Need help planning my off-grid system

Re: Need help planning my off-grid system

Reality wise, you probably would be best off paying $20k for the power lines and, when you are ready, installing a grid-tie solar system (and I would suggest that solar hot water for domestic and space heating possibly should be installed before solar electric--in terms of the best bang for the buck).

From very rough numbers I have played with before comparing the cost per kWhour between Grid Tie and a true off-grid system roughly 3-4 times the cost per kWhr (between $0.25 vs over $1.00 per kWhr).

You appear to be near Reading California, so you should have good sun most of the year--good for solar.

Also, I believe the current California rebates only pay for Grid Tied systems--not for off-grid systems. It is very possible that you could pay for the Utility lines with the money you save in batteries and inverters, and the extra money you qualify for in Grid Tie rebates (worth around 1/3 of the total system costs--installed, retail at ~$10 per watt of solar).

Connecting power lines, I would hazard to guess, is only going to get more expensive and more difficult (permits, growth limits, zoning, etc.) as time goes on.

I understand the pull for a true off-grid existence... But the price of maintenance (generators, batteries every 5-15 years, fuel, etc.) will probably equal, or cost more after all of the accounting is done, than bringing utility power in and using Grid Tie solar. If you will need emergency power (storm, grid failure, etc.)--Generally, it is cheaper to install a backup generator for those few times it would be needed (keep fridge/freezer/aquariums running) than to create an entire off-grid home/installation.

In any case, your #1 job should be conservation in building the structure to take advantage of the sun, shade, lighting, thermal mass, and new technology equipment (heat, cool, appliances, conservation life-style, etc.).

Reading through Home Power Magazine and working with your architect/designer to avoid some very basic mistakes made by many people (i.e., too many windows, improper use of thermal mass) will be worth it with respect to future costs and livability.

My 2 cents worth anyway.

-Bill

Re: Need help planning my off-grid system

The off-grid system becomes more complicated because of the nature of the beast--storing and providing energy based on the needs of the batteries and the appliances. The addition of a generator anyway for winter weather, and the inefficiencies of conversions from solar to DC to battery to DC to AC (typical system).

The Grid Tie Inverter can be around 95% efficient converting DC solar to AC utility power--but so can the charge controller... However, with the Grid Tie the power goes either directly to your appliances and/or out through the meter (spinning it backwards) out to the grid--with just a little loss in the wires.

With the Off-Grid system, much of the solar power goes from the panels, to the charge controller, and then into the battery--that battery (if wet cell lead acid) costs you about 20% in losses right there. Then, there are the losses of re-converting the energy back into AC--another 10-20% loss there... And lastly, since you can only store about 3 days of usable power, any extra power generated (during summer, you guys on vacation, etc.) is forever lost--beyond that what the battery can store (Grid Tied, you can "store" up to one year's of energy--depending on your net metering plan--you can even "pre-use power" in one season and make it up back in another system--something that a battery system cannot do). You add up the losses because of battery/inverter losses and other issues--the solar panels have to be 1.5x to 2x as large with a off-grid/battery system.

And, there are the variation between summer and winter generation--for you system (see below for exact numbers), from a 3kW system, you can get ~500 kW during the "sunny months" and down into the ~150 kWhrs in the "dark months"... So, you need to make those losses during the winter--either by cutting back on energy usage (OK, if you have heavy summer loads, like A/C), or, more likely by a fuel driven generator (and maybe alternatives like some wind power too)...

Generator power (gasoline) will probably cost you on the order of $0.50 to a $1.00 per kWhour just for fuel--let alone the costs for wear and tear.

You add the costs for batteries (and their replacement every hand full of years), the extra equipment required (chargers, inverters, generator, maybe wind-turbine), and long periods of cloudy weather (fuel costs).

I went through one winter 23 day period where my system, which could generate ~10 kWhrs per day (on sunny winter day) average just 3 kWhrs per day, and I went 3-4 days with less than 1 kWhr per day--that December I only generate less than 1/2 the predicted amount of electricity because of bad weather (and I live in a realtively sunny area of the state).

