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Question DetailsAsked on 1/19/2017

If I use air conditioning for 1500 sq.ft. 50 hours /week for 8 weeks each year, is 16 SEER smarter than 13 SEER

The longevity factor: this question is for 15 years and this would be 400 hours per year of usage. Total: 6,000 hours

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Answered 1 year ago by Member Services

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You don't give enough info to say what size (tonnage) A/C or specific brand but you did give SEER (Seasonal Energy Efficiency Rating or Ratio), but without electric rate all I can do is give a rough guesstimate - and show you how ot figure it with your actual numbers for your utility rates and the actual units proposed. But you are talking a very small number of weeks of usage per year, and few hours per day - a minimal A/C demand area, so I would guess up front that higher efficiency is not worth the added cost you are likely to be charged.


Note I said the amount you are likely to be charged for the higher efficiency unit - the actual unit cost difference for those two SEER ratings is likely to be only a couple of hundred $ - but what they charge you might be as much as $500-1000 or more additional for the higher efficiency rating, because higher SEER ratings are a fad thing - people commonly think they are getting a lot more benefit than they are (except in high cooling demand country), so the higher efficiency HVAC units are priced up by distributors and installers way more than the actual incremental cost - sometimes as much as 5-10 fold over the actuall incremental cost to them.


You are talking 6000 hours of usage - and I am assuming this is A/C operating hours you are talking (since you said 50hrs/week), not total hours when the A/C is "active" - ready to cool when the thermostat demands it, which is typically about 3 times actual operating hours.


So - on to the calculation. For a normal A/C with 13SEER rating, that means the number of kilowatts of electricity used wpuld be 1/13 of the number of thousand BTU's of cooling produced. With a 1500SF house, running A/C only 8 weeks per year, you are talking a minimal A/C environment so probably about a 2 ton unit (though maybe smaller depending on outside temp and humidity) - which is 24,000 BTU's of cooling per running hour, or 24,000/13 = 1846 Watts (1.846 Killowatts) of electricity (roughly) per hour of use. Multiply that by 6000 hours of lifetime usage and you get 11,076 KWHr (kilowatt hours).


Now - run same numbers with 15 SEER and you get 1600 Watts per running hour, or 9,600 KWHr of electricity usage over 15 years.


Now - the cost of electricity, which you should be able to get off your electric bill - include all per-kilowatt costs including any regulatory fees or taxes or such which are based on the amount of power used, but NOT including your fixed basic monthly service fee or any taxes/government fees/etc which are a flat rate per month. DO include any cost-of power-surcharges - they are added to the base electricity rate per KWHr. If in doubt, call utility about what charges are fixed and which are proportional to your electric usage. And if you have split usage billing (higher rate for higher increments of electricity usage) use the highest rate you normally pay at. If you have peak demand billing rates, I would use the mid-range or average of the rates probably, unless you run A/C only at wakeup time and for a few hours right after you come home, in which case use the peak period rate.


At the nationwide rate of 12¢/KWHR, then, at 13SEER you would be paying about $1329 in electricity over 15 years to run the A/C - a pittance compared to normal A/C operating cost, which is commonly that much in a couple or few months in many parts of the country. At 16 SEER, that would be about $1152 - so only about $177 estimated energy consumption difference - likely far, far less than the difference you will see in bids.


Plus, you need to figure, even in this low interest environment, some value on the additional up-front money you would be spending on a higher SEER unit - if you assume say 5% time value of money (minimum you should get in a broad-based index fund, for instance), the $177 savings over 15 years would be beneficial only if your added up-front capital cost for thge higher efficiency unit was about $122 or less - or $151 at 2% assumed interest which you could have gotten on the additional money invested in the higher SEER unit. Not very much at all, compared to high A/C usage areas where that number is commonly in the many hundreds of dollars or more.


You did not say if this is a replacement unit, or brand new - if brand new, considering the low usage factor, you might consider one or two window or portable units (one in living area, maybe one in bedroom if nights are hot and sticky during the A/C season) - much lower initial installed cost, though granted more like probably 5-10 year life rather than 15 or so - but a possibility if a seldom used item and looking at substantial installed cost for a conventional unit. Ditto to a "swamp cooler" - a portable evaporative cooler, if cooling need is low and intermittent and heat, not humidity, is the issue. Work very well in desert type country, useless in high deep south or eat of mississippi type humid summers because they work by evaporating water in a fan unit to cool the air passing through them 10-20 degrees typically.


Not to throw another wrinkle into the picture, but if your area is generally mild and never or rarely gets down to freezing range and never to say below about 15 or so, it might be a heat pump (which looks like and is installed like an A/C) might be of interest - costs more than an A/C but provides both cooling like an A/C, and effective heating down to about 35-42 outside air temp typically, pulling the energy for heating from the outside air with high efficiencies - commonly around 2-5 times as efficient as direct electric heat or gas heat. Of course, more likely to be of interest if in a mild heating demand area (generally not effective if you have true winters because they have to use direct electric heating coils at expensive electricity rates, or supplemental direct gas heating). Also, if your existing presumably forced air heating unit is coming up to replacement age, a combined heat pump system might be of economic interest - less likely to be so if your existing heating unit is not very old.


One other very important factor in deciding whether to go with an upgraded or base efficiency unit - is how long you realistically expect to be in the house, because generally you do not recover a measureably greater amount of the capital cost of a higher efficiency unit at resale time, so if moving every 3-5 years like the average american, you may never recover much of the energy savings from a more efficient unit - except sometimes where it contributes to a 4 or 5 star energy rating for the home.


One other thought, though for a 1500SF house might or might not concentrate the cooling too much in just a couple of places - if this is a new A/C install, AND you do not have existing forced air furnace and ducting to carry the cold air around, you can commonly sanve quite a bit of money by avoiding running ducting from the A/C and using a so-called "mini-split" A/C unit - where the outdoor unit has pressurized coolant tubing running to from 2-4 wall-mounted A/C fan and radiator units that then produce the cold air at those points - less distributed than normal ducted system, but in many cases (especially houses without exposed basement or crawlspace to run the ducting) can be cheaper by quite a bit and a lot less to near zero disruption of interior finishes to install them. Talk to your vendors or google for the difference in ducted and mini-split A/C units, and for the difference between A/C and air-sourced heat pumps.


More discussions about types of units and efficiency and payback determination, some thoughts on brands, and rough ballpark costs can be found in the many previous answered questions about HVAC systems in the Home > HVAC link under Browse Projects, at lower left.

Answered 1 year ago by LCD




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