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PostPosted: May 15th, 2020, 11:41 am 
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Sunsets are better on the western side looking towards Aberdeen Proving Ground. They can be really spectacular. When I used to drive home from work I had to make sure that I was not distracted by them since the road goes right next to an almost vertical cliff that is about 100' high.

ray


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PostPosted: May 15th, 2020, 1:58 pm 
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About a year ago I went through the exercise of solar and battery backup vs a generator for our new house currently under construction. First you have to realize that the most common type of inverter used for solar DC to AC conversion is micro inverters for many reasons too involved for this discussion. The problem with micro inverters is that they sync to the utility line frequency and when utility power drops, they stop functioning. So, to have solar with backup capability you have to have a stand alone inverter that can also sync with utility power but will function in a stand alone mode when utility power drops. In order to size that inverter, you have to know what the total load will be. Then you will probably find out that you cannot run everything on the house on any reasonably sized and priced inverter. You will be faced with internal house wiring changes, a transfer switch, and a safety outside disconnect switch to prevent electrocuting firemen if they respond to a house fire. In my new house I will have two load centers, one only for utility power and the other for backup power through a transfer switch.

Once you determine the total load you will run off the backup, you will have to figure out both peak and average load. The peak load will determine the size of the inverter and the average load will determine battery size depending on how long you want to run. The common metric is KWH (kilowatt hours) a measure of stored energy. For example, let’s say your average load will be 5 KW and you want to run for 48 hours. Not counting on significant solar contribution because you might have an outage in the middle of a period where a storm blocks the sun for 48 hours, you would need 5 X 48 = 240 KWH worth of battery storage. Then you have to account for that by the efficiency of the inverter. Even assuming a very efficient inverter you should plan on 90%. Therefore, the KWH you need would be 240/.9 = 267 KWH rounded off to around 275 allowing for wiring and other losses. Then figure out the cost of lithium batteries to provide that amount of storage. A 12 volt 100 AH deep cycle lithium battery will cost between $750 to $1,000. Each one provides at maximum 1.2 KWH assuming 100% efficiency which will probably be 80% at high discharge rates and allowing for safety to battery life by over discharge. So, the battery would provide 1KWH of usable energy. To get the storage you need, the battery cost would be at minimum $75 X 267 or over $200K. The batteries would also need a storage space and maintenance. Obviously, the scenario of 2 days at 5KW does not make any sense from a battery backup scenario. Even if you only wanted to run for 5 hours, you would still need in excess of $20K worth of batteries. It would be cheaper if you used SLA (sealed lead acid) batteries but their life is much shorter and the % or rated AH capacity at high loads drops more than lithium. And we did not take into account the inverter cost.

Looking at a propane fired generator, the costs are much different. First, I would recommend a propane generator instead of gasoline because propane does not need treatment or refreshing like gasoline. Second, your propane tank can serve double duty if you have an outdoor gas grill. No more lugging tanks to Lowes for exchange.

Looking at a reasonable generator that is 10KW, allowing many more loads to function, the price of the generator with a 16 circuit load center/transfer switch is around $3,000. That generator at 50% load uses about 1 gallon per hour. The cost of propane is running around $2.25 per gallon so the cost to run the generator for 48 hours would be $108. BTW, this should tell you what a bargain utility power is. The good thing about a generator is that you can have backup power as long as you have propane. A propane supplier can help you pick the correct size of tank. At our new house that is almost all electric but with a propane range top, propane fireplace, and outside propane connection for a grill, we will have a 22KW generator and a 500 gallon tank, overkill for your application.

Here is a link to the generator I mentioned:
https://www.electricgeneratorsdirect.co ... 02402.html

I hope this is helpful and that I have not overloaded (pun intended) you with info.


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PostPosted: May 15th, 2020, 2:41 pm 
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Joined: January 14th, 2015, 11:15 pm
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So the key is the usual electrical power usage which I can look up. We already have propane for heat, cooking and clothes dryer and because of its yurt shape our house is very energy efficient.

You might find the following link interesting for solar / battery calculations in Australia. An assumed power usage of 25KWH pe.r day is significantly less than your assumption but it is a different climate.

https://www.solarchoice.net.au/blog/is-home-solar-battery-storage-worth-it-april-2019-update/

The climate is very temperate where we are moving to and there are cooling sea breezes so I am not expecting to use much electricity at all except for my amplifiers!

