Off-Grid, Please comment on my plan

machineman

Sometime next year my cabin will be finished and I will start building the power systems. The place is completely off grid. I made this spreadsheet to figure how much capacity I need. My target is 200Kwh/month. I will do a 48v PV grid. Do those spreadsheet numbers look right; 1.6Kw PV per day and 4444ah needed for >50% charge with 2 day reserve?

Also I will not be living there full time so can I reduce the number of PV's and let it take longer to fully charge the batteries? Or maybe thats an incorrect assumption. For charge controller and inverter I've been looking at that new Xantrex XW all-in-one system http://www.xantrex.com/xw/index.html

BB.

Re: Off-Grid, Please comment on my plan

I am not quite sure if I understand everything in your spread sheet... So a few comments on what I think I do...

1. Solar panel efficiency... Generally, in warm climates, a conservative estimate of solar panel "efficiency" (STC panel factory rating vs real life, warm weather, and average dusty panels, etc.) would be around 75-80%.

There is also the efficiency of charging a battery (flooded cell around 80%, and AGM around 90%).

And, if you are doing everything with inverters, you have losses there which give efficiencies around 80-85%...

The overall solar panel to 120 VAC appliance (conservative):

1/0.75 (STC panel rating) * 1/0.80 (battery chg eff) * 1/0.80 (inverter eff) = 2.1 (panels would have to be ~2x nameplate ratting * irradiance * hours)

2. Next, battery size--assuming that you want ~3 days of no-sun autonomy and only to 50% discharge (for long life), you would want 6x your average daily supported load. If the batteries are driving inverters, then you could increase their capacity by 1/0.80 or 1.25--but this is probably getting lost in the details.

Of course, if you have optional loads (washing machine, TV, or even A/C--may not need A/C for cloudy weather), you could size the batteries down.

My worst December (south of San Francisco--2 years worth on my system) was almost a month of 4.8 kWhr/day vs the next year average of 7.5kWhr/day of "daily" potential. And, in that stormy month, I had a lot of 1kWH/day times too. Fortunately, my system is a 3.5 kW (solar panel, 3 kW PTC--California) rated Grid Tied system... So, I never had to think about what to do during bad weather.

Also, you power requirements may vary dramatically between winter and summer--so, you have to compare your sort of worst case summer and winter needs to ensure that your panels are sized appropriately.

3. Backup power... If you need the power, there will be times when there just is not enough sun. It sounds nice to have a monthly average of sun--but in real life, you can only store 3 days of sun--and there will be times (depending on where you live) where you will go days and possibly even weeks without sun.

So, you will probably need a generator (gas/diesel/propane) for those time. Wind can help as it, many times, generates power when sun is not available (stormy weather). If you have alternate power sources, you can probably save a bit (at least initially), on the size of the solar panels.

Use your generator for bulk charging, when needed, and use the solar for topping off/equalization charging.

Adding solar panels to an old installation is usually better than undersizing the batteries and trying to add new batteries later--mixing used and new batteries together tend to cycle the new batteries (vs the old batteries) more--and you end up with both the new and old batteries failing about at the time when the old batteries are at end of life.

4. I would suggest you work out the approximate $/kWhour cost for your power too. Almost always, it is cheaper to install the most energy efficient equipment and the most insulated/"environmentally site efficient" home first, so that you are conserving every bit of power you can, vs trying to "buy" your way into solar power...

5. You say that you will not be living in the cabin full time... Solar power has its best payouts for full time living--or at least the 6-9 months when the sun is out (excluding winter) when you are capable of using (virtually) all of the power generated.

Any power generated and not used while you are away from the cabin is lost forever (but the fixed costs for the generation equipment, aging of batteries, etc.) is still there.

You can play some games--such as, if you are only going to be there on weekends, size the panels down so that 7 days of sun will charge for 2 days of use (2/7 = 29% of daily use). Size the batteries for 2 days of use (instead of 3--2/3=67%). The batteries may not last quite as long the solar panels are rated 3% or more of battery capacity, they will probably charge fine and last long enough (sulfation--hardening of sulfates that occur when a lead/acid battery is normally discharged--will kill flooded cell batteries if left discharged over any extended period of time).

