Off-Grid Living With Solar

We have made a number of attempts over the last 6 years but have not been able to find a cost-effective off-grid system to provide our customers.  This has finally changed with the introduction of the Tesla Powerwall 2 which we feel very strongly about.

The Basic Problem: Batteries cost a lot and store comparitively very little power.  This is true of all types of lead acid batteries as well as the newer Lithium batteries. In fact the lithium batteries cost much more per kwh stored but have a longer lifespan (in the end lithium is still usually more costly that lead). A smaller secondary issue is that the equipment for off-grid is more elaborate than on-grid so costs more to purchase initially.

An Example of the best we could do before the Powerwall 2: We have worked out an example system.  This is a 3600 watt system so it is moderate in size and has decent capacity for small to medium applications.  Importantly, this is a simple, preconfigured system so it is possible for a DIYer to install.  Equipment includes: inverter, charge controller, breakers, enclosures, distribution panels, control panel).  However, we've combined it into a single unit so you just need to connect grid-power(if available), solar panels, batteries, load panel connection.  Estimated cost of the equipment is $5000(not including the solar system itself).

A 3600w system can provide 80% of it's rated maximum capacity without shortening the life of the equipment so that's 3600w * 80% = 2.9kw maximum draw.    

This will need solar panels to charge it and this system makes use of 3600w of panels which is 12, 310w panels (Total 3760). 

Now the batteries: A set of four 105AH lead-acid sealed batteries will cost $1400 including the necessary cables.  This does not include the shipping cost of the 300lbs of batteries. If instead, we choose high maintenance wet cell batteries then the cost is considerably lower at $800 for a set of four.  Wet cells need monthly toping up of distilled water and also can vent hydrogen so need a vented enclosure. Wet cells are high maintenance and you would need to learn some chemistry to maintain them correctly.

What do four 105AH lead-acid 12volt batteries provide?  The basic formula is Amps * Volts = Watts.  So one 105AH battery at 12 Volts = 1260Watt Hours or 1.2kwh.  Since there are four batteries this is 5kwh.  There is a small conversion loss but we'll ignore that for now. See this calculator.

Next to consider is how much you plan to drain the batteries.  Draining them 80% will give only 300 charge cycles before they wear out (about a year of full use).  40% will provide 600 cycles (about 2 yrs of full use).  Based on this the maximum draw should be 40%.  Four 105ah batteries can provide only 2kwh of power when 40% drained.

I should note that you may well not make full use of the 80% or 40% drain every day so the lifespan could be considerably longer.  One the other hand, if you have several cloudy days in a row the solar panels may not fully charge the system, so you might draw the batteries down further or need to curtail power use.

Using the 40% drain figure of 2kwh seems prudent for the next calculations.

Matching the 13.5kwh of the Tesla Powerwall  You would need seven sets of four 105ah lead acid batteries to provide the 13.5kwh storage of the Tesla Powerwall 2.  

If a set of four sealed lead acid batteries costs $1400 and you need seven sets then the cost raises to $9800 plus the $5000 for the equipment.

If you want to have a ten year lifespan system you will need to replace the batteries four times at a cost of $9800 each time.  This brings the total 10 yr cost to $54,000.  That's insane

Maybe you can double the amount of batteries from 28 to 56 and reduce the drain to 20% which would extend the life of the batteries so you only need to replace them once but that's 14 sets at roughly $20k each time which is a 10 yr cost of $45,000.  Any you'll need a lot of space for this pile of batteries.

The Powerwall is 85% Cheaper!

Now instead lets talk about the Powerwall 2.  It costs about $7000 fully installed with a 10 yr warranty and zero maintenance.  This is a genuine breakthrough and costs 85% less.  So, even if you only need 8kwh of storage the Powerwall is massively cheaper and more reliable.