Gerard Foret

So here's the major differences between Lead-Acid and LIPO.

Depth of Discharge. LIPO can discharge to 20% without harm, lead-acid 50%, meaning for a desired capacity, it would take 30% more lead-acid for the same use.

Weight: On average, including Battery management, LIPO weigh 75% less than lead-acid

Charge cycles: LIPO will endure at least 4 times the number of charge/discharge cycles of lead-acid.

Note that LIPO batteries are rated in USABLE capacity, not total capacity. Lead-acid are rated at total capacity. To maintain the appropriate level of discharge (50%), you need twice the capacity of lead-acid to equal LIPO.

So, let's do some math:

Lead Acid: approx. $0.11/watt (this is what I paid for 4-6v crown lead-acids for my 24v bank). A bank capable of cycling 7 kw (meaning a 14kw bank) would cost $1,540. If properly maintained and cycled daily, this bank would provide about 2 years of service before needing replacement (or losing significant capacity). Over 10 year this bank would need to be replaced 5 times for a total cost of $7,700. Let's say you can nurse it and get twice the life (4 years), the cost would still be $3,850.

Tesla's 7kw pack has a $3,000 price for the same 10 years.

Regarding the higher voltages, we've become conditioned to use DC at low voltages but the benefits for higher voltage are smaller conductors, less line and conversion loss.

Please give me a link to where you can buy Li-ion for 20-25% the per kwh price of the tesla. I'm looking at converting my camper from lead-acid to lithium and would love to find a price lower than $0.42/watt

Steve Farmer wrote:I'm confused about the hype around this.

You can already buy lead acid or Li-ion batteries for around 20-25% of the per KWh price of this tesla pack.

I've been trying to find out the warranty details. Both are warranted for ten yrs. The 7KWh pack for "daily use" the 10KWh pack for weekly use.
If Tesla has made a Li-ion pack that really can go thru 3650+ recharge cycles then I am impressed.

I wonder, and it's only a guess in lieu of them publishing much detail on their warranty, if they expect to be replacing batteries in the pack under warranty well before ten yrs are up but the 4-5x overpricing is going to cover this, especially assuming that many/most customers won't cycle the pack every day and that costs to tesla will fall over the ten years.

The battery bank will give you a total of 12kw of energy. Typically, we never want to discharge lead-acid below 50% so that gives you 6 kw usable. I size my battery bank to provide 3 days without solar input so in your case that's 2kw/day available.

As long as your have a little more than 2 hrs of solar insolation per day, you should be able to top off your 50% drained bank in one day.

I've got inverter envy. That's one sexy inverter and seems appropriately sized for the the rest of the system.

Nice!

Austin Laureski wrote:I wanted to run the system by you all before buying the rest of the components. First I'm building an off grid system. I already have 12 x 255 watt Canadian panels for a total of 3000w. In the future I want to add another 1500w. I want to build a 48v battery bank. I don't know my power needs yet but currently we use about 300kW to 600kw a month with normal living on grid which includes the a/c in the summer. For the offgrid house I want to add a 12v system for lights off the 48v bank using a step down converter or by another charge controller to a 12v battery.

Here's what I want to get let me know if it will work or if it's just not going to work right.
8 x Trojan t 105
Midnight Classic 150
Conext XW+ 5548 By Schneider
Golf Cart DC Converter 48V to 12V Step Down from China

That's what I am thinking so far what do you think

Samuel,

Let's start with the 10 watt panel. I'm going to make a few assumptions as we go along but I think you'll get the idea of the general principles involved.

A 10 watt panel tends to mean it will provide 10 watts at a given voltage when being exposed to unobstructed sunlight. So, let's say the given voltage is 12 volts. Most off-grid solar systems utilize batteries to store the power until needed, and deliver it in larger quantities for shorter periods of use.

A battery is a bank where watts can be stored until needed. You're assumptions regarding a battery which can deliver 10 watts for 100 hrs would also deliver 20 watts for 50 hrs is correct.

With most solar designs, we start with the needs (or loads) as the base requirement.

So, let use your message and start some calculations:

LED lighting:
Very efficient, especially if using 12v lights which do not require an inverter. I use 3-10 watt/12v LED floods inside my barn, and 2-20w floods outside the barn on a motion sensor. These lights are used only during tending the horse at night, or late or early feeding, and the outdoor motion controlled lights seldom run for more that 10-20 minutes a night. Since most devices uses that watts number to indicate watt/hours, here's my power requirements for my lights assuming all lights will be used for 20 minutes a day:

70 watts/20 minutes= 23.33 watt/hrs per day.

So now I need to convert that to numbers that match battery rating which are typically AH (amp/hours). I use a web based ohms law calculator for this and get 1.94 AH per day. Let's say 2 to make the calculations easy.

We also have to consider how many days we want the system to work without sunshine. I use 3 days. So I need 3 times my needs to be available in the battery when I need it. That's 6AH. When using lead-acid batteries it is wise to only drain them down to 50% of their rating to prolong life. That means for the above example, I should purchase a battery with 12AH of capacity.

