Discussion of Off-Grid systems

Despite repeated efforts, we do not currrently offer either off-grid or backup power solutions.  So we've started this section to discuss the challanges of completely off-grid and hybrid systems.

We get a number of calls from people interested in Off-Grid / Backup Power systems.  On two occasions we have invested considerable time and money in developing off-grid / backup options but when we offered them for sale few customers were willing to purchase them because they were very costly, we can't blame them.  

We are still investigating off-grid systems to find a way to provide a solution that at least some customers would consider at a somewhat reasonable cost.  We are not currently hopeful we will find a solution anytime soon.

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 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: 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.

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

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

Now the batteries: A set of four 105AH lead-acid maintenance free 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. 

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 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.

What can that power?  A KWH is 1000w of power for 1 hour or 100w of power for 10 hrs. If you figure out the power use of all the equipment you plan to use and the number of hours of use each day you can compute your battery needs. 

Plan for a set of four batteries for each 2kw a day you need, we can supply a number of battery sets for a system to meet your needs.

For example an average, modern refrigerator uses about .9 kwh a day. (Here's an idea, put a couple gallon jugs of water in the freezer and fridge to store cold.  Then put the fridge on a timer so it's only running when the sun is shinning.  In this way you are storing cold instead of electricity so less batteries are needed.) Being clever is part of the fun.

A modern 60w replacement LED light uses 10w so it uses 100w or.1kwh if used for 10 hours a day.

A modern laptop uses 40w so using it for 5 hrs is 200w of .2kwh. (Change the laptop battery using the day using the sun is hitting the panels and there is no need to provide house batteries for it) 

A well pump uses a lot of power but for a very short period of time so this can be supported as well.

List every piece of equipment and get a Kill-O-Watt meter(On Amazon) to record it's daily power requirements.

Some other appliances:

  • Cable box (No DVR) 27w
  • Boiler pump 100-400w each
  • CFL Light 13w
  • 5k btu AC 500w
  • DVR 45w
  • Coffee Pot 950w
  • Electric cook element 500-1500w
  • TV 50" LCD 200w
  • TV 50" LED 100w 

You can imagine that buying the most efficient appliances is much cheaper than adding extra batteries.

So, that's what we're working on right now.