electrical questions

Re: electrical questions

You're confused because it is confusing. Feel better now?

First off, there are "nominal" Voltages and "actual" Voltages. For instance you may have a PV with a "nominal" Voltage of 12, but it has at least two other "actual" Voltages: The Voltage Open Circuit (Voc) and the Voltage at Maximum Power (Vmp). For the same "12 Volt" panel the Voc may be 23 and the Vmp 17.5.

And then it gets more confusing.

In between the solar panels and the battery bank (assuming off-grid) there's a charge controller. This could be one of two (three, actually) different types: Pulse Width Modulated or Maximum Power Point Tracking. The PWM just connects the panels "straight through" until certain battery Voltage set points are reached, then it starts to pulse to maintain that set point. The MPPT can take a much higher Voltage array of panels and "down convert" it to the relevant charging Voltages.

Which is where we get into more differences. A "12 Volt" battery actually gets charged at about 14.2 Volts, and then settles back to "float" at about 13.8. Furthermore, there is another available charge state called "equalization" which pushes the Voltage up to 15 in order to force the individual battery cells to "even out" or equalize (hence the name) as batteries don't always stay at the same Voltage per cell as they should.

So for the example "12 Volt" system you could have a photovoltaic array of 17.5 Vmp running through a PWM charge controller to bring the batteries up to 14.2 Volts for charging, or you could have a "24 Volt" array of 35 Vmp running through an MPPT controller to do the same thing. Just as an example; not written in stone.

Hopefully I've given you some understanding of the source of confusion. Does it help or is it more confusing?

Re: electrical questions

To add to what Zeuspaul said and possibly further confuse you ...

There is also an efficiency factor in array size and going up in Voltage when using MPPT controllers. For a small array (<400 Watts) there's not much gain to be had from MPPT. Also, the efficiency drops off as the array Voltage goes up in relation to the system Voltage. Usually it's best to keep the down conversion to a factor of 2, as in a 24 Volt system charging from a 48 Volt array.

And yes there are some "not quite 24 Volt" panels out there with a Vmp that is too close to 24 to do any good for charging the system.

To really get a handle on this you have to start with loads, and design from there. Then you can see how different configurations work out for the proposed need and what the advantages/disadvantages of each potential design are.

Re: electrical questions

You're skipping some very important steps. The first being answering the question "how much power do I need?" You have to get that worked out before you can determine how to supply it.

Off-grid systems are battery based. The panels and charge controller re-charge the batteries. Until you know how much battery you need you can't determine what panels/controller you should get.

Growing a system over time is one of the most frequently asked about aspects of solar. It's nearly impossible to do. It's best if you look at it long-term, and see what the power requirements might be down the road and then see if there's any practical way to build up to that "one stage at a time" or if it's going to require a complete re-work.

There's a couple of reasons for this. One is that aging batteries don't mix well with new batteries, so if enough time has lapsed you have to buy an all new battery bank even though there may be years of usable life in the old ones. Another is that as over-all loads increase it is better to go up in system Voltage rather than try to supply 4 kW off 12 Volts - the current draw gets enormous and difficult to deal with. If it's lots of small loads you can sometimes get by with adding parallel systems.

But you really need a long-term plan, starting with your present loads (including both maximum draw at any one time and total Watt hours used per day) and extending in to what you want to include en toto come the future.

Re: electrical questions

puresolar wrote: »

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Is this doable? I know that some items would need to be swapped as the system increased, and went from grid tied to gridless. I know I would also need the batteries once I was off the grid. It is my understanding that while I am on the grid though that I would not need the batteries as all the power I make and dont use goes straight to the power company.

Whoops! System incompatibility! Error 404!

There's quite a difference between a Grid-tie system and an Off-grid system. Different inverters, for one thing. The basics:

Grid-tie. Solar panels feed directly to a grid-tie inverter, which sends whatever AC it can produce to the house wiring. Anything not consumed by the household loads is sold back to the grid. Providing such an option is available in your area. This requires permits and the co-operation of the local utility.

Off-grid. Power is supplied by a battery bank which is recharged by solar panels regulated by the charge controller.

That said, there are "hybrid" systems available (look at Xantrex XW inverters) which can feed the grid and also supply battery-based "back up" power if the grid goes down. Normal grid-tie inverters shut down with no grid to synch to. Of the three, the hybrid is probably the most expensive, followed by off-grid and finally grid-tie.

Now here's where planning comes in. If you know you want an off-grid system capable of 'X' Watt hours per day, there is no reason you can't build it in your own home and shift loads to it by taking them off the utility power. You'll save on electric bill, but you won't be selling anything back to the grid. In $ terms it's a lousy plan, but if the end goal is to get a system with all the bugs worked out for that remote cabin - it works. You just have to be sure your inverter AC Out is never connected to the grid wiring.

I also hope you don't mean 800 kilowatt hours per month. It's doable, but talk about expensive! Normally an off-grid set-up would be 30 kW per month to 60 kW per month. Some are "full size" household systems, but with off-grid power costing around $1 per kW hour over the life of the equipment it gets expensive real fast.

You can start getting a handle on your plan by purchasing a Kill-A-Watt meter and start checking everything you use now on utility power. You'll get a very good idea on what uses the most power and where you could cut back. About $30 for the meter, and you'll be seeing things to shut off right away. Probably pay for itself in the first month!