Inexpensive Home Building

Cut through the jargon and nonsense of home building and house construction by starting from zero dollars and trying to figure best-value bang-for-your-buck when choosing construction methods or construction materials. My research might answer some of your questions and at other times perhaps you have the knowledge or experience to post the answers to my questions and thereby help others too. The goal is an affordable and sustainable home for all.

Sunday, February 04, 2007

Candles Beat High-Tech Solar Photovoltaic?

Candles=solid-fuel energy sticks

It’s difficult to beat the price of grid power if you already are connected but new home builders might face the choice of paying to connect versus paying to set up an off-grid system.

The first step is to “daylight” (maximize natural lighting) as described in the last post and here’s why:

The Myth of Photo-Voltaic Solar Power?

Solar power provides about 1,000W (1kW) per square meter at the equator's sea level. However, typical photo-voltaic (PV) solar panels are only 15% efficient so a square meter of solar panels provides only 150W. Further, even compact fluorescent lights (CFL) are only 20% efficient so 150W of CFLs provide only 30W of light. Incandescent light bulbs are only 5% efficient so 150W of incandescent light bulbs provide about 8W of light—less than 1% of the solar power that hit the solar panels.

So, the expensive “solar” system is 97-99% inefficient at providing electric light.

(For a similar reason, PV solar-electric water-heating is less efficient than passive solar water-heating which puts the water tank in direct sunlight.)

Then consider that the sky is not always clear and sunny. Solar panels will generate less in winter and more energy in summer because of the number of daylight hours. However, even a seemingly clear day might have particles that reduce available power to 80%, moderate clouds can reduce available power to 33% and heavy clouds can reduce it to 5% (a 45W panel would provide about 2W).

What do you get at the end of the day? The Southwest USA accumulates the equivalent of about 5 full sun hours per day. However, much of the US population should expect only about 3 hours or less of full sun equivalent per day (at 3 hours, a 45W solar panel would provide 135Wh per day, which might power a medium TV for 1 hour per day) . Northern Michigan should expect only 1 full sun hour per day (a 45W panel would provide 45Wh per day, which might power a 60W light bulb for about a half-hour per day after accounting for system inefficiencies).


For night, let’s say that you need 5 hours per day of artificial lighting and your location and setup give solar panels the equivalent of 5 full-power hours of sunlight per day (likely only in the very favorable Southwest USA).

Which is the best way to spend $350 for lighting?:

  • PV Solar-Panel System: $350 can buy a 45W solar-panel kit from for $200, the required 300-watt inverter for $50, and a good Trojan T-105 battery for $100, to provide 225 watt-hours (Wh) per day, indefinitely (actually, you would need a pair of 6V T-105 batteries for a common 12V system but you could buy cheaper batteries too).
  • Tea Lamp Candles: $350 can buy 4,375 5-hour (5h) 40W "tea lamp" (a.k.a. "tea light") candles at about 8 cents each, which is almost 12 years of 5 hours of light per day. The 4,375 candles total 21,875 hours of light and 875,000Wh (875kWh) of energy. At a consumption of 225Wh/day, the candles would equal 10.65 years of solar panel output at the assumed optimistic rate. The tea lamp candles cost 40 cents per kWh, which is higher than typical grid power's 10 cents per kWh but remember we're comparing to the infrastructure and operating cost of PV solar energy.
  • Food Warmer (Votive) Candles: $350 can buy about 3,100 10h food-warmer (votive) candles at about 11 cents each (cheaper per hour than the tea lamp candles), which is 31,000 hours of light, which is almost 17 years of 5 hours of light per day. We need to know the wattage of votive candles to calculate cost per kWh. If for the moment we assume that the food-warmer/votive candles are the same power as the tealight candles, 40W, then $350 of food-warmer/votive candles would provide 1.24 megawatt-hours (1.24mWh=1,240kWh) of energy at 28 cents per kWh. I hope to get a definite wattage for this calculation.

Update 3/6/07: I found a tealight candle price of 4.375 cents each: $350 would buy 8,000 candles for almost 22 years of light (5h/day) and, at 200Wh each (40W * 5h), provide 1.6 megawatt-hours (1.6mWh=1,600kWh) of energy at 22 cents per kWh.

Cost Effectiveness

The PV $350 is more expensive than the candle $350 because the PV has to be a 100% up front investment before you get the first watt (with an interest/inflation factor in the payback period) while the candles are more pay-as-you-go ($10-$35 at a time). Remember that you might have to replace a solar panel or battery, which would postpone your break-even date.

Backup Logic

Backup power should be independent of what it is replacing, or have an inverse relationship to it. The obvious disadvantage of solar-electric lighting as a backup to natural solar lighting is that you are trying to squeeze more energy from the very thing that is disappearing on you.

Are candles powerful enough for your needs?

Even a 40W candle can provide all necessary ambient light for a room. You can focus candlepower with parabolic reflectors or Fresnel lenses (techniques used in flashlights and automobile headlights). The first lighthouse lantern room, the 1696 Eddystone Light in England, used candles to warn ships at sea.

Energy Storage, Reliability, and Safety

A candle, as a day-or-night, on-hand, on-demand energy supply, does not vanish from your drawer the way the Sun can "vansih" from the sky. A candle, as a storage battery of energy, does not have the discharge (energy leak) issue that an electric battery has. A candle, as a solid fuel, is stable and stores without high-pressure, liquid-spill, or volatile-gas dangers. You must act safely with any system, whether it is open flame or battery acid.

This very preliminary review suggests that candles are a viable alternative to the commonly recommended PV-CFL combination for lighting.

Use a votive candle to warm your coffee and light the room at the same time (replacing a 20W CFL and 20W electric hot plate)?

Please post the best $/kWh candles in Comments. Thank you.


At 2:51 PM, Blogger Jennifer said...

you must be talking about paraffin candles - not soy candles

soy candles will run you a little more $ wise but they burn nearly twice as long!

I like the idea of heating my coffee with a votive. I wonder who we could get to make up a jig for that?...

At 12:14 AM, Blogger J at IHB and HFF said...

Hello. Yes, I meant paraffin. A simple wire stand for the coffee mug would be best.

Do you know the watts and lumens (measurable with a photometer) of different soy candles?

Thank you.

At 2:05 AM, Blogger J said...

Yes, I have heard of soy candles but never have personally tried it.
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