Determining how much solar power is enough

Designing Solar Power Systems
This page is for FM translators (those FM transmitters for out in the sticks where city transmitters can't reach), but the details apply whatever your needs: determine how much you're going to draw 24 hours a day, figure out how much sunshine you get in a day (usually about five hours), then put 24 hours of your needs in a battery every day whether it's sunny or not.

That means you need a lot of solar power going into your batteries, and you need a lot of reserve in your batteries. Going totally solar is extremely expensive. This will help you determine if it's worth it for your application.

Philip, who came up with only 5 hours of sunshine per day? That seems extremely low to me.

That's probably an average. SOCAL will be an exception.

But an average of what? Where in the U.S. do you not get more than 5 hours of sunlight per day? Obviously rain/cloudy days aren't as bright but the sunlight is still there.

I grew up in the Midwest, lived in Colorado one summer and the rest of my life has been spent here in SoCal but even in mid winter in St. Louis we got over 5 hours of daylight.

There are also some pretty cool tracking devices that follow the sun as it moves across the sky which greatly increases the amount of useable solar power.

At any rate, solar is only going to get more economical, more feasible and more efficient as time and technology marches along.

Warning, IMNAE, so hopefully nomad or some other expert can check my numbers.

Its a matter of angles. Solar Panels produce 100% power when the sun in pointing directly at them. Once the angle becomes too oblique, the output drops fast.
So that rules out much of the morning and the evening.

Then, unless you are on the equator, you have to take your latitude and weather into account.

I am not against Solar, think it is a great tech. Bang for the buck is still low, but improving.

If your goal is conservation, reduce usage with better appliances, better light bulbs, and better habits BEFORE you look at solar. You get more bang for your buck, and when you DO go solar, you need to spend less on panels because you reduced your requirements.

Most Solar Panels run~$5.00/Watt.
Since grid power in the Midwest is $70.00/1,000,000 Watts
(Megawatt) you will need to use that panel a LOOONNNGG time to break even on a cost level...

Solar calculations also need to take into account the operational requirements throughout the year. Higher latitudes complicate the design to point that solar power becomes useless for many months during the winter. The cost of the installation is prohibitively expensive when looked at in the short term or even the medium term and even the long term (although the long term for grid electricity supply needs careful analysis as well as it might not always be available)

Solar Calculation for domestic Freezer application

From this previous calculation a 240W peak power solar PV with MPP regulator, high efficiency inverter and a 200AHr Battery installation at my location will produce over a year about 170-200KwHrs or typically a cost saving over using grid tied electricity of about £29-34 or $43-51. The installation cost would be around $2500 so would take around 50-60 years to pay for itself using current grid tied pricing. Solar PV could not hope to be cost competitive with burning fossil (previous stored Solar energy) and nuclear fuels on a large scale as the energy has already been accumulated. Grid Electrical energy is just a means of releasing previously stored solar energy rather than a means of collecting solar radiant energy, storing the energy then transmitting that energy to the customers load. Grid tied electrical energy companies who provide this electrical energy have for the most part have no means to provide long term energy once the previous naturally accumulated solar energy store has run out. They are also doing a good job at wrecking the environment at the same time trying to get to these ever diminishing energy resources. Drill baby drill has its down sides.

http://news.bbc.co.uk/1/hi/world/americas/8651624.stm

It is probably better to use naturally available hi energy efficiency solar energy accumulators at high latitudes during the winter months. They are inexpensive but just need a greater installation area. They also make great air conditioners during the summer months and even can provide humans with nutritional benefits and construction materials.

That's answered in the article:
"Since according to NOAA the sun averages 5.5 hours of 1000 W/m insolation at our proposed site above Vail ... " with more detail at
http://www.rbr.com/radio/ENGINEERING/95/13603.html

Solar power follows a bell curve of efficiency through the daylight hours. Based on the rated output of the panel, the average yield of watts generated over a period would allow only so many net hours a day total, not that the sun only shines for 5 hours, but that the energy generated equals a fixed number of watts per day. A 500 watt panel could be generating 50 watts between 7:00 am and 8:00 am, which is (roughly) equivalent to 10 minutes at the rated output.