This has told be how much battery I need to power the stuff I want to power, when I want to power it.

Now I can calculate how much solar panel I need to make this all work. I want the system to be able to recharge from a 50% drain in 1 day. Using the same ohms law calculator, I need panels that can put back 6ah at 12 volts in 1 day.

It gets tricky here because each location is unique and has different sun exposure issues. In solar terms, we call that insolation, and you can find the insolation for your area using google. Mine is an average of 4.5 hrs of sun a day.

I need to put 6ah back into the battery in 4.5 hours. Using the ohms law calculator I find out I need 72 watts in 4.5 hours to equal 6ah. That's 16 watts/hr. I need at least 1 16 watt panel to recharge my system.

This approach applies to all off-grid systems using batteries. Start with the work to be done, determine the battery capacity and the amount of backup power (how many days without sun), and then calculate how much solar panel is needed to recharge the system.

You can use the same calculations for the 12v pump. The blender is going to be a little more complicated unless it's a 12v bender, and I'm thinking it's going to be a power pig.

Also, for any moderately small system you'll find you'll need a solar charge controller to prevent battery over-charging and manage the battery discharge.

Samuel Morton wrote:Good evening everyone,

This is my first post and forgive any ignorance regarding my electrical knowledge.

I am thinking of buying some solar panels for my shed roof which I hope will power (not full time but as and when said appliances are needed) a LED lighting strip and a small water pump to help me wash my produce (and maybe a blender and phone charger).

The solar panel system I am looking to buy is 10watt 12 volts and I was just wondering about the following things:

- what appliances could this power?
- if this solar set-up can power a 10watt appliance for 100hours (for example) would a 20 watt appliance be able to be used and would last 50 hours with this set-up?
- if I buy two systems and have two batteries and connect these together will I then have 20watts and 24volts?
- Will 1 panel take twice as long to charge two batteries and would two panels charge a single battery twice as fast?

Thanks for getting back to me,

Samuel

This is correct as well. The goal is watts (work that can be done) and higher voltage systems do that the most efficiently. On small systems it's of little consequence other than battery health and safety. My small barn system (250w) is 12 volts (single 12v deep cycle). My 1kw system running refrigeration is 24 volts (4-6v Crown 235 in series). On large, multi-kw systems 24 volts or higher is the norm. Another reason for this is the charge controller(s) can only handle X amps in charge capacity, and a parallel bank requires a much higher charge current than a series bank for proper charge rate. Both deliver the same watt/hrs of work minus inefficiencies.

Charles Tarnard wrote:Everything said above is correct, so if you understand those posts but not this post, then ignore this post.

Voltage is a measure of potential, or differential. With each 12v cell you have a difference of 12v from terminal to terminal.

When you put the terminals in series, you are increasing the differential from the most negative point to the most positive point (in other words, adding them up). In this case the cell with the lowest current capacity (the panel with the least sunlight, the battery that is weakest) limits the whole set in amount of current they can deliver.

When you put the cells in parallel, all the positive terminals are on the same point and all the negative terminals are on the same point so the difference across all the cells has to be the same as the difference across any one cell. In this case each cell can deliver current across the terminals in the same way so current capacity can be added up.

Hope that wasn't totally confusing.

chad duncan wrote:
You lose less power to the resistance in the wires when you run at higher voltages.

Here's the why:

While solar panels are rated in watts of output, watts is actually a product of voltage x amperage. Where voltage is pressure and amperage (or current) is flow. Think of your wiring as pipe to handle this flow.
So, think about water in a pipe and I want to fill a 5 gallon bucket from an 10 foot elevated (volts/pressure) water source containing 20 gallons of water (watts) . I can use a large pipe ( higher amps/current/flow) and fill the bucket in a certain amount of time. Or, I can elevate the water source to 20 ft (increase pressure/voltage), use a smaller pipe (lower amps/current/flow, and fill the bucket in the same amount of time. In this example the higher pressure (voltage) allows me to use a smaller pipe and still get the same amount of work done. Copper is expensive. By using higher voltage I can reduce my wire size and save money. The same theory applies to battery bank voltage. Notice on inverters that their efficiency goes up when using 24 or 48 volt inverters vs. 12 volt inverters.

Secondly, especially in longer wire runs, higher voltages experience less voltage drop than lower voltages. Since the current is lower (and the voltage/pressure is higher), there is less reaction to the electrical resistance of copper wiring, meaning more juice from the panels getting to your controller (or from your battery bank to your inverter).

Lastly, higher voltage allow for longer cell strings in the battery bank. There was a recent heated discussion about the importance of reducing parallel strings to improve battery bank health and safety, meaning it is safer and healthier for a battery bank to contain 4-12 volt batteries in series and operate at 48 volts than it is to have 4-12 volt batteries in parallel and operate at 12 volts.

C. Hunter wrote:(but google is giving me too much crap to find the info I need after 20 minutes of sorting advertising from info.)