Can you build an off-grid system--sure. It is neat (and) expensive project. Is is cost effective--less clear.

Over the years, I have read many articles (back when solar panels and the electronics were much more expensive than today), and there seemed to be a sweet spot at about 100 kWhours per month, if you can get your monthly usage down to that level (or below), the equipment is not that expensive and the maintenance (cleaning batteries, watering, size of the wiring, etc.) is reasonable... 10 kWhrs per day is a pretty large system--especially if that system needs to generate 10 kWhrs per day winter and summer.

From the link I posted in another thread (solar calculator), a 3 kW rated system system for 96013...

kWH Month
==============
163 January
247 February
370 March
421 April
495 May
501 June
550 July
519 August
450 September
344 October
168 November
133 December

---

4,363 TOTAL

On average, you needed 10 kWhr/day = 3,600 kWhrs per year... A 3 kW grid tied system with net metering can do it pretty nicely.

However, if this was an off grid battery system, you average uses at 10 kWhr/day because 15+ kWhr/day or 450 kWhrs/month or 5,400 kWhrs per year...

Your off-grid 3kW system (with about ~3.5kW of solar panels) could (at best) only supply 100% of your power needs from April through September and just 30% of your needs during December (or less in bad weather)...

Again, depending on your actual seasonal loads, the numbers may be better (or worst)--and these are averages. The real numbers for any particular period can be much worst (or somewhat better).

IMHO, the 1.9 kW of solar panels would not be enough power for a 10 kWh/day (really 15kWhr per day) load. And seven strings of "smallish" batteries would be a cabling/maintenance/operational nightmare (again, in my humble opinion--using much larger batteries would help here).

Wind-Sun's store has price listings for all of the basic components, and good FAQ's in the various categories (charge controllers, inverters, batteries, etc.) where you can read up on their suggestions...

In the end, comparing 20 years of off-grid solar costs (cost of purchase and maintenance) vs 20 year amortization of utility line + power costs (with optional grid tie solar to reduce exposure to ever increasing electric rates)--I would be hard pressed to ignore that utility feed...

One way to look at the utility feed costs... assume 20 year payback period and 10 kWhrs / day usage...

$20,000 / (10 kWhr/day*365d/yr*20yr) = ~$0.27 per kWhr + $0.12/kWhr electric rate PG&E E1 rate @ 300 kWhr/month
effective rate of = $0.39/kWhr (for 20 years)

$0.39 per kWhr is not cheap--but it is still cheaper than anything you can generate with an off-grid system.

Plus a 3 kW $30,000 grid tied system will be eligitable for ~$8,000 in CA state rebates (plus another ~$2,000 in Fed tax credits). Something that a off-grid system (at something like twice the installation and maintenance costs--SWAG) is not even eligible for.

I hope that this helps give you some ideas.

-Bill

Re: Need help planning my off-grid system

Sorry Mike--I guess I sound like a broken record here at times.

One way to think about solar (or any complex engineering system) is to look at everything as a block... The batteries--a block of energy storage. The inverters, a DC to AC conversion block. The Charging block, several sub sections--one that draws and regulates power from solar panels, another one that draws power from a fuel powered generator, and others--like Wind Turbine). And there are losses as power passes through each of these blocks (battery ~20%, solar charger ~5%, inverter ~10-20%, etc.).

Realistically, the only block in this setup that can be damaged by improper use is the battery itself. The other parts (charging block, inverter block, etc.) will operate based on available energy and storage space--but will not be damaged by these normal operations.

So, focusing on the lead acid battery... The issues are; 1. battery is temperature sensitive, 2. produces hydrogen gas and is filled with sulfuric acid, 2. should not be deep discharged (typically, below 50%), and 3. should be "fully charged" ideally within a day or three of being discharged.

So:

1. The batteries ideally run around 70-80F-- if it is too cold, they can freeze (and break the case). A battery freezes at lower temperatures if it is fully charged. Battery charge voltage is temperature sensitive--so having a charge controller that monitors the temperature is important. In general, for every 10 degree Celsius increase in temperature, the battery life is cut in 1/2. A battery forms sulfates as it discharges, and these sulfates begin to harden after approximately 1 day and hardened sulfates do not recover during recharging (from what I have read). So recharging a battery after use is very important.

