How Much Watt-Hour/Day Capacity is Needed for Family of Two to Move to Off-Grid Living?

a0128958 · May 2015

Bill (BB) says: "A 500 Watt*Hour per day system is actually quite capable. Can run a laptop computer, a few lights, charge some cell phones. ..."

My brother-in-law and his wife are moving from Houston to an outlying area near Seattle. They're going to change from hard core 'city-slickers' to out-in-the-wilderness dwellers. His question to me is, what's it going to take to live, family of two, in a structure that is advanced WRT air infiltration and insulation, grid connected when grid power is available, and up to 3-4 years off-grid.

He knows I have solar panels (8 kW grid-tied battery-less / 36 panel system - details at http://www.neukranz.com ) so he's asked me for some help.

My fundamental question is, how much capacity it needed?

I know 'it depends' on how frugal he can adapt. Let's assume he can adapt pretty well, but, he's not willing to live like a cave-man. Somewhere in between.

For those out here who have been living a few years off-grid, what's a good 'mid-range' to design to? Something that requires life-style change but not something that forces it to an extreme. Budget is not as key of a concern as being able to live 3-4 years off-grid and at least not be miserable during these years.

Can anyone help?

Many thanks!

Best regards,

Bill

BB. · May 2015

My typical guesses:

1 kWH per day (30 kWH per month)--Computer, lights, radio, cell phone, small water pressurization pump
3.3 kWH per day (100 kWH per month)--Above + refrigerator + Sat TV + washing machine + well pump (near normal electric life--Albeit highly energy conservative)
10 kWH per day (300 kWH per month)--Above + more energy usage (central heat, extra fridge+freezer, more TV entertainment, irrigation, etc.).

A 3.3 kWH per day system is expensive, but does not stop your heart and can be self installed/maintained. A 10 kWH per day system--Recommend professional help or at least a fair amount of electrical/contractor/construction experience.

Of course, when you move to the north west -- There is a whole lot less sun unless you are east of the Cascades....

Lets try a 3.3 kWH system near Seattle--Using the standard rules of thumb. He can cost out the system and get an idea of where he stands.

Assume 2 days of battery storage and 50% maximum discharge. A 48 volt battery bank (note, this system would be OK at 24 volts too--Picking an inverter and batteries will help decide 24 vs 48 volt battery bank):

3,300 Watt*Hours per day * 1/0.85 inverter eff * 2 days storage * 1/0.50 maximum discharge * 1/48 volt battery bank = 324 @ 48 volt battery bank

Next, sizing the solar array... Both on the minimum charging requirements for battery bank, and the load+hours of sun per day.

Recommend 5% to 13% typical solar array charging rate for battery bank... With 10% or more for full time off grid:

324 AH * 59 volts charging * 1/0.77 panel+charger derating * 0.05 rate of charge = 1,241 Watt minimum array
324 AH * 59 volts charging * 1/0.77 panel+charger derating * 0.10 rate of charge = 2,483 Watt nominal array
324 AH * 59 volts charging * 1/0.77 panel+charger derating * 0.13 rate of charge = 3,227 Watt "cost effective" maximum array

And then there is based on hours of sun and season. Using PV Watts for Seattle, fixed array tilted 48 degrees from horizontal:

Month
Solar Radiation
(kWh/m²/day)

1
1.54

2
2.50

3
3.71

4
4.37

5
5.31

6
5.52

7
5.88

8
5.17

9
4.98

10
3.00

11
1.76

12
1.26

Year
3.76

If we toss the bottom three months of the year (will have to use genset and/or cut way back on energy usage). Gives us 2.50 hours of sun in February (break even month):

3,300 Watt*Hours * 1/0.52 off grid AC system efficiency * 1/2.50 hours of sun per day average (Feb) = 2,538 Watt array "minimum" based on above hours of sun suggestion

So, around 2.5 kWatt array would be a good minimum...

Note that a 324 AH @ 48 volt battery bank (or 648 AH @ 24 volt battery bank--same energy storage) would support a ~3.24 kW [fixed to kW, was kWH which was a typo] AC inverter (typical maximum) nicely with flooded cell batteries.

