Quote:
Originally Posted by uncle_bob
Let's say you need to run 50A at 120V. That's about 1/2 what a big trailer with everything turned on will run (it runs 50A at 240V). At 12V, that will be 500A. Unless you really like buying new batteries, you will need at least 1000AH on your battery stack. (discharge at C/2 rate). You also will need at least a 6KW inverter. |
I think you are misunderstanding the loads involved here. My 3400sqft house (fixed residence, not AS) uses 5879watts to operate 2 full A/C units at the same time and the entire rest of the stuff in the house. Watts ÷ Volts = amps, so in my case that is 5879/240 = 24.5amps, or 50amps on 120V, or 500amps on
12V...but my home A/C is also a whole lot more BTU than an AS has (5.5 tons, ~66k BTU @ 13SEER/11.18EER).
The way to determine your A/C power needs without actually metering while in use is to look at the BTU rating and efficiency (EER). BTU ÷ EER = watts. I don't have an AS A/C to look at the name plate for the EER rating, but lets assume its in the 9'ish range. So 15,000 BTU ÷ 9 = 1667 watts, or 13.8amps @ 120V. The name plate rating of "30 amps" for a single or "50 amp" for a dual A/C is to handle the in-rush of startup, not to actually sustain operation. As a reference, these 2 A/C units on my house require a combined 80-amp 240V feed. By these calculations my home A/C uses 5600 watts, or 23 amps...nothing close to the 80amp required to "start" them. You absolutely need to plan for that inrush current when you side your inverter and max rate of your batteries, but not for capacity.
The tricky part is actually determining load and run cycle, or what % of the day the A/C will actually be operating in a cooling capacity. In my case I lose some ability to use my house as a reference point for consumption over 24-hours as my EV car that takes 13kwh after my commute skews the numbers more than a bit and my power monitoring is for the main feed to the house, not per device.
On the heating topic, the same numbers work for a heat pump...you couldn't ever dream of heating on resistance heat coils, you must use mechanical efficiency to have a chance. What we need is to get the inverter heat pump technology into our RVs, then we can get 15.8EER. A BTU is a BTU, regardless of how it is generated. You just need to look at the efficiency of the heat pump to make the same calculations. A 15k BTU A/C with 15.8EER would only draw 950watts, or 80amps @
12V.
(Obviously you need to account for power loss in the 12VDC to AC conversion depending on the inverter used)
You are right that going 100% battery is a big task, perhaps someone could just find a way to transport enough solar and a Tesla PowerWall (or competitor), that would be 14kWh of energy. All of the battery options for something like this will get a lot better in the next couple of years. EV cars place massive strain on batteries (very high rate of discharge and charge), and batteries will start to fail to meet the EV needs but would be entirely adequate for residential/RV use cases with much lower rates of discharge.
The secondary market for EV batteries as we get more of them on the road will help shift all of these discussions, my compact EV carries over 18kWh of battery in a fairly compact structure. This is why every major EV manufacturer also has a product to compete with the Tesla Powerwall, including BMW and Nissan. Imagine if AS added a location to store these cells into that pancake frame structure with the fresh/gray/black water tanks.
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