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The 10 RE Commandments

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The idea came up not long ago in IRC to bring "the ten commandments" of Renewable Energy, or "RE", together, and put them all in one place so that the newer victims of the addiction can reference them as the guidelines for the entire process. I don't know about the rest of you, but I would have loved to have something like this for reference along the way when I was starting out. ;)

What I'd like to do with this is throw out there what I've picked up on personally and remember from recent conversations in IRC as a seed, and then refine it as time goes on and the vets add to it.

Please feel free to comment with additions and/or contentions/corrections... I don't claim to have it all immediately accurate right out the gate. Any comments will be combed and integrated, but left unmodified as comments.

Mods, feel free to edit the list directly as you see fit, at any point in time. I'm not looking at this as the typical thread. A quick comment as a reply just to point to the addition/correction would be adequate.

I only ask that major questions that comprise more than clarifications that can be used to tweak the list be carried out as they usually would throughout the rest of the forum, in the appropriate category that covers them. If a given topic becomes too engrossing, it may need to be "split" so that the replies can remain primarily about refining the list. We want to help, so help us help you. ;)

The document is laid out in a manner targeting off-grid systems. The scope required to cover every use of an RE system greater than a single thread like this can cover. The concepts are similar for other systems, but certain aspects may differ and/or omit/include components not mentioned in this article.

With that said, I hope this turns out as a very valuable addition to the collection of knowledge that is already about.


1. Determine Your Needs

The first thing to accomplish is to determine what it is your new system will be doing. You can't power your house on a VW cranking battery trickle charger panel, and likewise, it's financially irresponsible to shell out tens of thousands of dollars to run your turbo charged yard light collection.

Your needs here need to be realistic. Are you looking to just experiment? Do you have something specific you wish to run on RE as a dedicated system? Are you looking to build a cabin and need power but there's no grid because the location is so remote? Maybe you're planning on going all out, and wish to run your entire house from wind, solar, and hydro and pull the plug on the electric company forever.

Regardless of what it is you want the system to do, you need to define its purpose with fairly rigid boundaries before you begin.

2. Research Your Loads and Reduce Consumption Requirements

Once you've established what you want to do with what you'll be generating, you need to know what you'll be powering.

Take the time to determine the nature of your loads and minimize the power requirements to accomplish any and all given tasks within the system.

Production is only part of the equation in renewable energy. Reducing what you need is a very important step that is often overlooked by the newcomer. A common and expensive mistake is to open an electric bill and attempt to base requirements from that. Stop right there!

There are several problems with this approach. First, an on-grid home is almost NEVER anywhere close to optimized for RE as it sits. There are a lot of things that can and should be done within the home first to reduce consumption. Lights are a big one. While incandescent lighting has been falling out of mainstream use for some time now, another problem crops up from the more efficient lighting. One tendency is to "let them burn" because they're using less than their hot filament counterparts. This mentality is not explicitly restricted to lights, and will get you onto trouble with RE. Turn them off.

Another big issue is that there are 5 major offenders in the electric department in a domestic setting that can be dealt with in ways better suited for renewables, and the impact on system cost is significant. Space heat, air conditioning, hot water, cooking, and refrigeration all require considerably more energy than most of the other items found in a typical home.

Hot water (or "DHW") and space heating in particular both have very substantial potential for savings because they can be produced cheaply and relatively easily from commonly available components, directly from the sun. Don't underestimate this.

Savings in cooking involve the methods used to prepare food, and vary depending on what your desires and needs are. Microwaves for example pose very significant savings in the energy requirement department over an electric range/oven, and even a toaster oven. There are other methods that technically fall outside the "renewable" category, such as propane for ranges/ovens, and it's not unheard of for even the most "complete" RE system to include these options for food needs.

Refrigeration is a requirement (see load categories below) in any domestic system 99.9% of the time, and the key there is being creative with how it is accomplished, combined with the most efficient equipment possible to make it happen.

Air conditioning. Oh, a big mistake. You can have it in play, but be prepared to pay dearly for the extra production, conversion, and storage requirements that come with having it. It's generally considered a luxury.

Anything that is seasonal (space heat and air conditioning for example) needs to be taken into account as well. The demands on these sub systems do not remain constant year-round, and so there are ways to optimize the overall system to account for this.

Other loads just need optimizing. Laptops instead of PC towers, Smaller TVs instead of bigger ones, and so on. One saying that you'll find sprinkled throughout in one form or another in the RE world is "A watt saved is a watt made". They all count.

3. Categorize Your Loads

Ok, so you've got your loads minimized, converted to another type of source that is more economical (space heat, hot water etc), and all laid out in front of you.