Thanks for all the detailed info. When we sell our currrent house I will pass on your info to the new owners.

ray


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PostPosted: May 15th, 2020, 2:57 pm 
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5KW average load is about 4x our usage in an all electric house....

Roscoe

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PostPosted: May 15th, 2020, 4:30 pm 
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Best thing is to go through your last 12 months of electric bills and figure out a reasonable average. However, when you are working in a short time span you may have either more or less than the average. Once that is determined, the calculations can be re-done.


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PostPosted: May 15th, 2020, 5:08 pm 
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tomp wrote:
Best thing is to go through your last 12 months of electric bills and figure out a reasonable average. However, when you are working in a short time span you may have either more or less than the average. Once that is determined, the calculations can be re-done.


No need w/my provider, they provide the high/low & average usage over the last 12mo on every bill. 1000KWH/mo for our house. If we had gas heat, hot water, stove & dryer, it'd be 1/3 of that... In a long-term outage, the loads I HAVE to be able to run (Fridge, water pump, lift system pump for the septic) to stay in the house total <5KWH a day.

Roscoe

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PostPosted: May 16th, 2020, 3:02 pm 
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For general info this is an analysis I did for my generator guy on the load going to each load center. It may or may not help you. Note that this is only for major loads and not general lighting or regular 120 vol outlets. The ones going to the generator have a "Y" in the On Generator column and three loads are marked for load shedding if necessary. Note that there are a lot of high powered loads in this house. The generator guy said the only load that would have to be load shed would be the 7.5 KW supplemental heater for the 2 ton heat pump. BTW, there are two load centers each capable of 200 amps.


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Home genrator load risk analysis By generator connection.pdf [26.67 KiB]
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PostPosted: May 17th, 2020, 2:27 pm 
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Joined: March 2nd, 2013, 2:43 pm
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Location: Potomac, MD
I thought I might put in my two cents for a somewhat different perspective.

I have ten solar panels and battery backup with lithium iron phosphate batteries. I installed this system at least seven years ago after a couple of storms left me without power for 10 days or more at a time. I installed 6.7 kWh worth of batteries. Ever since I installed the system I have never had the grid go down for more than a couple of hours at a time. Pepco started aggressive tree pruning. I do have natural gas which runs most of the furnace, water heater, dryer, and stove/ovens. One kW electric is still needed for the furnace blower, and the ovens use glow plugs.

I rarely use air conditioning, but occasionally run a wall unit from my solar inverter during the peak of the day. There are some stretches for as much as a month or more at a time in the late spring, summer, and early fall that I have been able to go off grid and run exclusively with the solar.

Now here is the real difference between myself and Tom. I use vacuum tubes in my audio system(s) and Tom uses SS.


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PostPosted: May 17th, 2020, 3:04 pm 
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https://www.youtube.com/watch?v=Jzds7cFKChM

These would be handy when the power goes out.


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PostPosted: May 17th, 2020, 3:45 pm 
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Location: Parkville, Maryland
Just a quick note here -- residential utility bills only indicate consumption in kW/hours. To size a generator you need to base it on the connected demand in kW. Most appliances' nameplates will indicate full-load amperage at 120-vac. The sum of all the desired appliances' (you wish to have on emergency power) current draw x 120-vac will give you the demand in kW. Most generators sold in Europe are rated in kVA -- here in the good old USA they are rated in kW (the power available for use after the generator efficiency is taken into account). The needed run time is a function of utilization because the household loads typically are alternate loads and do not all run simultaneously. That said -- you size the generator for the demand and decide how long you may need the emergency power in hours or days or weeks. Generator manufacturers can advise run time based on the amount of fuel you'll have available. Most generator power ratings are based on an expected load at any one time so for purposes of guesstimating you could use a utilization factor of .72. Say a 50-kW generator would probably be faced with an ongoing load during an outage of about 36-kW. The typical amount of work the generator is expected to perform determines fuel consumption. Where did .72 come from? Several years of analyzing telemetry from installed energy-management systems.

i hope this helps.

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