Also, around a 3.2 kWatt solar array maximum for that battery bank would be a typical recommended maximum "cost effective" array.

Generator wise... Using "typical" numbers of 10% and 20% rate of charge for a 324 AH @ 48 volt battery bank:

324 AH * 59 volts charging * 1/0.80 typical battery charger eff * 1/0.85 genset derating * 0.10 rate of charge = 2,811 VA minimum rated genset + ~32.4 amp rated charger
324 AH * 59 volts charging * 1/0.80 typical battery charger eff * 1/0.85 genset derating * 0.20 rate of charge = 5,622 VA nominal rated genset + 65 amp rated charger

That is where I would suggest aiming at.

Don't get me wrong, a 100 kWH per month is a bare minimum amount of energy for a full time off grid home with lots of conservation (in my humble opinion). The least I ever got my bill to for my home (moderate climate in summer) was around 175 kWH on summer month--and I use a lot more normally (200-300 kWH per month) and full natural gas for cooking/central heating/hot water/drier.

Another way to run the home... Run a 0.5 to 1 kWH per day system for evening/night usage--And run a genset during high power usage (cooking, cleaning, water pumping to cistern, etc.).

If the home will have utility power in 3-4 years--It would be difficult to justify a full off grid system cost that would get mothballed or relegated to emergency backup use.

-Bill

Alaska Man · May 2015

I agree with Bill, if you are only going to use it for three years buy some batteries an inverter and a genny. Forgo the Solar. Batteries will get you through the night even let you watch a bit of LED T.V. here and there. With my AGMs in the depth of winter when we have no sun, we run the genny about 4-6 hours a day with our set up. Right now. I haven't had to run the genny for a few weeks.

I suggest AGMs because they can take a higher rate of charge, which means less fuel because of less genny run time. Also less wear on your genny.

westbranch · May 2015

the killer will be refrigeration, a new fridge,WITHOUT all the nice to have add-ons , like ice maker, water cooler, large size (~20 cu. ft....) bottom freezer.... will use ~ 1 KWh +- per day. Plan for 1.5 for leeway. But unless he really wants to re-program his usage patterns, as said above, forget the 3-5 yrs on batteries. Not saying it cant be done but it will need commitment!

a0128958 · May 2015

The above responses are exceptionally helpful. So much so that I backed up to make sure I understand the 'big picture.' Here it is:

Length of Time: 15 years

Energy Sources Installation: Professional (vs DIY)

Structure Overall:: 2000 - 2500 SF, with passive solar design, air infiltration and insulation subjects all well focused on.

Land Characteristics: At least 2.5 acres; No streams.

Structure Characteristics - HVAC: Heating will done with one of these alternatives (1) masonry heater, (2) wood stove, (3) pellet heater (requires power), (4) radiant floor (requires power for water pump). Ventilation options include (1) designing the structure for appropriate amount of natural ventilation, (2) use of a heat recovery ventilator (requires power). AC is out of scope.

Hot Water: Solar Thermal Panel system. No tankless water heater - the electric ones consume considerable power and don't work all that great; the gas ones require a reliable source of propane which is not assured for this property (see below).

Available Fuel Sources: only solar (PV and thermal) and wind are guaranteed accessible. All other energy sources are available too, but unreliably. Electricity from a grid connection will be initially and occasionally available, with periods of up to 3-4 years of unavailability. Propane and diesel will both initially be present on site but resupply might never be possible.

Potable Water: Collect and store from rain water. Well and pump installed, but not generally in use, for backup alternative.

Most representative large city for outside temperature, outside RH, and insolation modeling purposes: Seattle.

Available Locations for Solar (PV and Thermal) Panels: If there's enough roof top area and it's oriented southward and it's shade free then solar and thermal PV panels will be roof mounted. Otherwise enough clear-cut space exists to enable solar PV panels to receive full insolation if not appropriate to them on the roof.

Minimum Requirements: (1) Wood or pellet stove for heat. (2) Reliable source of drinking water via well with pump, or, rainwater collection system). (3) Refrigeration for food & medicines / basic freezer for ice. (4) Hot water.

If it's not listed above then it's not a minimum requirement.

