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rykertest
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Posts:35
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| 11/14/2008 1:31 PM |
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I know I'm gonna mess up explaining this but here I go. I've been looking online trying to learn about SP. I guess I don't get the inverter output numbers. When doing a calculation of how much power I need to run my house to date lets say, do I just need to concentrate on the wattage the inverter says it puts out?
If I need 4000 watts to run everything in my house (just using that number as an example), and the inverter says it produces 5000 watts, then I have a 1000 watt surplus correct?
I have a feeling I am missing something basic here. Thank you.
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Dana1
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Posts:390
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| 11/14/2008 6:00 PM |
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Battery-to-AC inverters, are specified by their maximum sustainable output power. But it's a regulated output- it only delivers the power required to maintain the output line voltage, won't go full blast until/unless there is that much load being drawn from the line. If the load turns off or turns down, the power output of the inverter drops accordingly. You need sufficient battery & wiring on the DC side too- the inverter can't deliver more than it can suck in.
Grid tied inverters are a somewhat different beast- they convert whatever DC power is available (direct from the solar array, no batteries involved) and feed it onto the power grid. It's output spec is also a "maximum-sustainable" number, but it can only deliver that amount of power when there is sufficient DC streaming in from the solar array.
Read the fine print though- some types of loads (mostly motors, particularly AC & refrigeration motors) require dramatically more power on startup than when they're running. There may be some amount of short-duration burst-power that can be drawn from battery inverters beyond the sustained output draw to help your motor-start conditions. You may need the full "surplus" and then some to reliably start some loads. (Peak startup power or locked-rotor current is something to consider when buying the appliances that you intend to run on any off-grid system.)
Any clearer yet? |
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rykertest
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New Member
Posts:35
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| 11/14/2008 7:32 PM |
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Posted By Dana1 on 11/14/2008 6:00 PM
Battery-to-AC inverters, are specified by their maximum sustainable output power. But it's a regulated output- it only delivers the power required to maintain the output line voltage, won't go full blast until/unless there is that much load being drawn from the line. If the load turns off or turns down, the power output of the inverter drops accordingly. You need sufficient battery & wiring on the DC side too- the inverter can't deliver more than it can suck in.
Grid tied inverters are a somewhat different beast- they convert whatever DC power is available (direct from the solar array, no batteries involved) and feed it onto the power grid. It's output spec is also a "maximum-sustainable" number, but it can only deliver that amount of power when there is sufficient DC streaming in from the solar array.
Read the fine print though- some types of loads (mostly motors, particularly AC & refrigeration motors) require dramatically more power on startup than when they're running. There may be some amount of short-duration burst-power that can be drawn from battery inverters beyond the sustained output draw to help your motor-start conditions. You may need the full "surplus" and then some to reliably start some loads. (Peak startup power or locked-rotor current is something to consider when buying the appliances that you intend to run on any off-grid system.)
Any clearer yet?
Yes somewhat clearer thank you for such a detailed reply. I'm trying to go over a list of things that will require a lot of power, like washing machine and dryer, fridge, water pump, etc. Our water heater and stove and furnace will be gas or propane. I will almost certainly be grid tied despite wanting to be off the grid. My goal is to at some point pull the plug on the power company but wanting and getting are 2 different things. lol
All of my appliances are going to be energy star, except maybe for my fridge. I am having an old fashioned glass/latch door fridge made, and I don't have the specs on that yet. I wish I could do a 5000 watt system, but may have to settle for 3000 to start.
I guess even if I use the grid power with the initial start up power draw, maybe the SP can run it normally and that would still cut way back on my power usage.
My utility company currently isn't too keen on net metering and it's actually cost prohibitive to hav that set up. So if I have solar panels that are grid tied with no battery back up and it is not set up for net metering, where does any surplus power go? Space?
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Brock
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Posts:388
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| 11/15/2008 9:06 PM |
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There are really three main groups of inverters.
Grid tied (sine wave) with no batteries, they simply take the generated solar power and "invert" it to match the grid and push it back to the grid, only one connection. They disconnect from the grid in 1/30 of a second in the event of a grid failure and do nothing until the grid comes back up.
Grid tied (sine wave) with batteries or also referred to as hybrid systems. This is what I have. It has a "grid in", "generator in" and "load out". This inverter can be set to sell back to the grid out of the grid in side or only feed the excess solar power to the load side of the inverter. So if you don't have a net metering situation it can be set up to work that way as well. The big advantage of a hybrid system is if the power grid goes out everything on the "load" side of the inverter will keep running as long as you have sunshine or a charged battery bank and the grid in side is disconnected from the grid, again until the grid comes back up. The disadvantage is cost, these are typically the most expensive inverters and you also have to maintain and eventually replace a battery bank. In a Hybrid system if you consuming 5000w on the load side and 3000w is being generated from the array, then the inverter will just take 2000w from the grid. Typically the battery bank on a hybrid system is smaller than a full blow off grid system since they rarely run on battery power alone and the batteries are not cycled as much typically giving you a longer life span.
Motor starting surges aren’t a big deal while grid connected since it can borrow those big very quick hits from the grid as long as the grid is there. For example if the grid is present I can start my geothermal heat pump, then disconnect from the grid and keep running, but if the grid is out and my heat pump tried to start, flash, out go all the lights. I have a 6000w inverter with a 24kw battery bank and only 1000w of solar but set up to eventually handle 6000w of solar. The starting surge is big; typically you will need an inverter at a minimum of four times the rated load as the motor you’re trying to start. So say your fridge is rated at 300w your need a minimum of 1200w of inverter to start it. Or in my geothermal heat pump case it draws about 4000w running and I would need 12-16kw of inverter to start it.
The last is standalone (modified sine wave or sine wave), it can't sync with the grid and thus can't back feed to the grid. Some have build in chargers that can charge the battery bank via grid or generator power and most can pass grid power through them if it is present. But the loads are either connected to the grid or the inverter at any given point, more like an overblown computer UPS.
Unless you looking at something temporary I wouldn’t even consider a modified sine wave inverter, while much cheaper they can fry some electronics and actually cause motors to consume between 15% to 40% more power due to the wave form. This not only uses more power but can overheat and eventually burn out the motor as well.
Over 20 years typical numbers for a grid tied PV system are in the $.25-.33 / kw range, a hybrid system is in the $.50-.66 / kw range (battery replacement every 10 years) and a full off grid is in the $1.00 / kw range (bigger battery replace every 5-10 years). |
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Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1kw solar PV setup, 3400 sq ft |
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Dana1
Registered Users
Basic Member
Posts:390
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| 11/17/2008 10:59 AM |
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If you're going grid-tied, no need to worry about peak-loading from motor startups, etc. The inverter needs to be sized to handle the actual peak output of the array and little else matters.
In a non-net-metering situation any surplus power (beyond your instantaneous load) generated by a grid-tied system is effectively donated to the power company (the grid-at-large acts as the load for the surplus.) Since it's unlikely that your peak power output will correspond with the demand-load of your house, there is very little economic benefit to you for a grid-tied system with NO payback from the utility for the surplus power you use.
I'm not sure how/why it's been made cost prohibitive to go net-metered with your utility, but for sure there are no consistent across the board rules established within the US or Canada. (It's a situation that cries out for national standards.) Without net-metering it may be more cost effective to set it up a battery based system with additional switching smarts to automatically switch critical circuits over to the grid at times of insufficient battery power. (Seems silly and more expensive to have to take that track though.) |
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