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Topic: What draws too much power for solar (Read 193 times) previous topic - next topic
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What draws too much power for solar
Is there anything besides air conditioning and the microwave that draws too much battery power for solar to keep up?  Secondary to that, how much solar is enough to be comfortable boondocking (assuming decent weather and maybe using an electric blanket and heater)?

Thanks.

Mimi
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Re: What draws too much power for solar
Reply #1 –
Mimi, that is a pretty complex question.  If you max out your solar, using every available square inch of roof space, and have a battery bank to match, that yields one answer.  If you have one or two panels from the factory, that is a very different answer.  I don't know of anyone who can use solar to power an electric heater.  Here is the math (I hope I get this right).  My electric heater draws 1500 watts of power.  Assuming that it runs for a total of 4 hours overnight, a cool but not cold night, that 120 volt heater is drawing 12.5 amps at 120 volts, or 125 amps at 12 volts.  To power JUST the heater for those 4 hours, you would need 500 amp-hours of battery capacity if you were to drain the batteries.  To avoid damage from dropping the batteries below 50%, you would need 1000 amp-hours of battery capacity, or something on the order of 10 batteries.  Usually there is a 1 to 1 match of solar panels to batteries, so you would need 10 panels and a LOT of sun.  My mid bath can only fit six panels.  So, you would not be able to run a heater like mine using solar.

I have 5 panels and 6 batteries.  I can run my DVR for 6 hours, my CPAP machine for 12 hours, my TV for 4 hours, my coffee grinder for 1 minute,  and my computer for 2 or 3 hours, plus a few hours of LED lights, which will draw my batteries down to about 80% by morning.  On a normal full sun summer day, my panels will recharge the batteries from 80% to 100% by late afternoon.

To determine your solar needs, you need to determine the energy requirement of everything you might use - every device, every light, every recharger, etc.  Estimate how many hours you will have each device plugged in.  Then, a little math will allow you to determine how many watt-hours you plan to use.  Take the watts of each device multiplied by the hours for each to get the watt-hours per device.  Add up all the watt-hours to find the total demand watt-hours.  Divide the total watt-hours by 12 to determine your demand amp-hour needs.  To avoid damaging your batteries, you should avoid using more than 50% of your battery capacity.  To take good care of your batteries, make that more like 30%.  Look up the capacity of the batteries you are considering, and multiply the battery capacity by 0.3 to get your design capacity per battery.  Divide your number of demand amp-hours needed by the design capacity per battery to get the approximate number of batteries you will need. 

If you don't want to do all that, you still need to make a list of all the devices, how much energy they draw and how many hours you expect to use them.  Take the list to a quality Solar Panel provider-installer.  I used AM Solar in Springfield, OR.  They will crunch the numbers to tell you how many batteries and panels you will need.

Like I said, it is complex.  Forget about powering your microwave, your AC, a radiant heater, a hot plate, most percolators or other heat-producing appliances.  They probably draw too much. 

Remember that the farther north you are or the farther from late June you are, the less efficient solar panels will be, and you get little energy before mid-morning and after mid-afternoon.  If you are under the trees (nice in summer) your solar panels will produce little power.  Same if it is overcast.  Bottom line, solar is great, if you are parsimonious in your energy demands and it is sunny summer weather.  Otherwise, solar will probably not meet your needs.

Ken F in NM


'08 MB
Re: What draws too much power for solar
Reply #2 –
Quote from: Opuntia  - 
(assuming decent weather and maybe using an electric blanket and heater)?

Don't plan on using an electric heater. On the lower temp setting it will draw at least 600W, which thru an inverter translates to about 55 Amps from 12V. This will draw fully charged batteries down to 50% in a couple of hours. An electric blanket will be too large a drain if used more than just to pre-warm your bed also. Note too that your 350W panel is rated at about 72 degrees at the equator at mid-day, and these conditions are not found in the real world. In the winter, it will deliver about half its rating, using an MPPT controller, at our latitudes.

The key is to find ways to maximize resource efficiency - LED lighting, propane for heating and cooking, etc.

Steve
2004 FL
2013 Honda Fit
Re: What draws too much power for solar
Reply #4 –
This link to the "Education" pageon the AM Solar website offers information on how to calculate your power needs/wants and which components are necessary to make it happen.

RV Solar Education

Joan

2003 TK has a new home
Re: What draws too much power for solar
Reply #5 –
All of the foregoing posts (when you take the time to wade through them) are spot on!

But you need to keep in mind one hard fact... solar panels do not 'Power' anything, nada, no way. Their sole reason for existence is to charge the batteries! Nothing else.

Once those batteries are charged then you can think about how you will be using the potential power that they provide.
Steve S.
Lazy Bones & Cedar
2004 30'IB (Island Bed)
Yesterday is History, Tomorrow is a Mystery
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Re: What draws too much power for solar
Reply #6 –
To increase battery use, consider lithium batteries.  A good chunk of RVers have done this and have knowledge about it. You get much more energy from your batteries and they are lighter, but they are pricier.    The link Joan provided talks about battery types.
Jane & Scott
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