So, that gets back to how you use a system. Discharging and charging the batteries within these limits, and they should last pretty well. That would mean that if you have more that a few days of no sun--then you would have to fire up your alternate charging source to recharge the batteries and power your daily loads during the time the sun does not shine.

2. The battery storage area needs to be ventilated to prevent hydrogen build up and explosions. The hydrogen gas is formed during the equalization charging (converting the water in the cell to hydrogen/oxygen--electrolysis--during the last bit of the charging--and especially during equalization).

3. Monitoring the battery change by just watching its voltage is prone to errors... The battery should sit for several hours, without loads/charging currents, then the voltage accurately measured (against a temperature correction chart). The better method is to use a battery monitor that measures current into and out of the battery bank. It is much more accurate and can be read for capacity at any time (under load, charging, etc.).

I am more of the school that a few, very large batteries in series, is preferable to a bunch of "smaller" batteries in multiple parallel strings. Filling fewer/larger battery cells, less cabling, higher voltages (lower current--12 volts vs 48 volts systems).

Getting a bunch of parallel batteries to current share can be pretty difficult. Cables, connections, etc. should all be identical for each of the parallel banks for the batteries to properly load share. One battery manufacturer I read said that they found series connected banks to last longer than parallel connected banks.

Wind-Sun, the owner of this site, themselves said they don't see any life difference between parallel and series connected banks in terms of life--so, if properly designed, either will probably work OK for you.

The inverter (DC to AC conversions). There are many good suppliers out there that have many options for true sine wave inverters (vs modified square wave)... But they are not cheap. You best need to define the loads you have first, then pick the units that might meet your needs (120, 240, low standby loads, efficiency, connections with charge controllers and generator controls, etc.).

The charger block--now that you know the downstream requirements (load, inverter, batteries, losses), you can size the charger. Solar panels are probably the highest cost per watt--but if you have good sun and a place to mount them, they are probably the least maintenance/lowest long term cost component of the system. But, there is a ~4:1 output between summer and winter months in your area (and mine). And it can be made worst if cloudy weather (or recently, major brush fires) reduce your capacity.

And only you can decide if you want the solar panels to supply 100% of your load during the summer, but 25% of the load during the winter--or if you have seasonal/optional loads... A/C during the summer and lower electrical use during the winter. Optional loads--washing, drying, vacuuming, TV, etc., that you can load shed during periods of low sun..., will all help you reduce the absolute size of your solar array.

Solar panels themselves have issues--voltage (and power) drops during hot weather (need good air flow for cooling, more panels if in hot area). Voltage changes between cold and hot weather--need to look at the extremes to ensure that charger/grid-tie inverter can accept the range of voltages.

Wind Turbines, if you have the land to mount them, and the ability to service them are a good backup to solar--in many areas, the wind is high when the sun is not shining (stormy weather). They are mechanical units so there is more maintenance required (getting the turbine down off a hundred foot tower has always been my question--renting a crane vs a tilt up tower and those issues/limitations--but I don't have the land or wind--so it is not something I am real familiar with).

Generators--your choice, gas, diesel, propane, natural gas... Look at the fuel flow rates vs kWhours generated. A good Generator/Battery system is probably the most efficient. Running a 10 kW generator to run a 200 watt TV is not great. Running TV on batteries for a few days, and running the 10 kW generator to pump 7.5 kWatts back into the batteries (bulk charging) and using the solar to do the final charging/equalization (where currents/power use are relatively low)--a good thing.

You have visited a few off-grid homes in your area--so you have seen the equipment required.

Do you have specific brand of equipment you like/dislike? Many of the manufacturer websites have pretty good diagrams/explanations of how it all fits together.

Also, building a small solar system first (1/2 - 1 kW of panels, a few good sized storage batteries--or even golf cart batteries, with charge controller, battery monitor, inverter) and getting some runtime under you belt (use it like a generator for your home construction site, or to power part of your existing home), would help too.

-Bill