Avg Daily Load to Design To: 6.67 kWh per day (200 kWh per month) (This is a judgment I made using BB's posting, budget available, and my estimate of how much I think my brother-in-law and wife can adjust to an energy limited environment).

I think my next step is to decide on system technology needed to simultaneously accommodate grid power, solar PV power, batteries, wind generator and electric diesel generator availability. Which then affects design items such as how long batteries must go before reaching 50%, how many resupplies of batteries will be needed.

Many thanks!

Best regards,

Bill

mike95490 · May 2015

Wait. Grid power is at the site NOW, and expected to be gone in 5 years ??? Better look up Grid Abandonment Charges for your area. (Basically, the town promised the electric utility x Thousand rate payers, and if you disconnect yourself, you pay a penalty fee so the utility does not loose money. Also depending on local code, structure may get red-tagged for not having utility supplied power) That mess aside....
What about the house air system - does it have a powered Heat Recovery Air Exchanger ? Tankless water heater ? Those both can have high electric losses, where being on grid does not matter, but 200W load 24/7 is killer for batteries.
Suggestion for heating - Look at a Masonry Heater (site built, 4,000 lb of brick, totally passive. http://mha-net.org/ I added a hot water loop on mine for 80 gal of 90F water) pellet stoves have electric motor, need "pellets" and electric ignition system, again, tough on batteries.
Water source - pumping can take a lot of power.

a0128958 · May 2015

Thanks Mike. I learned something new here, something that happens a lot on this site that I'm thankful for. While what you wrote doesn't apply to my situation (see next paragraph) I do have a friend near Ft. Worth that did 'pull the plug' on the utility last year. I'm not sure he's aware of a possible 'grid abandonment charge' - I'll brief him now that I'm aware of this. Thanks.

The property for this subject, near Seattle, will be in a remote, unincorporated area. Current property negotiations clearly include the fact that grid power may or may not continue to be available, and/or grid power may come go over time, absent as long as 3 - 4 years at a time. This is simply what needs to be designed for.

House Ventilation: Maybe. If a Heat Recovery Air Exchanger's power requirement fits within 6.67 kWh/day budget, then yes it will be present. Otherwise, the structure will be deliberately designed to allow for needed ventilation naturally, without any appliance.

Hot Water: Solar Thermal Panel system. No tankless water heater - the electric ones consume considerable power and don't work all that great; the gas ones require a reliable source of propane which is not assured for this property.

Heating & Cooling: Thanks for the suggestion on an alternative for heating. I looked at the Masonry Heater site. Wow! This idea goes to the top of my list of alternatives for heating! And, didn't that pellet heaters have a power demand. That pushes this alternative near the bottom.

Potable Water: The owner is comfortable with the effort and rigor to collect and store rain water. Pumping from a well may still need to be implemented, but as a backup alternative to when something may go wrong with rainwater.

WRT system design: I'm now looking at this article to help educate me: http://www.bluepacificsolar.com/grid-tied-battery-backup.html . Looks like I'm needing what they refer to as a 'grid tied with battery backup system'

I've updated the initial requirements posting of mine with the above clarifications.

Many thanks for the help! Hopefully illustration of the design steps for a complex system is helpful to others.

Best regards,

Bill

a0128958 · May 2015

BB. wrote: »

Note that a 324 AH @ 48 volt battery bank (or 648 AH @ 24 volt battery bank--same energy storage) would support a ~3.24 kWH AC inverter (typical maximum) nicely with flooded cell batteries.

Bill B: I don't (yet) understand how you got from 324 AH @ 48 VDC (which is 15.6 kWh) to a 3.24 kWh AC inverter. I understood everything written up to this sentence.

Thanks!

Best regards,

Bill

BB. · May 2015

Sorry Bill,

I made a typo there... The max inverter rating (maximum inverter load) should have been 3.24 kWatt (not kWH)...

A good rule of thumb for a "reliable" off grid power system with flooded cell batteries is around 100 AH @ 48 per 1 kWatt of AC Inverter rating (also works well as a rough design maximum for solar array Pmax rating).