These need to be categorized as to their importance and usage patterns. This makes a big difference in the final sizing of the overall system as a whole.

Critical loads:

Refrigeration, space heat (to a degree), and even hot water can be examples here. This is probably the most important list to get right, as the system has to be designed so as to support them full time, regardless of weather conditions affecting production. The list will vary, but try to keep it as small as possible without overlooking anything. You can affect cost significantly right here by keeping it minimal, but you don't want to find out later that you left something out that has a major impact on your life.

These are also referred to as your "base load", things that will run 25/8/366.

Normal loads:

These are things like lights, computers, pinball machines, you get the idea. They get turned off when not in use (ok so a pinball machine runs all the time, humor me), and aren't considered critical to your wellbeing as a human in the intended environment.

The majority of your list will be in here. The requirements of this category can also be a little more difficult to calculate as a whole (because there are so many, and usage patterns can vary so much).

Generally, be pessimistic here, meaning overestimate usage by a reasonable margin. If you're looking to convert your on grid home to off grid (and aren't doing something like relocating to do so), one thing that can help out here is logging the time you spend per day on the computer, how long lights burn (a yearly average is best in that particular case), and so on. It will make your calculations more accurate and your growth/waste margins easier to define.

Transient loads:

These are loads that spend most of their time in an unpowered state, such as a microwave, washing machine, and so on, but draw significantly more than trivial amounts of power when they are operating.

This calculation is somewhat about overall usage of course, but plays more of a role in selecting certain components to be able to handle these loads properly without causing an overload condition in an inverter, leaving everyone in the dark for example.

While you'll want to be mindful of the demands in terms of consumption over time, this is more about raw addition.

4. Know The Environment

We're making progress here. Now you know what your loads will need to do their job when called upon. A quick peek at the new and improved electric bill can help you confirm that your calculations have some merit, but your environment has production limitations, unlike your electric bill. Let's take a look at what some of those are and why they are important.

Different areas of the world have vastly different characteristics in what is available to get your energy from. Some areas are very well suited for solar, while others receive so little light that the amount of panels (of either type) would be prohibitively expensive to consider as a source.

Likewise, wind doesn't exist in much more than trivial amounts in some areas, while in others, anything that isn't bolted to significant chunks of concrete is at risk for becoming a candidate for the next "gone with the wind" remake.

Hydro is a special case that most of us don't have access to, simply because you'll generally need a river, stream, creek, or the like flowing through your property. Then there are restrictions with what you can do with the water when its there in some places.

There are numerous resources throughout the internet that can clear up any questions you may have about your geographic location in general.

However, this is only part of the equation. The local conditions will vary from the more widespread averaged data available on the maps and such. To correct for these discrepancies, you'll need to make observations and take measurements over time (typically at least a full year) to get an accurate idea of your worst case requirements to make up the difference.

Be mindful of things like trees, buildings, and even laws. Just because the maps say you live in a suitable wind location with plenty of sun, it doesn't mean that in your particular situation that you're going to be able to extract all of that light from the sky or erect the tower to put a turbine on.

Finding out about a law prohibiting turbine towers after you've bought all the framework and all the components to build a HAWT is not the time to discover its existence. Do your homework first!

Another aspect to consider with RE is that sometimes, there is the potential to produce too much power. This has to be dealt with appropriately and presents in several forms depending on the subsystem generating the power. Wind turbines can overspeed due to excessive wind for example, and a method to shut the turbine down to minimize or prevent damage altogether must be in place and ready for actuation at all times.

Because of the unpredictable nature of the environment, RE systems inherently are also capable of producing too much power. On systems not using a dedicated charge controller (typically on smaller designs), the voltage of the batteries must also be prevented from going over design limits by some means. Every properly designed system regularly crosses into this state, and therefore the batteries must be protected in some form or another (usually by a device called a "dump load"). On systems using a full featured charge controller, a dump load may not be necessary as a protection device, although may be in place as part of another aspect of the design.

Batteries can also overheat from the environment (direct exposure to the sun, lack of proper ventilation, etc), as well as from normal charging currents if there is nowhere for this heat to go. They also need to be protected from extreme cold and kept from freezing, a condition that is generally considered to be fatal for the lead acid chemistry. A battery is progressively more susceptible to freezing as it is discharged further and further. This condition must be avoided at all times.

For Flooded Lead Acid (or FLA) cells, hydrogen production must be accounted for. The gas is produced during charging, particularly as they approach full, and is generated in copious amounts during a process known as equalization (an occasional and intentional controlled overcharge).  The hydrogen can accumulate and form extremely explosive mixtures that must be properly ventilated to the outdoors to prevent a dangerous condition.