Energy could be viewed as the accumulation of effort, and watts (power) being the rate of accumulation. You can say 500 watts for 5 hours, or 210 watts for 12 hours (or whatever elapsed time is from dusk to dawn), or anything in between, including a varying rate over time (the bell curve) but the net will still must work out to 2500 watt hours generated, or accumulated.

Rule of thumb is that in good conditions you can estimate solar panel power output as the rated output for four hours each clear day. Figure six hours a day if the panels are adjusted to track the sun.

Practical usable power from battery banks is typically a third of their rated charged capacity in amp/hours. A bit more if you use batteries with friendlier charging/discharge curves, a MPPT charging system, use twelve volt output directly or a high efficiency inverter. But seldom do you get more than half without shortening the life of the battery bank.

That isn't the final word, and such rough estimates won't replace a detailed engineering study, but it is remarkable how many times such a quick and dirty estimate is within a few percentage points of some very expensive professional analysis.

Yeah, I'm still waiting for my Oil Burner to "Pay for Itself" - same goes for my washing machine and water well. I did the math, and I must have made a mistake, because it says that those things are a cost, not an investment.

Now, if you're expecting your home-based power generation system to have a LOWER OPERATING COST than your grid-fed system, well, no, it's not reasonable to ever expect a decentralized system to operate at a lower cost than a large-scale production facility.

If that were the case, it would somehow and some way eventually be cheaper for you to make your own automobiles, to grow your own food, to make your own lumber and to make your own medications.

It's not. I grow my own food - to an extent. But I could never grow enough corn or wheat on my own. I can't smelt metal. I can't make plywood.

Photovoltaic systems are still appealing - but not so much from an individuals economic perspective.

Cost for line power from the POCO (power company)is roughly $.06 to $.10 per kWh depending on where you are and what they are using for fuel at the time. Solar is running $.10 to $.15 per kWh. Mostly more to the high side. It would seem to be a losing proposition.

But there are other considerations. Like how much is it worth to you to be able to completely avoid summer brownouts and outages and being independent? Also, many POCOs buy back power so while you're using little power your meter spins backward. POCOs typically spend extra money on peak load generation. They are willing to pay a premium to cover peak load requirements. Currently utility buy-back is the standard rate but computerized meters and changes in the regulations may see POCOs paying peak rates for power they buy back from you during peak times.

The ability to remove your house from peak summer load motivates many sun belt POCOs to subsidize solar installations. Last time I asked the local company was contributing a dollar per watt of solar pane installed up to 20,000 watts.



It also has to be noted that future trends are that central POCO generation is going to get more expensive. The shift from coal, resort to expensive nuclear power systems, or the need to pay for coal and other high CO2 sources, is going to see the price per kWh rise.

Whereas solar systems are becoming ever more efficient, easier and quicker to install. Purchase price per watt of generation capacity is going down as efficiency is going up. Yes, a bank of solar panels is a major investment but longevity is expected to be 12 to 20 years. A losing proposition on its face the kicker is often tax breaks, or subsidies from the POCOs or government.

There are also sometimes unexpected returns. A local family covered the south side of their roof with panels and saw a 10% to 15% reduction in air conditioning cost due to shading from the panels. A benefit even the POCO engineers had overlooked.

The average cost of residential power in 1970 was $0.02 per kWh. Today, it's four times that. Just since 2000, rates have escalated 6 to 8% per year. Extrapolating rate increases at 8% per year (if they don't go higher than 8%), we're looking at $0.17/kWh in 10 years, $0.37/kWh in 20 years.

My use of 1528kWh last month would cost, respectively, about $260 in 2020, and $565 in 2030 (flat rate, no reduction for off-peak hours).

There are always more things to consider than today's knee-jerk-reaction prices.

Sue

I would be happy with a small solar battery installation.
Something that would put out 500-1000 watts@12v an inverter
that would handle any 120v load I can connect to it ( 2 sockets )
and a battery that will support my home for 4-6hours for brown outs. I figure I would only need to run my freezer for 1-1.5hr/day
to keep my food. The fridge would likely take twice that because it will be used more often. other than those items I can shut down our home as needed to cut consumption. Lights are CF, comp.
runs thru a battery, cell phone charger, no heat needed we have a fireplace.
I checked out mysolarbackup and the power it provided for the cost
was prohibitive, IIRC the internal battery only has the capacity to run my freezer for 15-20 min until it is totally dead.

