Estimating add’l kWh for mini split heat to size solar system
AndrisSkulte
| Posted in General Questions on
We’ve sized and added four 1:1 Mitsubishi hyper heat mini split ASHP’s which should cover all of our heating load in the winter. Using an average of 700 gallons of heating oil in past seasons (for heat and hot water), I’d estimate 600 gallons for heat.
Is a COP of 3.0 close enough to estimate the add’l kWh of electricity to run the mini splits instead of the oil boiler?
600 gallons * 139600 BTU/gallon / 3.0 COP / 3412 BTU/kWh = 8,183 kWh add’l(yikes!)
8183 kWh / 12 months = avg 682 kWh add’l per month (130% of our current average 525 kWh/mo)
Am I on track?
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Replies
Also put in a factor (< 1.0) for the efficiency of the oil furnace.
Jon - Thanks! Forgot about that one... Guestimating 85% AFUE for the non-condensing boiler that's clean.
600 gallons * .85 AFUE * 139600 BTU/gallon / 3.0 COP / 3412 BTU/kWh = 6955 kWh add’l (slightly less yikes!)
Averages out to 580 kWh/mo when divided by 12 (for Connecticut's net metering arrangement which resets yearly).
I have solar in CT and didnt think you were able to size your system any higher than your total last year kwh usage? Have you spoken with any solar companies? You might not be able to install that much extra capacity.
I think you CAN build extra capacity if you show them the math on your conversion to heat pumps, but it's definitely something to discuss with the solar company ahead of time.
Many (or most?) local solar installers have rules of thumb about how many addition panels it takes to cover so many tons of heat pump, but like all rules of thumb there are many exceptions to the rule. That's why doing the adjusted math on the actual fuel use (and not the total tons of ductless) would be important to avoid overshooting by a large amount, unless you're also planning to buy a plug-in electric car.
What Jon R said.
To get a handle on derating for efficiency...
If you're only burning 600-700 gallons/year in a oil boiler it's probably getting on to a 3x+ oversize factor, and with such a low duty cycle the standby losses are just killing it on as-used AFUE efficiency. Even if the burner is still tune-able to 85% combustion efficiency, the as-used efficiency is likely to be in the low-to mid 70s unless the boiler jacket is well insulated AND is operated with a heat-purging boiler controller.
See Table 3: https://www.bnl.gov/isd/documents/41399.pdf
(Its clearly not 85% as-used AFUE!)
Also, on the more termperate warm edge of zone 5 such as the CT coast you're likely average closer to COP 3.5 (an as-used HSPF 12) with right-sized equipment, so using a COP 3.0 for estimation purposes would be conservative.