5. Calculate Your Losses

An easy to ignore aspect of RE while surfing the web looking for panels, inverters, batteries, wiring, and the like are the losses incurred throughout.

These will vary depending on the exact nature of your system, but certainly must all be taken into account.

The efficiency of the various components become a collective issue (and especially where there is serious money involved), and can be scrutinized at each component and these added up to obtain the overall system efficiency. When electric is the only thing involved, it all will come down to two numbers. Overall system efficiency, and subsequently a number representing the extra input you're going to need to make up for efficiency not adding up to 100%.

Without having everything in front of you to make the calculation, an accurate representation is very difficult, if not impossible to achieve.

Instead, a "typical worst case scenario" number is derived from what's been seen in various systems throughout on average. The "safe" magic number is 66%, resulting in a production requirement of 150% of what you ultimately use "at the outlet".

It's not a trivial amount, and a fair bit of that is represented by losses in the battery due to the range of "State of Charge", or SoC, that they are operated in.

There are also pieces of equipment that are designed to help improve the efficiency of the system as a whole. Tracking controllers to allow solar panels to follow the sun across the sky are one example. Their electronic companions, "Maximum Power Point Tracking", or MPPT, a device that adjusts itself constantly to get as much power as it can out of solar panels, wind turbines, and hydroelectric generators is another.

6. Calculate Your Storage and Reserve Requirements

You know what you need, how much you can make, and what you can make it with.

Now there is the matter of storing it.

To do this properly requires taking a lot of factors into consideration that are not always (pronounced "usually") properly taken into account.

Varying needs will have varying calculations and therefore results here, but the example presented is based on a "typical" system's design characteristics.

A common reserve is 3 days (72 hours) with little or no input. Most RE storage is accomplished with lead acid batteries, which have some important properties to consider to ensure their longevity. After 3 days, an alternate means of input is needed (see next section), but to minimize reliance on outside energy sources, the batteries are sized to allow for a certain amount of extended discharge on occasion without doing significant damage.

To do this, all of the above factors brought to the table thus far are plugged onto a series of equations that size up your storage needs.

Let's use numbers that are as round as possible to make the math as easy to follow as we can. These numbers do not represent actual data that may be encountered, but are solely used as place holders to demonstrate the calculations that need to take place to size a bank properly.

Let's say you have a worst case load of 10kWh per day. Vets, stick with me here, you'll get your chance... :)

To remain healthy, lead acid needs to remain above a certain SoC. This number is expressed in percentage remaining in the battery, and is ultimately what is used as the deciding factor in sizing.

There are many, many battery types out there, all tailored a little bit differently than the next, and thus will have somewhat different design characteristics specified, this number being one of them. For our battery, we're going to use 50%. Its generally considered a safe value, and is commonly accepted as "getting your moneys worth" from a bank.

A common mistake is to just calculate the bank's requirements looking at the above 2 numbers. You would quickly find out that you weren't even close, and the damage that would ensue would not take much longer and have not much different effect than taking your bank out back and shooting it with your favorite firearm.

There are other factors that need to be taken into consideration. One of those is the the losses.

Using the 150% above (again, playing it "safe"), the effective load suddenly increases to 15kWh/day.

Now let's push that up into the 3 days of no input. Now we're at 45kWh.

We're not done. The bank needs to be sized for a maximum of 50% SoC after those 3 days according the qualifications imposed by the manufacturer of the batteries.


I'd like to pause for a moment to drive an earlier point home. RE vets? Commence laughter. Newcomers, care to guess why?

Because that's one horrendous battery bank. If you were to lay down the kind of cash that you'd need to on a new bank of this size, you'd find you might need a second mortgage to pull it off.

The lesson repeated? Shrink consumption needs. 10kWh is conservative for a family of 3 living on grid. On RE, it would break your wallet, possibly irreparably. :(

7. Size Up Production and Backup Sources

Now the various numbers acquired previously are used to calculate the nominal system voltage, the size of the inverter, and the backup generation sources.

Nominal system voltage its frequently a point of contention among the RE crowd. It has even gone as far as to start all out wars in a couple of cases on the various forums covering the topic.

I won't go into the hows and whys surrounding the power ranges and voltages associated with them other than to say that there are numerous factors that go into deciding which voltage to choose. Because it is a system-wide consideration that affects virtually every component, cost of implementing one vs another and the power levels involved are the two main considerations taken into account.

The rough ranges used are as follows:

12V: 0 - 2.5kW
24V: 0 - 4kW
48V: 4kW and up

There are higher voltage systems in use, occasionally DIY, but they generally apply to very high power systems and involve custom built components such as inversion solutions and charge controllers. Most homebrew systems will be one of the 3 voltages listed above.