6. Security--some folks here have used the "security" bolts to secure their panels and systems. But, if this is an insecure site, I would be hesitant about installing a maximally sized solar array--It is just too easy for that stuff to walk away or be damaged by lowlifes...

Generators (in the short term) can be less expensive and easier to secure and replace if this is an issue.

-Bill

machineman

Re: Off-Grid, Please comment on my plan

Thanks Bill... I forgot to mention that I will have some sort of back-up generator, probably propane. The generator can probably kick in to fill the lower sun hours during winter. I live In San Jose not to far from you, the cabin is 250mi north in the mountains east of Redding. There will be no A/C and heat will come from a wood stove. Major appliances will be efficient refrigerator, washer, power tools, TV, computer. Stove and Dryer will most likely be propane. At first I will be there weekends only but some years later I hope to be there >50% of the time. I'm starting from nothing so I want to build it with the correct capacity for the long run.

I'm trying to work out what is needed to get 200Kwh/month and don't want to under or oversize my system (Primarly the cost of PV and batteries).

Thanks,
Tom

crewzer

Re: Off-Grid, Please comment on my plan

Machineman,

While I didn’t spend much time reviewing the spreadsheet, I have to admit that I didn’t find it particularly useful.

200 kWh/month is ~6.7 kWh/day net. Assuming 60% overall system efficiency, the system will need to be rated for 11.2 kWh/day. Assuming a minimum of 4 hours/day of “full” Sun in Sacramento (PV array tilted at latitude + 15 degrees; most months, except December and January), you’ll need an array rated at 2.8 kW STC. Allowing for various operational quirks, 3.0 kW should work.

See: http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/sum2/23232.txt

Assuming 6.7 kWh/day net and 90% inverter efficiency, the batteries will need to supply 7.44 kWh/day. For three days of autonomy and a maximum discharge of 50%, the battery bank will need to be rated at ~45 kWh. For a 48 V battery bank, that would be ~937 Ah.

There’s more detail work to do. But, this should give you an idea of what you’ll need.

HTH,
Jim / crewzer

Windsun

Re: Off-Grid, Please comment on my plan

And don't hold your breath on the new Xantrex stuff either. At a trade show last year they said "will probably start shipping at the end of January".

At a trade show in June, they said "will be shipping some items by the first week of July".

Well here it is 3rd week of August, and so far it appears that nothing in the XW series is ready to ship.

If you are not living there full time, 200KWH seems a bit high.

westbranch

Re: Off-Grid, Please comment on my plan

We have a similar situation in an isolated cabin. Also starting small with the objective of living there in a few years. Currently over-battery'd and under-panel'd. In process of fixing this.

For the interim, as we are not ready to install a "full size" array, (more construction to come) we have a propane fridge and a small 1000w Honda true sine wave gen. Lights are track lihgt straight 12 v LEDs. We use a small mod sine wave inverter to recharge a laptop & rechareable power tool batteries (DeWalt electronic charger- see comments here also re burnout by MSW...waiting). Once paneled up, will go for a larger true sine inverter capable of powering 12 amps of 110v into a skillsaw or similar...

The propane fridge eliminates somewhere in the neighbourhood of ~ 1 kw per day of demand. That is a lot of panels that will sit unused when you are not there, as stated below. Something to think about.... see some of the strings here on fridge power comsumption.

PS It took over 2 years to use up a 100 pounds of propane. The fridge is rated to use about 1 pound per day but we used about half of that... fridge is shut off when away.

have fun with the set up...
Eric

icarus

Re: Off-Grid, Please comment on my plan

I think that you are looking at a lot of loads that could be reduced. As mentioned above, propane fridge being a big one. One thing I haven't seen mentioned is a gasoline powered wringer washing machine. We have one c1960 that works great. Up dated to modern honda power. Very effecient use of water (wash the 2nd load in the rinse of the first etc) and the wringer gets cloths much drier than a spin cycle washer). Takes a bit of time, but heck, there is a cost to everything. Burns 1 cup of gas or less per load.

Icarus

BB.

Re: Off-Grid, Please comment on my plan

I also agree that you should be aiming at well under 200kWhrs per month--full time.