Can someone explain the whole why of wiring panels in parallel vs series? I've been watching the solarhomestead videos and he's got his set up in series (I think?) and I know that's something to do with 12v vs higher voltage, correct?

Trying to learn all the things. Think my head may explode.

I've read (but have no experience with) modified square wave inverters being hard on refrigerator and freezer compressors which is why I went with Pure Sine Wave. It sounds like you've been running your fridge with modified square wave with no problems.

That's exactly how my project started. I wanted backup for my fridge and freezer in case of a power outage. After designing to the capacity I needed, I realized that I could use it all the time, not just when power was out.

Depending on the size and power requirements of the well pump you may be able to make it work. The system I'm putting in to run my fridge (which is the real power pig) and freezer is costing me right at $3,000 and I'm doing the install myself.

Karen Crane wrote:
Is there a way to have both?
When there is no power, I would want the well pump to still work along with the fridge.

Karen,
Lead-acid batteries are extremely recyleable (>80%). The acid is easily neutralized (although will typically neutralize itself over time) and over 50% of the lead supply comes from recycled batteries. Battery replacement/management thru recycling is an inherent part of off-grid solar ownership. Depending on the battery choice, well cared for battery banks last between 5-15 years. One exception is Nickel-Iron batteries. Very expensive but come with a 10 year warranty and some installed 80 years ago still work.

Wood for electricity presents a few problems mostly having to do with energy conversion. You'd either be burning the wood to produce stream to spin a turbine to turn a generator, or heating wood to create wood-gas, to run a generator. Both involve several energy conversion transfers, each reducing efficiency. Most consider wood to be a resource for producing heat, and as a construction material. Specifically when used for heat, hardwoods are chosen which are slow growth trees. Rocket mass heaters have been proven to be the most efficient currently available approach to heating a space with wood.

Solar Panels over their lifetime produce 7 times the amount of energy that went into their manufacture. The also have less than 30% of the carbon footprint of fossil-fuel electrical production.

With current panel prices right below $1/watt, a DIYer would be hard pressed to make their own panels with the same durability and output as commercial panels for less than $1/watt.

The first step in migrating to off-grid power is to understand your current power usage. A rule-of-thumb is to take your current monthly electric bill, multiply it by 260 and that's what you'd have to spend on an off-grid solar package to live the same way you live now. Additionally, take 20% of than number and you'll be spending that on battery replacement every 5-8 years. Most of us who grew up with cheap power (and electricity from power companies is VERY cheap compared to having to make it yourself) have had to slowly change how we live to reduce our electrical usage. If my monthly electric bill is $150, I'd have to spend $39,000 for off-grid solar. If I can change the way I live and reduce that to $75/month, I'd only have to spend $19,500. This is a ballpark estimate but I think it's pretty close.

Once I started looking at power this way, I have begun looking for places where I either waste power, or can use an alternative energy source. As an example, I now burn wood in a buck stove for most of my heat. I harvest some wood, and purchase some as well. Over the 3 years I've been doing this I have reduced my electrical bills in the winter by $50-$100/month. On average I've been spending $200/year on wood, and saving $400-500/year on my electrical usage. I've also gotten the benefit of physical activity (cutting, splitting, and hauling firewood is physical), and we know that should we lose power, we have a reliable source of heat. In the summer, I do most of my cooking and canning outdoors on our deck using propane. The propane is costs me about the same as if I was using the electric stove, but keeps the heat outdoors, reducing the load on the air conditioner. In the winter, it's the opposite. I do the cooking indoors and function stack the heat used for cooking to help heat the house.

My approach has been to go small scale, and pick specific applications to use with off-grid solar. My horse barn is 300' from my house and when the power line between my house and barn failed, it was the same price for me to install solar on the barn, rather than replace the cable. The system I'm currently installing will run my freezer and fridge completely off-grid and also give me a little extra (such as run my indoor aquaponics system pumps.)

Karen Crane wrote:YES YES...need this resource!
I have no clue about what is a watt, Ohm or anything but want to get off the grid.
Would be interested in making solar panels as it is much cheaper I've heard.
No clue on how to do it.
No one seemd to be addressing theproblem when the batteries wear out.
What happens to all that acid that is inside?
Can the batteries be "rehabbed"? Or do they need to get dumped at some point?
Just saw something about making electricity with wood?
Would this be any better? Could have a big wood lot with sustainaboe wood growing and ise that as it recycles itself.
Anyone know anything about wood to make electricity?

The inverter I went with is the Cotek SK-1000-124. This is a pure sine inverter, with sleep mode. 2 year warrenty. $364 with free shipping.

Markus Loeffler wrote:Hi,

I am looking for a solid and reliable 24VDC MSW inverter for 120V outputting 1000W. I own a PowerBright 24V, 900W inverter which does a very good job but I recently recommended that same model to 2 friends and they both had a very bad experience: one was bad out of the box and the other died after a week of usage. So either there is a quality control issue or they did a new revision of the circuit board.

Any suggestions on good inverters?

Markus