Yeah, well, I can't afford a detailed engineering study, so it's nice to get a more nearly accurate ballpark estimate instead of relying on the numbers published by the manufacturer.

So far as powering the freezer, you can either get enough solar panels to produce enough power coupled with enough battery capacity to keep it running continuously or you could increase the battery capacity so that a smaller solar panel will keep them charged enough for short term, part time usage.

You don't need a whole house system to run only a couple of circuits.

Any pre-built or pre-assembled kit is gonna cost more than doing it yourself.

It's fairly simple to figure out the needs for a single use emergency system, it's watts, amps and ohm's law.

One quick and easy thing anyone can do for use in an emergency, pick up a pack of solar powered lawn decoration lights with LED bulbs. For $50-$100 you've got a temporary, battery powered lighting system for use when the grid goes down. Keep the batteries charged up or replace the ones that come with such a kit with some low loss Eneloop type batteries and Bob's your uncle.

The problem with a lot of comparisons of replacing grid fed power is that the infrastructure that is required for the grid fed systems are rarely taken into account. I've read of numerous examples where people thought that it would too expensive to use solar power, the only fair comparison would be to include the hidden costs of those power poles and lines outside your home, along with the guys in the bucket trucks who come along periodically and keep the system working.

There's a similar disconnect when talking about using alternatively fueled vehicles, when one adds up the real costs of the transportation system right down to the roads which are subsidized, then you realize that the alternatives don't actually cost that much more at all. I realize that the alternative fuel vehicles would be using the same roads but there's a huge amount of unseen and usually uncounted costs that have been paid for to allow us relatively cheap fuel and power.

The bad point of a kit system is that they invariably, good capitalists all, get their profit margin off the top. This is far less an issue in mass produced systems where wholesale purchase of parts more than makes up for the profit margin. But solar is still a small sector of the energy market and while it is growing even large marketers are selling systems by the hundred, as opposed to the thousands.

On the other side kit systems assembled by major retailers have a lot of experience and engineering behind them. Buying one you often pay a bit more but you also sidestep a lot of mistakes and teething problems. They are sort of the equivalent of training wheels. A small starter system that has parts selected to assemble easily and work well together, that is pre-engineered for you, backed by customer support goes a long way toward making getting started in solar easy. It makes parting with the cash a little easier.

A small starter system is often a good way to get your feet wet. Once you get to know the technology such small systems can be added on to. Or they can be re-purposed for use on the barn or cabin if and when you install a larger system on the house. Or they can be reworked to make a portable emergency power system to keep some combination of communications, lights, a small refrigerator for medical supplies running.

Example- Harbor Freight has a 45 watt kit for 199.00 US
(3*15 Watt panels - 12 volt @ about 1.5 Amps each)

includes panels, controller, 2 DC lights NOT BATTERIES

at 6 hours full light, that is 270 Watts possible, more likely 220 watts per day

I found 75 AMP hours 12 Volts for 40.00 each (military surplus, used 2 years of 10 years rated, AGM, Woo hoo!) Bought 4, so 120.00.

So, we have 300 AMP/Hours, or 3600 Watt Hours storage. (300 Amps * 12 Volt= 3600 Watts)

I have an inverter already (no extra cost) But to buy a new one capable would be ~100.00 (1000 watt continuous, 300 watt surge)

If I use 50%, or 1800 Watts, I can run my 18Ft/ Fridge (500 Watts) for 3+ hours. It would take me 8-10 days to recharge the batteries to full. Cost 320.00 ( or 420.00 with Inverter)

500W for a Fridge!! Your energy bills must be enormous especially if you need aircon to remove the heat generated by your fridge.

Thats 4380 kWhrs/year!! That would cost me around $800-900 a year to run in the UK.

The Vestfrost SE225 A++ Chest Freezer @ 8 cubic feet is available for 172 kWhrs/year whilst another Vestfrost SE225 operating as a chest fridge would work out around 36 kWhrs/year. The Vestfrost SE225 also has a 50 hour specification for the internal temperature rise from -18C to ambient temperature.

http://www.builditsolar.com/Projects/Conservation/chest_fridge.pdf

Total Yearly energy usage would be less than 250kWhrs/year or around 16-17 times less than your current refrigeration.