My home, with 2 adults and 2 kids can get under 200kWhrs per month--with natural gas for heating/hot water/drier/no-A/C. That is with a fridge and a freezer, plus computers, printers, central heat, 32" TV, radios, alarm, intercom, etc.... Much of that is newer energy star appliances and putting everything on power strips to turn off the standby power.

Get yourself a kill-a-watt meter for ~$30 or so, if you don't have one... And, if you really want to drive your significant other(s) crazy--try an energy conservation kick at your current home and see how much you can save/need.

-Bill

machineman

Re: Off-Grid, Please comment on my plan

My plan is to have a shop where I will use power tools often. Thats how I came up with the 200Kwh/month number. Maybe that can be suplimented using a propane generator and I can go down to 100Kwh/month. I will have to re-think the numbers.

icarus

Re: Off-Grid, Please comment on my plan

You may find it much cheaper in the short and the long rung to run your shop tools off your generator. Since you are not grid connected, and therfore have no way to bleed off any over capacity, the size of the battery/array banks will have to be quite large.

Think about it this way. A 3kw honda eu generator, with auto idle function, so it doesn't have to run full bore during the times no tools are turning costs less then 3k. It uses very little gasoline. Even if you use it several hours a day, the fuel costs will be small. When I am in my shop full time, the motor loads (saws/lathe/drillpress) etc don't run more than ~30% of the time. Still big draws for a panel/battery bank, but tiny for the generator.

I also use a gasoline water pumpt to pump the water to a gravity feed tank to the house. A few litres of gasoline a month vs a much bigger battery/panel set up.

To build a system to carry those big motor loads gets expensive in a hurry. I'm not saying it can't be done, but I think your money might be better spend making the rest of your project more effecient. My guess that the size of the load for the shop will be so big that much of your solar input will go to waste.

Just one guy's opinion.

Icarus

machineman

Re: Off-Grid, Please comment on my plan

crewzer wrote: »

Machineman,

While I didn’t spend much time reviewing the spreadsheet, I have to admit that I didn’t find it particularly useful.

200 kWh/month is ~6.7 kWh/day net. Assuming 60% overall system efficiency, the system will need to be rated for 11.2 kWh/day. Assuming a minimum of 4 hours/day of “full” Sun in Sacramento (PV array tilted at latitude + 15 degrees; most months, except December and January), you’ll need an array rated at 2.8 kW STC. Allowing for various operational quirks, 3.0 kW should work.

See: http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/sum2/23232.txt

Assuming 6.7 kWh/day net and 90% inverter efficiency, the batteries will need to supply 7.44 kWh/day. For three days of autonomy and a maximum discharge of 50%, the battery bank will need to be rated at ~45 kWh. For a 48 V battery bank, that would be ~937 Ah.

There’s more detail work to do. But, this should give you an idea of what you’ll need.

HTH,
Jim / crewzer

Your calculations match what I had in my spread sheet if I change my efficiemcy to 60% which equals 11.2 Kwh/day of PV. The battery ah matches yours if I enter 48v battery bank. I good sign that a sort of know what I'm doing...

More to come on this when I recalculate my needed wattage and price out propane genertors for my shop plus extras. This project is a year away so maybe the new xantrex system will show up by then or maybe something better.

crewzer

Re: Off-Grid, Please comment on my plan

Good news! 8)

Greetings from Rawlins, Wyoming, where I'm waiting for a new alternator from my truck...:grr

Regards,
Jim / crewzer

mike95490

Re: Off-Grid, Please comment on my plan

crewzer wrote: »

Good news! 8)
Greetings from Rawlins, Wyoming, where I'm waiting for a new alternator from my truck...

Are they on sale there ??? ( runs and ducks )

crewzer

Re: Off-Grid, Please comment on my plan

ROTFLMAO!!

My wife and I are on a mini-vacation in WY. We dropped our younger son off at the Univ. of Wyo. (junior year history major), and we're supposed to be spending a few days visiting friends and tending to our property. The truck's "Check Battery" light came on yesterday when we were about 30 miles SE of Rawlins, and a local diagnosis indicated the alternator went bad. A replacement is due in this morning :cool:, so I'll do a swap and we'll hopefully be on our way. We head home later this week.

Wish us luck!
Jim / crewzer