How much solar power do I need to keep water from freezing?

The problem:

Chicken water in the winter freezes overnight with temperatures down to -25. How to melt water with solar power so I don’t need to plug in a big energy expensive animal water bowl which  – though thermostatically controlled somewhat – will run 24 hours a day if it thinks its not keeping the water up to temperature.

The rant and the rhetoric:

It is true that in the grand scheme of things, a heated livestock water bowl only takes 70 to 100 watts of power I believe, however its $50 plus tax. The question is, how do i use renewable energy AND build something for less money than buying a heated water bowl – and how do I get two heated water bowls even possible for less than half the cost of the COOP livestock waterers for chickens?

The solution:

  • 1 DC timer – 16 amp throughput, 8 selectable settings with battery backup – Purchased off ebay for $7 (free shipping)
  • 1 12 volt soup / drink heater – Purchased off ebay for $4 (free shipping)
  • 1 already existing solar setup with 60 watts of solar panels at least – purchased from various places, this is my ongoing solar project and experimentation station.

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The math:

A Joule (measurement of energy) = 1 watt of energy for 1 second.

Water requires 334 joules of energy per gram of ice (don’t worry, i didn’t know any of this before writing this either, its all found here: http://www.st-andrews.ac.uk/~dib2/climate/energy.html)

I’ve got a 120 watt immersion water heater. How many joules does that put out?

120watts x 60 seconds x 60 minutes is the number of joules per hour it puts out or 432,000 joules.

If I wanted to melt 1 Kg of ice (I’m thinking an ice-cream tub or something full, nothing massive), melting that would required 334,000 joules of energy (334 joules x 1000 grams).

SO… Running the drink heater for 1 hour produces 432,000 joules of energy – and making the horrible assumption that there are no losses – I could melt an ice-cream container frozen solid at least once a day.

I have a 100 AmpHour battery, running that for 1 hour would suck 10 Ah’s out of it – nothing big. Running two would suck 20 Ah’s if I were t defrost 2 bowls. Again, not that big of a deal, I wouldn’t want to take out more than 20 Ah’s for the sake of battery longevity. As well, calculated in is that I only have 60 watts worth of panels hooked up right now. At maximum capacity, they would need to collect energy for 2 straight hours throughout the day to replace 120 watt-hours – and that never happens. So lets assume 50% performance to make up for some clouds, lower daylight hours etc, I would need 4 hours of sunlight.

My realistic number would really be that 120 watt-hour number where I use one immersion heater for 1 hour a day, and have all that replaced as long as the sun is out during the day.

This is my goal and theoretically at least it works. I’ve not worked in my losses like radiant heat loss etc. The ideal would be to melt that container of water once a day and due to its warmth, stay unfrozen until lunch maybe or end of a short winters day if I’m lucky.

I may need more solar on it to make up for losses and the reality of storms blocking the sun and snow piling on.

Think I’ll have any success? 🙂

For less than $15 I should be able to keep  water in a small bowl in its liquid state for part of the day.

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5 thoughts on “How much solar power do I need to keep water from freezing?”

  1. Did you do any tests in your area to find out your solar production? A 120W heater is actually a sizable load for just one battery. It can definitely handle it but you may not get the AH projections you were thinking due to Peukert’s law with respect to lead acid batteries. Make sure to account for wiring losses. Admirable project. Keep it up!!

  2. Hi, thanks for the comment! Not sure on how it compares to elsewhere, there is plenty of solar, thats not a huge issue. True on that, drawing more out faster means less Amphours to draw from. That being said, i had 110 Ah battery on it which still should have been fine, its the same issue i’m having, the batteries just can’t handle very many cycles, i’m finding they can’t stand very much, 2 to 3 years of regular use and they are losing capacity (lead acid anyways). Lithium Ion is a bit better but same issue really, you get more life but you also pay double. in the end its cheaper for the life of the product but still – you’d think battery technology would be much further along at this point.

  3. thanks for the comment. I didn’t no – I know the general amperage coming in per hour from the panel on a decent day and had a general idea of how much amp hours I would get on a regular day. 120Watts is a lot – and batteries indeed have less capacity if you take the same energy out at a faster rate there. All part of learning 🙂 I killed my lead acid and AGM actually…. but they are good still for a couple of smaller 120 volt LEDs right now for lighting.

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