A refrigerator/freezer doesn't draw power like that continuously, it's intermittent. I think your math is off a bit. If I estimated correctly, you could get about 36 hours of usage out of those batteries before they'd be at 50% capacity.

It gets more complicated when you have the panels running at the same time you have the batteries being used. You'd be adding less than you take out obviously but you'd still be adding some.

I could be wrong about the figures but I think you're low on how long those batteries would power the fridge.

You also have to figure in how much power you're losing with the inverter. Depending on how efficient it is, you could be giving up as much as 50% converting the 12v source to 120v.

Top of my head guess, I think you could get approx. 18-24 hours of use from those batteries at those power levels.

Somebody please double check the math...!!!

Most American Style refrigerators typically have an annual load of around 500-600 kWhrs/year for the A and A+ rated European market. So I would estimate around 2kWhrs/day worst case. So 18-24 hrs would be about right. But then again the refrigerator might be an older inefficient model where it could easily be 3-4kWhrs/day so 9-12hrs might be a typical run time using the batteries especially if the inverter is a low efficiency model i.e 50% even when used with a modern refrigerator design.

Is the 3 hrs quoted an empirical or experimental test?

Also remember that when powering a motor from an inveter you need to be sure its a true sine wave and sized big enough for the startup current (could be 10x) so you need a way more expensive inverter.

There are 12v refirgerators available, they are $$$ but available. I'm trying to bandaid mu old camper refrige (absorption type) to last a couple more summers and then maybe buy one of those 12v ones.

Thats why I have a WAECO SinePower MSP352 300W Inverter.

http://www.outdoorgb.com/p/WAECO_SinePow...amp;country=GBR

One of the best features of the Waeco Sinepower inverters is that they can be remotely switched on and off for control purposes (i.e. external thermostats with bang bang or hysteresis temperature control) for intermittent motor loads such as refrigerators and freezers without having the inverter losses when the refrigerant motor is not energised.

The 90% efficiency and the 700W peak load capability is another bonus and shouldn't present a problem for high efficiency refrigerators with motor loads less than 75-100W (continous).

DC refrigerators are certainly much more expensive and in certain circumstances with very high thermal efficiency models, can just be run with a solar panel even without a battery energy source or at least a much smaller one due to the lack of inverter losses. You have to weigh up the pros and the cons though, which are mostly dictated by how much and how reliable the solar irradiation is available and where the DC refrigerator is to be located within a domestic premises i.e pure thermal hysteresis control.

3+ hours of continuous run-time, so I would guess that I could easily keep the food cold for well over 24 hours- I plan on making an empirical test within the month if I can.


Honestly, the solar is more of a learning project; looking at buying an RV and attaching the solar to it- I hate generators at night, even the expensive ones are loud and smelly...

Between the 1.5 KW Generator and the solar panel, I should be able to keep the fridge and a fan or two running for nearly a week.

If power is going to be down longer than that, its time to move the family out for a while.

Yes/no/perhaps ... Most motor operated appliances run quite well on a simpler, and much cheaper 'modified sine wave', stepped wave, inverter. Only delicate control electronics not protected by their own power supply, like electronic ballasts, have real problems. I have run several refrigerators, a freezer, and a few air conditioners off of simple modified sine-wave inverters and all ran quite well.

I carefully monitored how hot they got when running, overheating is the major problem if they don't like the waveform, but after a few hours running they all were doing fine. I worried about it in a few cases because failing to reset the thermal limiter means the unit isn't running and you need to rescue the contents of the refrigerator or freezer. Otherwise, the unit overheating isn't the end of the world. All UL refrigeration compressors have thermal regulator switches, typically a Klixon device backed up by a fusible link, in case the first fails, to disconnect the power and prevent permanent damage.

Starting, inrush, current is typically about three times the run current for most residential refrigeration units, a lot depends on what time scale we are talking about, but high efficiency motors pull a bit more before saturating and, in theory, 10X is not impossible.

Most electronic step-wave inverters can double their output for the short starting phase, essentially drawing extra power off of internal capacitors for a fraction of a second, and most refrigeration units can be fitted with a 'hard-start' kit, essentially a large capacitor, to prevent starting current issues.

