Calculation Verification

[NYCsolar]

Hi There--
We are building a cabin and will install a solar system. But first we are staying weekends in our Airstream on our property.
My question is, how big of a solar system would I need?

Calculation of Watt Hours needed:

Desc	Avg Watts	Hours/Day	Qty	Total	Notes
CPAP Machine with Humidifier and heated hose	14	24	1	336
LED Lightbulbs (6) In Ceiling	1.25	6	6	45	From 430pm to 230am (worst case)
Clothes Washer 1 cycle (no dryer)					Not in coach
Cellphone Recharge (4000mAh or 4Ah or approx 48Wh)	18	10	2	360	5 per phone x 2
12V DC Fridge (average size, dual-zone)	45	16	1	720	Assumes on 2/3 of the time (16 hours) vs 24h
Internet Router and Modem	12	24	1	288
Laptop single recharge via USB-C (5000mAh @ 14V )				90	K
Laptop single recharge via USB-C (5000mAh @ 14V )				70	T
Microwave	1200	0.25	1	300
LED patio lights (typically 1W per bulb): 50FT string	25	5	1	125
MaxAir Fan (medium speed)	50	8	2	800
Small USB Fan	12	8	2	192
Bathroom Exhast Fan	60	1.5	1	90
Stove Exhaust Fan	15	1	0	0	none
15w LED Ceiling lights	15	12	0	0
Bed reading lamps (qty 2)	1.25	1	2	2.5
RV A/C	1300	12	1	7020	Assumes it's running 45% of the time during 12 hours of operation
Gas Furnace	50	6	1	300	propane powered, but has electric fan
Toaster	1100	0.25	1	275
Kettle (6 cups/day)				0	Propane
Total Watt Hours per Day				11,014	Watt Hours
Amp Hours Needed for one day >				918	(Watt hours divided by voltage 12v)
Number of days you want to live on charged batteries is >	Days of Autonomy	3	2753	Total Battery Amp Hours	needed for 3 days of Autonomy

Calculation of solar panels required:

Solar Panel Requirement	[ This calculation determines how many watts of panels you need to recharge in 1 day ]
Req'd Battery Amp Hrs >	2753	x	Voltage >	12	= 33045.5 watts provided by battery system, or 33 kw
	NE USA: Spr 4.8, Sumr 6.1, Fall 3.2, Wint 1.8 | Peak Sun Hours >	6.1	=	5416	watts that need to be generated per hour
	Include 20% inefficiency/losses factor >	1.2	=	6499	watts need to be generated per hour with inefficiencies
	Solar Panel Size Conclusion >		You need a combination of panels that add up to 6499 watts

Summary:
I need a battery bank capable of providing 2,753 amp hours of power
I need 6,500 watts of solar panels

Is my calculation and process correct? Any comments are welcome!!
Thank you,
Kevin

 

[NYCsolar]

Forgot to mention that there is no utility power on the property, so it will be an off-grid system that we will expand (if) as needed when the cabin is built.

 

[BentleyJ]

First, congratulations on producing a spreadsheet and designing the system around your needs rather than purchasing equipment and then posting questions related to how to make it work.

Based on the list provided, there are at least 3 HIGH demand appliances and potentially more if a coffee maker and/or an electric skillet or hair dryer is desired. All of the appliances don't have to be all at once but the system should be sized for some convenience.

I would strongly recommend you go with 48V inverter of at least 6,000W, 12V is very limiting in that once demand goes above 2,000W battery amp draw becomes an issue. You mentioned expanding the system as needed later. That is another advantage to starting with a stackable, 48V inverter. Its very easy to add another inverter to double capacity.

Would also recommend installing a soft starter on the A/C unit.

 

[NYCsolar]

First, congratulations on producing a spreadsheet and designing the system around your needs rather than purchasing equipment and then posting questions related to how to make it work.

Based on the list provided, there are at least 3 HIGH demand appliances and potentially more if a coffee maker and/or an electric skillet or hair dryer is desired. All of the appliances don't have to be all at once but the system should be sized for some convenience.

I would strongly recommend you go with 48V inverter of at least 6,000W, 12V is very limiting in that once demand goes above 2,000W battery amp draw becomes an issue. You mentioned expanding the system as needed later. That is another advantage to starting with a stackable, 48V inverter. Its very easy to add another inverter to double capacity.

Would also recommend installing a soft starter on the A/C unit.

Thanks for the quick reply. We just boil water for coffee and pour it through a filter, cook with propane, I'm bald and my partner doesn't use a hair dryer, so we're good on those points.

Your recommendation confirms what I've been learning about 48v -- seems like the only way to get simple battery expansion and enough power for AC. Many thanks!

 

[DougfromdaUP]

I second Bentley’s congratulations for planning before you buy, and I also think 48volts is the only way to go.

I encourage you to do some modeling of solar photovoltaic production instead of relying on the too simplistic hours per day stuff. Look at PV Watts, and see what your 6500 watts of panels will generate in November and December. You’ll have to either shed some loads or include a generator in your plan, I believe. But use the model (it corrects for average weather in your area) and see how much power you can expect to generate in the dark, cloudymonths. Where I live, six or seven terrible production days in a row happen every November, sometimes even more.

 

[robbob2112]

I have a resmed aircurve 10 asv - I use it off a battery pack every night. In 10 hours it uses around 250wh with the tank and hose heaters on.

Just real world numbers for you.

 

[NYCsolar]

Thanks @robbob2112 and @DougfromdaUP for the good suggestions. The road to our property closes at the end of November. It's passable usually through January, but February and March it's 3' deep in snow. So we would size the initial system for winter until the cabin is built and then expand to meet needs, so I'm looking into a Sol-Ark inverter or similar with a modular battery system of some kind... Thanks also for the accurate CPAP number!

 

[NYCsolar]

Oops, I meant we would size for fall until our cabin is built