Required off grid system size using current daily KWH usage

[Hitechluddite]

My future retirement home and current part-time home averages about 15.4KW per day with the worst day in the winter at 27Kwh. What size combination of Panels and battery would supply this load? If I take away the two highest usage months I could get by on a 20Kwh a day average. Is there a point that a small diesel generator would be more cost efficient for those days over 20Kwh?

 

[sunshine_eggo]

Solar hours by month for Columbia, KY with panels tilted to 38° (best year round):





Divide your daily kWh by that number, and that's the minimum PV Watts you need.

If that 27kWh is in December:

27kWh/2.93h = 9.2kW array.

Your battery capacity is sized as a multiple of your daily usage. If you wanted 2 days of battery backup, you'd get 54kWh of battery. Most go for one day and get a generator.

It's up to you to decide on how much you want to spend. Generator fuel programmed into an undersized system will eventually cost more than PV and battery.

You can run the analysis for your exact location using the PVWatts link in line #1 of my signature.

 

[Hitechluddite]

Fantastic!
Question, when I run the calculator in line one of your sig file using a 10kw array it returns a power generation of 13,496 Kwh annually but adding my annual consumption from the utility shows a usage of 5,611 Kwh did I do the calculation wrong?

PVWatts

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

pvwatts.nrel.gov

 

[shadowmaker]

Fantastic!
Question, when I run the calculator in line one of your sig file using a 10kw array it returns a power generation of 13,496 Kwh annually but adding my annual consumption from the utility shows a usage of 5,611 Kwh did I do the calculation wrong?

PVWatts

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

pvwatts.nrel.gov

If totally off grid you need to plan according to worst case scenario (December). I'd calculate it little different than eggo there did, but result is about the same.

My math: 31 (number of days in December) x 27kWh (your max consumption per day) / 75kWh (total yield from 1kWp array in December) = 11,16kWp

And yes this means your setup will produce more than needed for the rest of the year.

If grid tied according to your assumption of 20kWh/daily : 31 x 20kWh / 75kWh = 8,27kWp

BUT, you need to understand that these calculations are averages. At least in here sun can be hiding for weeks making all calculations close to useless. Yearly yield can be predicted pretty accurately, but weekly even monthly variation can be huge. That's why you need enough battery capacity/generator. Overpaneling (~20kWp array instead of 11,16kWp) helps and it might be the cheapest way to go in the long run as panel prices are really low.

Calculations doesn't take account of shadows, dust, snow, etc on panels which can drastically lower production.

 

[sunshine_eggo]

Fantastic!
Question, when I run the calculator in line one of your sig file using a 10kw array it returns a power generation of 13,496 Kwh annually but adding my annual consumption from the utility shows a usage of 5,611 Kwh did I do the calculation wrong?

PVWatts

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

pvwatts.nrel.gov

As @shadowmaker indicated, when you size for the worst case, you will over-produce for the year.

He also makes good points about the variability of the data. This is a planning tool and your actual results will be both better AND worse than predicted. 30 year average weather data is factored in daily, but average weather for a given day of the year doesn't occur consistently for that day of the year. Unless you have enough battery capacity to cover your worst string of days, you'll need a generator for a handful of days per year. What you want to avoid is having to run the generator for extended periods because you've not designed for the worst case.

"30 year average weather" means "they" look at each Jan 1st of each of the last 30 years, average them and then pick an actual day that most closely matches the calculated average. That's repeated for the other 364 days of the year.

My remote site in the high desert of AZ regularly outperforms the December hour number, but the reality is that there can be 3-4 days of overcast/snow where production is reduced to a small fraction of what I get on sunny days. The reason I don't have to run my generator is because when we're not there, we have about 4 days worth of battery capacity, and that's enough to get us through the rough patches... so far.

If you really want to get into the weeds, you can download the hourly data that shows how the individual average days behave.

 

[wdwtx2.0]

One good thing about overpaneling is you always have extra panels exactly where you need them in case any go bad.
Panels are about the cheapest component.

 

[Lt.Dan]

You can also look into reducing your consumption. This 9 times out of 10 will reduce your costs overall. Bringing that 20kWh/day down to 15kWh/day would be a drastic reduction in your requirements, especially if you're using an alternative to heating in the winter months.

 

[Zwy]

My future retirement home and current part-time home averages about 15.4KW per day with the worst day in the winter at 27Kwh. What size combination of Panels and battery would supply this load? If I take away the two highest usage months I could get by on a 20Kwh a day average. Is there a point that a small diesel generator would be more cost efficient for those days over 20Kwh?

Your usage was close to what I had when I first started monitoring usage. As Eggo stated about a 9Kw array will get you there as a minimum, I run 8.4Kw but do seasonal tilt, I do not overpanel.

