But this gives a good idea of how much it would cost to replace a $400 per month utility bill with a full off grid power station. And we have not even included the 10-30% incidentals (wiring, permits, taxes, engineering, labor if you do not do it yourself, etc).

So, if you could justify spending $240,000 (1/2 of the cost of a solar power system) on your home to reduce power usage by 1/2--Could you think of anything that you would do to replace your bill?

Lots (and lots) of insulation (lots of ceiling insulation), triple pane widows, awnings over windows, energy conseration appliances, a super efficient A/C / Heat pump system. Get your hot water/electric heating from a heat pump system. Reduce inside electrical usage (lights, TV, laptop computers vs desk top servers, etc.)--Less heat you bring into the home, the less you pay for A/C to move the heat back outside again. Etc....

Don't get two wrapped up in AC vs DC kWH--From a math point of view, there is no difference. From a practical point of view off grid power is typically at 48 volts DC and your large appliances (electric stove, electric water heater, A/C, etc.) all run at 240 VAC.

Say your peak load is 60,000 watts (just a wild guess):

• 60,000 watts * 1/240 VAC = 250 Amp Utility Service
• 60,000 Watts * 1/48 VDC * 1/0.85 inverter eff = 1,471 Amp "DC Service"

You need ~6x heavier wiring/larger circuit breakers/etc. just to run the "DC side" of your home's power system... Plus you still need the AC 240 VAC wiring side too.

Does any of this help you to understand why this much power ($400 per month utility power) becomes such a huge off grid solar power system?

Don't get me wrong, large systems are done all the time. It is just for the "average" home owner, off grid power does not save any money when compared with utility power.

In some places (like California) where we can pay upwards of $0.50 per kWH, installing of Grid Tied solar power system with a good net metered time of use plan, we can actually save money. A GT power system can cost $0.15 per kWH or even a bit less for solar power--But this system has no batteries and cannot run if the utility power has failed (the utility is, for all the world, like a giant AC battery bank for GT solar).

At this point, many areas GT solar can be a good deal (and 1/4 the cost of off grid solar)--However, there significant planning/building code issues, and many utilities are beginning to put major road blocks in front of new GT solar installs. (utility does not like GT solar as their other customers are significantly subsidizing the GT solar customer--another thread of discussion).

-Bill