For a 24 volt system, that would be 200 AH per 1 kWatt of inverter rating, and a 12 volt system would be ~400 AH per 1 kWatt of inverter rating.

The math I would use:

1,000 Watt AC inverter * 1/0.85 inverter eff * 1/48 volt nominal battery bank * 20 hour discharge rate = 490 AH @ 48 volts (20 hour discharge rate of ~5 hours per night for 2 days with 50% max discharge)
1,000 Watt AC inverter * 1/0.85 inverter eff * 1/48 volt nominal battery bank * 8 hour discharge rate = 196 AH @ 48 volts max continuous discharge (4 hours of discharge, 50% max discharge)
1,000 Watt AC inverter * 1/0.85 inverter eff * 1/48 volt nominal battery bank * 5 hour discharge rate = 122 AH @ 48 volts max short term discharge (minutes to hours)
1,000 Watt AC inverter * 1/0.85 inverter eff * 1/48 volt nominal battery bank * 2.5 hour discharge rate = 61 AH @ 48 volts max starting surge (seconds to minute)

So, you can see around 100-122 AH @ 48 volt battery bank would be about the maximum reliable discharge you should expect to reliably support a 1,000 Watt inverter.

-Bill

mike95490 · May 2015

Hot Water: Solar Thermal Panel system. No tankless water heater - the electric ones consume considerable power and don't work all that great; the gas ones require a reliable source of propane which is not assured for this property.

Maybe OK, but in my area, we had nearly total cloud cover this last Nov/Dec. No solar PV harvest (well, negligible) burned a fair amount of diesel in the genset and propane, you can get 20 gallon tanks that are still manageable to carry in the back of a car/van/truck. Many rural areas have "small frame" propane delivery trucks that can haul 200 gall or so in a load.

Dave Angelini · May 2015

To really do this right you need to know how the people will respond to this change. It can be a very hard to impossible do this for some folks. If they really want this they are going to put more effort in than you are as a family member. Good Luck!

SolarPowered · May 2015

When I take my kids,and lady camping I take my 12v, 125Ah solar rig with me.
The inverter usually is performing most of the day from laptop, to Bluetooth stereo, blender, and occasionally an induction cooker.
The induction cooker and laptop, will drain the battery faster than it charges on the 120watt panels I have the battery on.

I would say to get through a day a minimum of 2kWh

So you would need atleast a minimum of (2)125Ah batteries, 12V.

2.8kWh solar panels configured for 12v to optimize harvest efficiency.

BB. · May 2015

If they have grid available at the start--have them run their home like it is off grid--And use the utility meter (and/or get a whole house monitor) and see how much power they need for their lifestyle. After a few weeks/months of usage--Then they real numbers to start their design.

Even if they don't have grid power--A genset for a few weeks and a Watt Hour meter would be more information than they have today--At the cost of $5-$10 of fuel per day (or more, if they use a lot of power).

-Bill

pleppik · May 2015

I'm curious as to what the circumstances are where the grid power could go away for several years. Is there an easement or right-of-way that might expire?

a0128958 · May 2015

I thank everyone for their help. I learned a lot.

Probably the most fundamental is that what I need is an off-grid design. During the rare time I have grid power, I just view the grid power as simply another charging source. I.e., the structure always runs on batteries. And that I simply have many sources to charge the batteries: continuously available solar PV and wind turbine sources, and an occasionally available diesel powered generator an occasionally available grid.

Since NAWS sponsors this forum, and since I'm exceptionally appreciative to them for making this expert forum available, I took my business to them. Here's the design:

6700 watt-hours/day in Seatle climate

(24) SW-260 panels 260 W
ground mounted structure hardware
(1) MS4448PAE Magnum 4400 W, 48 V, parallel inverter, 60 A charger, 120-240 VAC output
( 8 ) S-600 Surrette 450 Ah @ 20 hr rate, 6 V batteries
(2) FM60 Outback Power MPPT Charge Controller 60 A 12-60 V, 48 V - 3000W

Other BOS parts.

Many thanks from everyone!

Best regards,

Bill

How Much Watt-Hour/Day Capacity is Needed for Family of Two to Move to Off-Grid Living?

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