Spend some extra on the battery bank and determine what the actual minimum Kwh/day you can get by with. If no laundry or other high wattage items are run, we can get by in winter for 5 days with running the propane furnace for heat. I am adding another 14Kwh of battery to the 58Kwh bank this year so we can get thru 6 days of no sun. It seemed everytime we had poor weather the system would switch back to grid at about 5.5 days.

If you routinely use over 20Kwh in winter, you will need a generator or grid backup unless array size is increased and a very large battery bank.

 

[Hedges]

my annual consumption from the utility shows a usage of 5,611 Kwh

"Required off grid system size using current daily KWH usage"

Does this mean it is grid connected, but you want to cut the cord?

My future retirement home and current part-time home averages about 15.4KW per day with the worst day in the winter at 27Kwh. What size combination of Panels and battery would supply this load? If I take away the two highest usage months I could get by on a 20Kwh a day average. Is there a point that a small diesel generator would be more cost efficient for those days over 20Kwh?

Of course there is a point where diesel is most cost effective than PV + battery. Especially if you get socked in for 10 days; 200 kWh of battery would be way too expensive.

People have reported $1/kWh using diesel generator. What would it cost you to keep the grid?

You can also look into reducing your consumption. This 9 times out of 10 will reduce your costs overall. Bringing that 20kWh/day down to 15kWh/day would be a drastic reduction in your requirements, especially if you're using an alternative to heating in the winter months.

What is your winter 27kW going to?
Fossil fuel heating would be lower maintenance than diesel feeding electric heat. More efficient (less fuel consumption) too. Or wood heat. Electric heat is fine once battery is full, if you've got it, use it.

Probably 50% to 65% of energy from diesel engine goes out the tail pipe or cooling system. Consider "CHP", combined heat and power. Before the radiator (if water cooled), plumb in a liquid-liquid heat exchanger. Run that hot water through a tube inside insulated exhaust system (arrange condensate drain to avoid rusting out muffler, may need one of stainless not carbon steel). Use the waste heat for the house while charging batteries.

 

[Hitechluddite]

My current home there is a 14X65 mobile home, the HVAC system is a new but bottom of the line 15 seer 2 ton heat pump. When I build permanent I am most likely going geo thermal. the house will be twice as big but a more efficient build with a higher efficiency system so I don't see my usage going up much if any. I checked with grid tie in and feeding the grid because I could allot 3 acres to panels if it would be worth the work and investment. the local REEC doesn't give much for power fed ( 2.7 cents per KWH) and the wait time for approval is over a year so I discarded that idea...

 

[Hitechluddite]

As for batteries I really like the thought of 24V tesla modules, they are plentiful and give lots of bang for the buck!

 

[Hedges]

Some people here have built around Tesla packs. But it is work, and that is a more flammable chemistry.
I would consider lithium ion for portable, less weight. LiFePO4 is attractive for fixed systems.

The EG4 PowerPro looks good so far (and it reportedly talks with my favorite inverter for closed loop)

So backfeed doesn't pay much of anything. But the grid could serve as your generator for a fraction the cost. Is it available?
Ideally get an inverter that supports zero backfeed and blends PV & battery with grid, only drawing as necessary.

One way I could do that with my SI + SB system is simply tell it to use grid as generator. Operate off-grid until about 60% DoD, then charge a bit from grid. Not all off-grid inverters can blend, I could tell mine to limit draw from grid to 15A if I wanted while using PV & battery to supply larger loads. I think some of the popular large AIO like SolArk and EG4 18kpv will also.

 

[sunshine_eggo]

As for batteries I really like the thought of 24V tesla modules, they are plentiful and give lots of bang for the buck!

Run.

DIY NEW LFP is cheaper and safer and has better operating voltage ranges. Tesla packs are typically 6S NCA/NMC, and they're not optimal for either 24V or 48V.

 

[xcentric]

I'd re-run the calculations based on different angles of panels - you don't want to optimise year-round collection (and have a surfeit in the summer by sizing for the winter) - you want to maximise production in the winter, and miss out a bit in the summer. So your panels will be angled more steeply to catch more winter sun. and you'll be down in power in the summer. However - all this makes a few % difference, maybe into early double figures - and so if you have space then a few more panels than the minimum is better.

But in the winter you can have days of bad weather, rain, snow, and basically no sun, so going off-grid completely can be costly to cope with all that - you either need batteries galore, or a generator for those times, or grid that you sometimes use. These are in most expensive and most environmentally friendly order first to last. If you must go off-grid, I'd buy a small generator for the really bad days, and if you want to go towards more and more renewables, batteries are likely to drop in price over time; generators less so......