Inverter Sizing

[Capebuild]

I'm (still) working on my electrical setup and am getting to the point where I'll most likely be posting many questions...(the more I research, the more questions come up, and not being all that elex intelligent is not helping).

I had originally planned for a 1000 watt inverter. I'm realizing that probably is way larger than I'll need. I'd use it (at it's peak) for charging 2 laptops, 2 iPhones (which can probably also just use the USB chargers, but looking at worst case scenario). I had thought maybe a toaster oven but I think that might push me back to something in the 1000 watt range. So just for the 2 laptops really, which will draw about 200 watts if plugged in at the same time.

Any suggestions for what size inverter might be best?

Thanks

John

[Tom&Shelly]

Can you charge your laptops from 12 VDC? (Maybe with the cigarette lighter style plug) That's more efficient than using the inverter to go from 12 volts to 120 VAC, and then back again (which is what a wall-wart laptop charger does).

Then you wouldn't need an inverter at all.

Tom

[tony.latham]

I can't help you with the inverter.

But Tom is right about trying to find a 12-volt cord to charge your computers. Using an inverter, to power another inverter (the box in the computer's cord) to power a DC computer will waste a lot of juice. (And I might say, a bit goofy. No offense meant.)

I've got a new Macbook that uses a USB-C cord for charging, so finding a 36-watt adapter for a 12-volt outlet was easy.

These stove-mounted Coghlan's toasters work well. $5.





Tony

[jgizzi]

I don't know what battery string you are running but the idea of a toaster running from an inverter is something that needs to have been planned for. Resistant heat asks a lot from a battery string. 1000 W toaster with 15% inverter waste is asking the battery for nearly 100A at 12V. You would need a robust battery string to support that.

The idea of going directly to the 12V source for charging the laptops and phones is a good one. Most all of these device can run on the 12V and then you don't need to pay the inverter penalty, and simplify things as well.

[Capebuild]

Thanks all for the responses. We can do without the toaster, I know that would be an energy hog. And I think I'll reconsider charging the laptops from the 12V DC. So maybe I can do away with having an inverter. That would save cost and wiring.

thank you!

[saltydawg]

I over built my electric system and bought a 700 watt inverter, for some strange reason i got sent a 1000 watt.

The extra stand by loss from a 700 to 1000 watt is not much, but honestly there is not many items that would be fine on a 1000 but would not work on a 700. The jump from going to 500 to 700 does cover a lot more items.

I vote that 700 would do fine for most people when they need to.

[Capebuild]

I put together an "estimated" list of devices I plan on installing, including their amperage draw and watts. (image attached). I've got a couple of questions:

1). I realize this amp total considers everything is powered on at the same time, which is worst case scenario..... but do these totals look like I'm interpreting all this correctly?

2). I'm planning on using 2/100 watt solar panels with MPPT charger (model not known yet, probably a Victron of some variety) and a Battleborn 100Ah lithium battery. Does the 200 watts solar look okay to charge the battery sufficiently to power these devices?

Any other thoughts would be a help. I'm not really sure how to interpret the 23 amps total draw... I'm just adding up the info provided on the retailer's/manufacturer's websites.

Thank you for the help.
John

EDIT: I'm not relying 100% on the solar for power. Just wanted to get your thoughts on this setup. Also planning on batt to batt charging off TV battery with Sterling charger or other. Also will have shore power hookup with Genius 10 amp charger.

[Tom&Shelly]

I put together an "estimated" list of devices I plan on installing, including their amperage draw and watts. (image attached). I've got a couple of questions:

1). I realize this amp total considers everything is powered on at the same time, which is worst case scenario..... but do these totals look like I'm interpreting all this correctly?

2). I'm planning on using 2/100 watt solar panels with MPPT charger (model not known yet, probably a Victron of some variety) and a Battleborn 100Ah lithium battery. Does the 200 watts solar look okay to charge the battery sufficiently to power these devices?

Any other thoughts would be a help. I'm not really sure how to interpret the 23 amps total draw... I'm just adding up the info provided on the retailer's/manufacturer's websites.

Thank you for the help.
John

EDIT: I'm not relying 100% on the solar for power. Just wanted to get your thoughts on this setup. Also planning on batt to batt charging off TV battery with Sterling charger or other. Also will have shore power hookup with Genius 10 amp charger.

Hi John,

Off the top of my head, the numbers look reasonable. Assuming everything is on at once is a good worst case scenario. Of course, it may not happen or even be possible, and if necessary, you can play games with that.

So, how to interpret 23 amps (or other amps draw). Folks always seem to mis-state amps and amp-hours on this forum. Amps is the current draw when the appliance is on, and when you multiply by the voltage (12 volts in this case) you have, as you did correctly on your spread sheet, the wattage. That's power. It is essentially what you are consuming at an instant in time. When you multiply that by a time unit, such as hours, you have energy: Watt-hours. Note that is what the power company bills you for, for your home. (Usually expressed on the bill in kilo-watt-hours.)

