Solar power for air condition unit

[hhn1960]

Hi,
I am new to the solar panels and I am just wandering if it is possible to use the air condition unit(approx. 18000BTU) with the use of solar panel. If it is, can anyone please explain the rough approximation of what amp of battery should be used,how many watts of panels should be fitted and which kind of inverter would be suitable?

Thanks in advance

[Sunking]

You bet you can do it, just need to know what the SEER of the unit is, and your location. But for now let's assume it is a efficient unit of with an SEER of 12, Power factor of .8, you live in Kansas City with a Summer July Sun Hour exposure of 5.3 Sun Hours, and will run the unit 24 hour a day at a 33% duty cycle (8 hours).

OK your AC unit will consume 18,000 BTU / 12 SEER/ .8 PF = 1875 watts. The Unit will run 24 hours per day with a on/off duty cycle of 33%. So the unit will consume 1875 watts x 24 hours x .33333 = 14.85 Kwh in a 24 hour period.

OK we need to make an adjustment factor for a battery system efficiency, and battery charge discharge efficiency, and inverter losses. So we take your daily Watt Hour usage and multiply by 1.5. This number will be used to determine solar panel wattage and battery capacity. So 14.85 Kwh x 1.5 = 22.275 Kwh is what you need to generate each day to run your air conditioner.

OK for the solar panel wattage needed take the above adjusted watt hours and divide by your location Sun Hours. So 22275 wh / 5.3 Sun Hours = 4200 watts.

OK for the battery capacity we need to first select a system voltage. In this case there is only one choice, 48 volts. Next if we want the bateries to last at least 5 years we do not want to discharge them more than 20% each day. This will allow us 5 years service and give us 2-1/2 days of cloudy weather operation. So to find the battery AMP Hour capacity we take the adjusted daily watt hour, multiply it by 5, then divide that number by the voltage. So [5 x 22275 wh] / 48 volts = 2320 AH @ 48 volts. About 7000 pounds of batteries plus a room all their own to claim home.

Next we need a MPPT charge controller. To find the amperage needed we take the solar panel wattage, and divide it by the battery voltage so 4200 watts / 48 volts = 88 amps. We can get by with a 80 amp charge controller.

Last we need a good quality True Sine Wave Inverter. To run a motor it needs to be 3 times the motor size in watts consumed to over come start up currents s 3 x 1875 watts = 5625, so round up to a 6 Kw inverter.

Now the fun and educational part starts; cost estimate to build this system.

Solar panel $3 per watt = $3 x 4200 watts = $12,600
Batteries $130 per Kwh of capacity = $130 x 111 Kwh = $14,300
80 Amp MPPT Charge Controller = $600
True Sine Wave Inverter $1 per watt = $1 x 6000 = $6000
Misc Material = $7500
Permits, Inspections, and Contract Labor = $9000

TOTAL COST EST = $50,000.

I told you it would be fun and educational right?

Ready to get started today? Or you could just pay that mean ole electric company $80 per month for what; 5 months per year to run that AC unit?

[Mike90250]

The best way to offset the electricity used by a household, is installing a solar Grid tie system. There are no batteries to recharge & replace, and about 95% of your harvest is turned into useful electricity. Battery systems can only achieve about 50-55% efficiency.
Of course, it's expensive and has a 10-20 year payback, depending on if you have rebates in your state (if the state is sill solvent).

Every dollar spent in conservation (insulation, seals/gaskets, lighting, energy star appliances) is worth 10 spent on PV.

Solar Thermal hot water preheat for your hot water needs is a good place to start, it has a very short payback (2-5 years) in reduced water heating costs.

Otherwise, to go a independent battery system, you need to know how much power the AC uses, in your house. If you have the AC installed, use a Kill-A-watt meter to measure it for a week, to get your average daily use.
Of course, the higher the efficiency of your unit, the less power it will need.