Inverter power capability calculation is fairly straightforward, and depending on what you are designing for, can be as simple as adding your 3 load categories together and adding some room for overhead and a little bit of growth (see "future" section below).

The other consideration with inverters is which type. There are two flavors. "Modified Sine Wave", or MSW, and "Pure Sine Wave", or PSW. A variant on PSW is "TSW", but means the same type of inverter, just called something different, "True Sine Wave".

In the 12V class, the majority of what is available is MSW because they're much cheaper to produce and purchase than PSW. In 24 and 48 volt, PSW dominates.

There are considerations to be taken into account with MSW that PSW does not need. PSW mimics the grid, and anything that can operate from the grid can run on an appropriately sized PSW inverter.

MSW's output adversely affects certain types of equipment. The affected equipment varies, but in general, transformers, induction motors, and capacitive power supplies are an accepted beginning to the list. Examples would be microwave ovens, refrigerator compressors, and certain small self-contained items that "hang from the outlet" such as emergency power failure flashlights, respectively.

Genset calculation is similar to the inverter, but needs to be able to run all of your loads if required, and handle the bulk charge rate of your battery bank at the same time. A loading consideration of about 80% is good here, so let's just make this dirt easy for math sake: If your requirements are 8kW (say, 4kW for your loads, and 4kW for bulk charging), a 10kW genset is a good fit.

8. Factor In The Future

If you end up within 20% of the next level up in power in the voltage list in section 7, consider going up to that next nominal system voltage. You're approaching the top end of what it is practical to do at the lower voltage. Upgrading later is a system-wide event, and is centrally costly. It makes little sense to buy for 12V when a small need later would put you up into the next bracket, leading to the need to buy new everything. The time to do it is before you build it, not after.

9. Get Friendly with Weather Forecasts

This is a crucial part of utilizing an RE system effectively.

You need to know what you're in for the following day in order to use your system efficiently. Its always best to use the power as its being made, so when possible/practical, schedule the items in your "transient load" list according to when the power is going to be available.

Knowing what is to come helps reduce battery wear and tear, lowers genset (or other alternate source) dependence, and increases the overall efficiency of your system. The battery is the least efficient component by far, so if you can use the power before its stored and without pulling it out of the bank, you're that much further ahead.

10. Train Those Using the Power

Last but not least, it does you little good to do all of the above and properly utilize your system if someone in the household is coming behind you and undoing your efforts by walking away from the computer for hours on end, leaving the lights on all over the place, and not being observant of weather conditions.

This is not to say that they need to have a full understanding of everything about the system and how it works, but having their heads wrapped around consumption reduction and how to tell (and utilize the info!) when the system is producing power at any given moment goes a long way.

Learn your system, watch what it produces, what you consume, and make note of what it can and cannot do, and make it known to those involved with its use.

If it seems to good to be true IT IS!

- Revisions -



Added to section 4, making provisions for extreme environmental conditions, the need for turbine shutdown, dumping, and hydrogen production/ventilation

Fixed error in section 6, removed "/day" from battery capacity calculations, the "day" part was is already calculated.

Changed power levels vs nominal system voltages listed in section 7.


Various typos, corrections, modifications to the formatting

Added section about MPPT and physical tracking across the sky to section 5.



Very good start - well reasoned. Now to nitpick :)

Section 4 - be aware of the problems that good RE conditions can bring - too much wind, overheating batteries in the sun etc
Section 6 - figures multiplied by 3 for 3 day operation but quoted per day still. Just needs the "/day" removed from the 45kWh and 90kWh numbers

I'll have another read at work tomorrow if I get a slack time waiting for a compile to finish ;)


--- Quote from: MadScientist267 on April 17, 2013, 08:22:47 pm ---12V: 0 - 1kW
24V: 1 - 2.5kW
48V: 2.5kW and up

--- End quote ---

I think I would change this to
12V: 0 - 2.5kW
24V: 0 - 4kW
48V: larger than 4kW

The reason is because most of the high quality 24V inverters (Magnum MS4024/MS4024PAE, Outback FX-series, Trace/Xantrex/Schneider SW/XW-series) go up to 4 kW capacity on 24V.  Breakers, boxes, etc.. become proportionately more expensive when you make the jump from 24V to 48V because you have to make the step from 60V rated equipment to 250V (NEC requirement).  So it's kind of hard to justify the cost of 48V equipment for somebody who does not need larger than 4 kW capacity.

In other countries this may be different.  But in the US and Canada, just meeting NEC/CEC requirements will add ~50% more cost to a 48V system vs 24V.


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