Math check please!

[SaltViking]

I’m looking at trying to calculate how much battery I would need for a off grid system. This system would be in the mountains of Tennessee so I’m not looking at desert sun.

The home used 770kWh last month. (Highest month for the year.)
If I multiply that by 12 = 9240kwh per year.
If I divide that by 365 I’m looking at 26kWh a day.
If I want 3 days of backup I’m looking at 78kWh.

So for the array from what I understand I take my 78kwh and divide that by 5? (For the hours in a day.) 15.6 kWh production of solar needed to recharge that system in 1 day?

 

[Warpspeed]

A whole lot depends on how cloudy it gets where you are, and for how many days in a row it remains totally cloudy.
Can you guarantee five hours of full sun and a clear blue sky after three days of solid total cloud ?

It gets to a point where in total cloud cover you get almost nothing. Doubling the number of solar panels gives you twice almost nothing, so that approach does not work. Doubling the battery size is probably not an economic proposition either, and you still may run short !

Give it your best shot, and be prepared to recharge your battery with a gasoline generator perhaps just a very few odd times in mid winter.
Its the only practical and economic solution to the whole problem.

 

[LithiumCanuck]

I’m looking at trying to calculate how much battery I would need for a off grid system. This system would be in the mountains of Tennessee so I’m not looking at desert sun.

The home used 770kWh last month. (Highest month for the year.)
If I multiply that by 12 = 9240kwh per year.
If I divide that by 365 I’m looking at 26kWh a day.
If I want 3 days of backup I’m looking at 78kWh.

So for the array from what I understand I take my 78kwh and divide that by 5? (For the hours in a day.) 15.6 kWh production of solar needed to recharge that system in 1 day?

If you calculate an average over the year, you could run out of power on your highest month and during lowest production months…

Sizing my system, I calculated my highest day and the worst day of production in winter (which is less than 4 hours of sun….) I sized the panels I needed, kept the number. I sized the batteries I needed and added one (but that’s me….. )

 

[robby]

For doing the Math on what the array will provide for your location each month of the year use PVWatts.
It really is very accurate in doing the math based on your array and location.

https://pvwatts.nrel.gov/

 

[SaltViking]

A whole lot depends on how cloudy it gets where you are, and for how many days in a row it remains totally cloudy.
Can you guarantee five hours of full sun and a clear blue sky after three days of solid total cloud ?

It gets to a point where in total cloud cover you get almost nothing. Doubling the number of solar panels gives you twice almost nothing, so that approach does not work. Doubling the battery size is probably not an economic proposition either, and you still may run short !

Give it your best shot, and be prepared to recharge your battery with a gasoline generator perhaps just a very few odd times in mid winter.
Its the only practical and economic solution to the whole problem.

We have a pretty good creek that runs through the property. Our hope is to offset power supply with a micro hydro generator.
On top of all that we have gas generators as our last resort. I’m not too proud to use gas.

If you calculate an average over the year, you could run out of power on your highest month and during lowest production months…

Sizing my system, I calculated my highest day and the worst day of production in winter (which is less than 4 hours of sun….) I sized the panels I needed, kept the number. I sized the batteries I needed and added one (but that’s me….. )

I like your math!

For doing the Math on what the array will provide for your location each month of the year use PVWatts.
It really is very accurate in doing the math based on your array and location.

https://pvwatts.nrel.gov/

Thank you. I will have to check it out on my computer. For some reason I can’t get it to work properly on my phone.

 

[MichaelK]

The problem here is that the consumption you see in your home is not really going to be exactly the same as with an off-grid home. We get so spoiled living in our suburban homes, with unlimited electricity available any hour of the day. When you HAVE to conserve though, you'll find you can get by with far less electricity.

When living in a suburban home, I was seeing at least 15-20kWh of consumption per day. In the same range as what you see. With my off-grid home, I consume far less. If I see > 4.0kWh I'm asking why? I don't feel that we're living a deprived lifestyle. We have lights, TV, internet, refrigerators and freezers, all within a budget of 3-4kWh per day. Peak of summer is higher with air-conditioning upstairs, but I've never seen numbers even approaching what I would get in the suburban house.

Can you go into more detail as to what your electricity goes to? Central air, electric stoves, and electric hot water are BIG users. If you can get alternatives for those, there's the potential for a lot of conservation. We use propare for cooking, and hot water. We use wood and propane for heating.

So, with a more conservative mindset, and alternative fuel sources, I'd say you might decrease daily consumption down to 1/4th of what you consume now. That would allow for a much smaller battery.

 

[Warpspeed]

Michael is quite right.
I managed to better than halve my metered consumption, without making any changes to lifestyle by simply monitoring every single appliance individually and figuring out ways I could improve things.
I found the biggest energy hogs to be the small things you might normally ignore, that consume small amounts of continuous power 24 hrs, not big things that are only used very intermittently for only a few minutes at a time.
The biggest overall improvements come from adding up a lot of very small incremental improvements.

There are too many examples to list all, but here are a very few.

I had two grid powered electric wall clocks, each used twelve continuous watts. over 24 hours that is 576 watt hours per day.
They were replaced with two battery wall clocks that each run for a year off one AA battery.

In the garage there is an electric garage door opener that has a low voltage dc motor. That is powered from a transformer that always runs warm, and draws 40 watts continuously 24 hours, just to power the radio receiver !
I modified that, so the radio receiver is now powered from a small dc wall pack, and it operates a relay to connect ac power to the big transformer and motor, only while the motor needs to run. That saves me 960 watt hours per day.

LED lights throughout the house obviously.
I replaced a 20 year old refrigerator with a new more energy efficient type.
Replaced an old CRT computer monitor with a flat screen LED monitor (this was a very long time ago!)

I have quite a few appliances that have transformers fitted that are continuously energized.
The on/off switch on the appliance does not disconnect the main incoming ac.
One example, my washing machine that uses a continuous 84 watt hours per day even when switched off.
That is almost exactly the same total power consumption as running a single full washing load wash cycle including spin dry on high speed.

The moral of the story is get used to switching appliances off at the wall outlet that are not being used. But you will never know what is actually happening, unless you run every household appliance through a watt hour meter for a day or so. It will be VERY revealing.
Its hard to believe but I cut my overall consumption from about 8kWh to just under 4Kwh.
That makes a huge difference when sizing a solar system.

The expense of replacing some old appliances will be easily offset by the savings in your main solar battery and solar panels.