You have to watch output capacities of the inverter but all I have seen have internal fuses to protect themselves so even if overloaded they don't blow up or let out the magic smoke. Keep spare fuses on hand. Practice replacing them a few times, they don't blow often but you don't want to have to learn during an emergency. Same is true of all equipment.

Many of the better manufacturers fit the start circuit of their compressors with one or more over-sized capacitors as a matter of course. This both serves to protect the unit, limit service calls, and eliminates annoyances with lights flickering when the refrigerator starts in a home with marginal wiring.

As far as efficiency goes for modern inverters even inexpensive modified sine-wave inverters only lose 10% to 15% of the power in the conversion from DC to AC.

A lot of inverter information dates back to the 60s when square-wave inverters were common, hard to find one now, and efficiencies were quite low. I have seen quotes of as little as 50% efficiency. Modern inverters are far lighter, more compact, cheaper, efficient, self-protecting, easier on the devices plugged into them.

Another bit of misinformation is that computers will only run on true sine-wave inverters. Definitely false. Most computers have switching power supplies that easily convert a rough modified sine-wave into smooth, conditioned and well regulated DC power. I have run my PCs on a cheap inverter just about every time the lights go out for a few years now.

I will add one proviso: It helps that the PC power supply is over-sized for the load the PC places on it. If the power supply was operating at close to its design limit its capacity to deal with a modified sine-wave would be compromised. Large, high-quality PC power supplies save me a lot of grief.

I'm considering building a PC around a power supply optimized for running off 12v DC to take advantage of better efficiencies. Possibly being able to switch between two power supplies for the best of both worlds. Currently, on line power, the 120v 60cycle gets converted into 12v, 5v and 3v rectified DC by the power supply. the conversion from 12v DC power, from batteries or vehicle, to the filters and conditioned array of power the PC likes is far less drastic and more efficient.

Small modified sine-wave inverters can do a lot of things the people think they can't. Even the sub-$80 700w units from the discount store will run your average refrigerator/freezer when hooked up to the battery in a car as long as the engine is running. I use one of two of those discount store units to run power tools on jobs where we just have to run them for a few minutes and have run refrigerators on them several times.

The larger 2400w modified sine-wave units have a lot more going for them but even those units are mere fraction of the cost of a small true sine wave unit.

I've had enough issues with running from inverters that I'm working on avoiding them as well. Even a small motor such as my electric razor didn't like the non perfect sine wave.

This is kit list I eventually ended up getting for a Solar Powered PC

Motherboard - POV Ion Atom 330 (replaced the dual core Atom 330 heatsink & fan with a Thermalright HR-05 Northbridge Cooler and Noctua 80mm Fan NF-R8 which is virtually inaudible).
RAM - Adata Vitesta Extreme Edition DDR2 1000+ DDR2 2GB.
PSU - 300W Silverstone ST30NF, Fanless, Nightjar, 20 & 24 Pin, ATX, 0Db.
HDD - Samsung F2 EcoGreen HD502HI: Silent 500GB 3.5" HDD HD502HI (still waiting for SSD prices to come down to justify absolutely silent operation, although the Samsung is pretty darn good - http://www.silentpcreview.com/article951-page4.html).
BD ROM - Liteon HOS104.
Audio Amplifier PCI Sonic Impact TIO board T class Amplifier 30W RMS.
Case - Silverstone LC10B Black Aluminum HTPC Desktop Case w/o PSU.

Power consumption works out around 35-45W, the computer is virtually silent (required for Audiophile purposes) and the PSU can be switched out with something like a picoPSU-120 12-25V DC-DC ATX PSU. i.e. the HTPC case has lots of room to drill the DC power cable connector for direct SLA battery operation (when I eventually get around to it) or to mount an appropriate sized SLA battery if need be.

I have a very old mini-itx board and a 60W power supply that looks like the PW-200V from http://www.mini-box.com/DC-DC
I used it in the early 2000's for an in car mp3 computer but have it setup a simple NAS device now. Plan now is to build something like an online UPS using a battery and solar charge controller with the low voltage dropout setup to switch to line power rather than disconnecting the load. Then run my server, wireless, etc all from 12v.