Solar Dump Load

Will your battery pack and inverter have enough power to manage the inductive kick of your motors starting for the refrigerators and freezer?

Attention!

Attention!

Meaningless units alarm: <...250 watts of electricity per hour...>!

Arroooooooooooooooooooooooooooooooooogah!

Huh? I understood it perfectly.

250 watts per hour would be 250 watt hours of electrical power consumed per hour.

Next you will be telling me that my 5 horse power pump that ran at full capacity for an hour didn't do 5 horsepower hours worth of work because I was not grammatically perfect?

BTW, how are those OCD meds working? Are you taking the right milligrams per dose or are you just taking them per day?

Yeah, I am confused about the "dump load", too.

All you need is the solar array, voltage controller/charger, and the battery array.

Then you can use voltage inverters to supply AC power from the battery array.

Probably should have made it clear that this is a back up supply for the frequent brown outs we have here. The cost of installing a full solar system would be prohibitive.

Usually the unplanned brown outs last for less than 2 hrs, planned ones for utility maintainance work are usually 4 to 8 hrs & I realise my batteries will have died well before that point.

Many thanks to all for the time you gave taken to assist me working this out! My load is 250 watts (max) the solar panels I'm planning to install are either 300 watts or 400 watts

Now that's been clarified, the batteries already have sufficient capacity.

A dump load is not needed.

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The Watts are not apple to apples.

A 100 Watt light bulb consumes 100 Watts of power at 120 Volts.

Your best case power out with a 400 Watt solar panel is 400 Watts at 18 VDC.

Now you need to convert that 18 VDC to 120 VAC, which even at 100% efficiency means that panel is only producing 60 Watts of power at 120 VAC. Actual power will be only 50 to 55 Watts after losses are considered.

So, just to power that single 100 Watt bulb requires two 400 Watt panels running full tilt. Note, that panels can not produce 100% power all the time because the Sun's angle through the atmosphere, clouds, shade, and angle of incidence vary throughout the day.

What?

my 100 watt solar panel @ 18VDC charges my 12V battery for 1 hour.

My 85% efficient inverter (12VDC to 120VAC) means I can run my 85Watt @ 120VAC light for 1 hour, of course barring any losses in cables (minuscule).

You are right. I realized that about an hour (after I woke up :) )after I posted it. Breakfast of Champions - eating your own words. :)

Keith, welcome to CR4!

Scrap the whole entire PV thing, get some magnesium, and a steam engine, add water, a voila, you have green energy (no CO2) on demand. Also, buy some large fresnel lenses, and focus the sunlight down really tight onto a Cr+3, Nd+3 doped YAG crystal and you have a solar-pumped laser to convert the magnesium oxide back to magnesium. Hey if they can do this at University of Tokyo, so can you.

To answer the actual question.
The dump load needs to be rated for the same voltage as the supply. E.G a mains rated kettle isprob no good as a load for 12v DC. The element will be designed for 240 V and may be too high a resistance (even when cold) for 12v...
On the other hand it may work... but without water it may just blow.
Del

The original poster won't have enough power from the panel as it is. The need for a dump load as it stands at the moment is abstruse.

Solar dump load...?

How to Calculate Kilowatt Hours: 5 Steps (with Pictures) - wikiHow

Thanks for the replies so far

I've been using the battery bank & the inverter for a week now. After installation I tested it for 2 hours, with all the appliances turned on. Nothing failed & after 2hrs the battery bank voltage had dropped from 12.6v to 11.7v. Another test a few days later with a 'plug-in' electicity meter showed that in the space of one hour I had used 250 watts, hence my statement that I consume 250 watts of electricity per hour.

I thought that if I could charge the battery bank with the solar panels at the same time as discharging them with the appliances they would happily remain at a good voltage. (You've all probably guessed that I'm not an electrician, My rational went something like -' The battery bank is like a water butt full of water. If I turn the tap on It empties at a rate of 1 gallon a minute. If at the same time I pour water into the but at the rate of 1 gallon a minute, then the butt will remain full.)

In the unlikely event that the batteries become fully charged during the day my understanding is that the Solar Charge Controller will send the power from the panels to a dump load, which prompted my question re using a resistive 240v device to accept the 12v 'overflow' (The water butt ir rearing its ugly head again!)

So I really need to know, is my water butt analogy flawed?

kebang

<...in the space of one hour I had used 250 watts...> ??? <...250 watts of electricity per hour...>

Please read the instruction leaflet for the plug-in electricity meter again. Then read the read-out again, and please stay away from meaningless units:

• Power is energy per unit time.
• Energy is power multiplied by time.
• Power per unit time is meaningless in this application.

Please advise further: the corrected result is important as it will help determine the correct strategy, tactics and recommendations, and has financial implications that may be important locally.