If you divide watt-hours by the voltage, you have units of Amp-hours, which can also be thought of as energy (at an implied 12 volts). Of course, you don't need to go to watts. If you assume all of your appliances are on for one hour, you've drawn 23.7 amp-hours from your battery. Notice that's how deep cycle batteries are rated, in amp-hours.

So, if you have a usable 50 amp-hours (as I do with my 100 amp-hour AGM battery, which should only be drawn down half way) you would have a little over two hours of energy, with everything on at once. Sounds a bit grim, so now you may want to consider how long each appliance will be on, and add that to your spreadsheet.

I'm eyeing the 12 amps draw to recharge the laptops. How often do they need recharging and how long for them to charge? I doubt they'll draw that much once their batteries are charged. And so forth...

Hope that's helpful.

Tom

[saltydawg]

I did this math before not sure if here or some where else, and it was an apple air book iirc.

The batteries in laptops are about 4400 milli amp hours at 12 volts are so ( 11.7 is nominal iirc ). Most laptops will run for about 5 or 6 hours on battery, so 4.4 amp divided by 6 ( best case ) hours is about .75 amps an hour. Figure most laptop chargers will charge the battery in an hour while being used, so figure 4.4 amps, plus the .75 amps. That get us to around 5.5 amps if being charged from a dead battery and being used but only the first hour.

So while 6 amps is a good number with a little room to spare, thats the absolute worse case load. Once charged the laptop should be using 1 amp or less per hour each.

[John61CT]

A DC boost converter capable of 200W

set to the voltage required by the laptop input

hopefully both in the same range, often ~19-21V

Will not cost much, and not as inefficient as going DC to AC and back to DC.

Buying one "car adapter" specifically compatible for each laptop likely not much more

and then no worries about the plug diameters and polarity issues.

[Capebuild]

Thanks for all the follow up. All this electrical set up is a real learning experience. I'm enjoying it..... just want to make sure I get it all figured correctly.
I'll post my final (or semi-final) plan when I've got it all figured out... and hopefully get your feed back. This is really out of my comfort zone... but learning a lot.

Thanks again.

John

[bdosborn]

You need a column in your spreadsheet that shows how many hours you plan to use each device a day. Here's a link to some spreadsheets to estimate your usage and get a handle on how big a battery you need.
https://newcontent.westmarine.com/content/documents/pdfs/WestAdvisor/Elecbugt.pdf

I estimated my daily usage and tripled it to size my battery, based on the fact that most of my trips were Friday through Sunday.

Bruce

[Capebuild]

Thanks for posting that worksheet, Bruce. I can see adding (or estimating) the hours the appliances will be used makes for more accuracy in planning the overall load and for battery sizing.

I have one other question, maybe I should make this a new topic...but my question is regarding monitoring both the solar system and its activity and also monitoring the battery state.

I've been looking at Victron's products and with 2/100 watt solar panels, I'm thinking the MPPT 100-30 will be my charger. I'm looking at the MPPT Control Monitor (the round one) to monitor the solar activity, running it through a shunt from the Battleborn 100Ah lithium battery.

I also want to monitor the state of the battery. So I was looking at Victron's BMV 712 Smart Battery Monitor. This also needs to run through the shunt...or a shunt......

My question is, is there a way of monitoring both the solar activity and the state of battery through a single monitor.... or what would be the best way to set up the two monitors I mentioned? I "think" having 2 shunt's, running each monitor through a separate shunt would be problematic and not the correct way to go about this. Looking for suggestions ... or more like... direction.

Thanks!

John

[bdosborn]

The monitor for the MPPT isn't required, the controller will connect to an app on your phone via Bluetooth. The controller also doesn't need a shunt, only the battery monitor needs one. While I have the 712 battery monitor, I rarely look at the meter itself. The 712 also connects to your phone via Bluetooth and the app is a lot more intuitive than the meter for me. You do have to toggle between battery monitor and MPPT controller in the app but I don't find it annoying. There are ways to show both MPPT and battery monitor readouts concurrently on a single screen but you'll need a raspberry pi or a display adapter like the Victron GX or a Cerbo . I use the phone app when I'm camping and it tells me everything I want to know about my battery SOC and the PV output. Make sure you get an MPPT controller that has the Bluetooth logo on it if you want to connect to your phone. There are demo modes in the phone app if you want to take a free test drive.

Bruce

P.S. I have a raspberry pi running the Victron Venus OS in my trailer that connects to WiFi when I'm at home so I can sit on the couch and watch my battery voltages.

[Capebuild]

Thanks a lot Bruce, I appreciate your follow up comments. I realize I can connect via bluetooth to get the readings on the MPPT controller and the BMV712, but I didn't necessarily want to rely on my phone to check the readings...so having the monitors would be more desirable. I just was just not really not sure if I needed both monitors to 1) get the solar activity reading and 2) get the battery state of charge. I now realize I do need both monitors. I did notice the Color GX component would provide a way to get both readings in one place.... but a bit out of my $$comfort zone, unit being fairly expensive.

Thanks again for your follow up.

I'm going to post my elex plan, but do it in a new post. If you have the time, I'd appreciate knowing your thoughts. thanks.

John