If the load is lower than the maximum power available from the solar panel, then additional batteries may not be necessary. However, what has been posted so far does not state this.

I don't think you need a solar recharge rate at the same rate of usage.

The reason is that you are not trying to remain off-grid, but to create a UPS (Uninterruptible Power Supply) for the duration of the brown/black outs.

What I would recommend is to first do a power budget, which includes the total number of maximum Watts used at any one point in time and the average number of Watts used.

Next, you need to determine the maximum amount of off-grid time you need to operate at your average load.

With that information you can build a battery backup system (basically extend the system you have) to meet those requirements.

Notice that the word "solar" never appeared in my suggestion. The reason is I think your off-grid power needs are measured in hours, not days. Solar is expensive. Even a 250 Watt panel is really only 250 Watts @ its rated voltage (usually 18 Volts at its peak), which translates to 20 Watts or less at the AC side of the inverter. That's a dim light bulb at best for a single panel that costs $500 or more.

You can see that you quickly need lots of solar panels to achieve parity with an average home consumption of 250 Watts (120 VAC) and can spend many thousands of dollars doing that.

Additionally, solar power is really only available a small segment of the day. Usually, this is given in Peak Hours. In Florida it is rated as 4.5 hours a day. That means I will average about 4.5 hours of full output over the course of sunrise to sunset (if the panel is aimed at the Sun). The power delivered by the panels needs to be in sufficient quantity to offset 24 hours of home use plus power that is lost in the conversion process.

As an example, my home consumes about 2500 kWh per month. I would need about 20 kW in solar panels to achieve off-grid power for my home. That is 10s of thousands of dollars to build a system that can power my house off-grid or supply power equal to the rate of discharge on batteries over a 24-hour period.

Since you do not need extended or full-time off-grid power, the cheaper alternative is to size your battery system to your worst-case needs and just trickle charge those batteries off the grid.

The other alternative is a simple generator, preferably natural gas to extend your current system's reach.

If it was me, I would simply add capacity to your battery bank and you can do that a little at a time until you reach enough capacity to manage your worst case needs.

I would also look into what loads you might be able to shed or do without during a power fail to extend your off-grid time.

OOps! No idea how that smiley got so big! (Now edited back to normal size)

Your load is going to vary according to what happens to be running at the time you are taking your reading....I would calculate the average load at 1kw per hour....Your battery pack@320x12= 3840wh your solar panel array is ok to maintain batteries under minimal load, but you will need a charger running off of grid current as a backup....once the circuit is broken by the charge controller, the panels stop producing electricity, so no dump charge thingy is needed....

Now that my incorrectly phrased question has been interpreted (Sorry about the confusion - especially to Mr. Slack who spent time doing maths which would have probably been unnecessary if my question had been posed correctly) my understanding is as follows:-

1. When installed the 400 watts of pv panels will start to charge the batteries at low rate before 9am, probably less than 150watts (daylight here is 6am - 6pm all year round).

2. Between 9.30am - 3pm the panels will be charging at their peak, so on a sunny day I might expect 275-350watts going into the batteries during this period.

3. After 3pm watts will start to decrease.

My conclusion, (& please correct me if I am wrong), The battery bank will remain near full charge if I use 250watts continiously between the hours of 8am & 4pm. In practise, in the early days after installation, I will take battery readings every 15mins after 2pm to establish the best time to switch the inverter off & revert to mains power.

The logic behind purchasing pv panels as opposed to doubling my battery bank is based on finance.

The 100watt panels cost 9,500 pesos, a 100ah deep cycle battery costs 10,000 pesos. The panel will last over 20yrs, the battery 5yrs.

In an 8hr 'maintainance' brown out, which happens during daylight on Sundays, the batteries will not be seriously drained

I have problems posting a reply. The system doesn't process my answers well.

The given values are "end product" 240- 250 VAC/ 60 Hz, after inverting. Tks

Many thanks for those figures, I find them extremely useful. Looks like I should produce 1.5 to 2 KW of electricity each day, sufficient to keep my batteries fully charged for 6 to 8 hrs.

The prices for your equipment are very good! Here in the Philippines the best price I have found for a 100w panel is equivalent to $230 (US)

Need to shop around a little more.....

...or you could get one of these and forget about the solar...It will keep the batteries charged when you have AC, and automatic switches to DC inverter when the AC goes away.

http://www.theinverterstore.com/3000-watt-inverter-charger-12-volt.html

Well my original post relating to a 'dump load' for pv power was obviously erroneous!

I'm very glad I posted it though as it has generated masses of valuable information & suggestions for the forthcoming installation; information which very probably would have escaped my consideration.

My thanks to all who have contributed their time & effort to assist me in this.

kebang