Electrical formula question

[Mike Davis]

If I have a 2.5 amp 120volt motor that I need to run 8 hours and all I have is a 300 watt 12v inverter,
how many 12v car batteries would I need to power it? And what size? Cold cranking amps or amp hours? Some batteries don’t list both specs.

 

[bowman]

You're pulling 300W (120V * 2.5A) with the motor for 8 hours, you'll need 2400W runtime power.

This example is calling for 1200 Watt-hours:

How long will a 1000W inverter last on a 100Ah battery?

72 minutes

12 v, 100 amp - hour battery has a label rating of 100 amps for one hour. Power in a DC circuit = VA = 12 x 100 x 1 hour = 1200 Watt-hours. Your 1000 watt inverter will use that much power in 1.2 hours or 72 minutes.

You're pulling 25% of that rated load, so you would get 5 hours from a single battery, 10 hours if you connected them in a parallel method.

Hoping @Michael Mathews chimes in to confirm or tell me I am way off.

 

[Craptastic]

Remember that the battery type matters if you plan on trying to keep those batteries healthy for longer life, and that matters by type of battery. Lead acid ones should not be drawn down below 40-50% of capacity before recharge. LiFePO ones tolerate draw down to around 20%. Yes, one is more expensive than the other but if you spend the cash for them you probably want to keep using them as long as possible.

 

[Mike Davis]

Ok, but since I need probably a 750 watt inverter to cover the surge at motor start does it still draw 300 watts or does the inverter draw 750 watts the whole time it runs?

In other words is there some overhead for the inverter?

I was thinking deep cycle since I don’t want to run a vehicle the whole time. Might as well get a small generator.

My thought is a battery and inverter will be quiet, not much heavier than a generator, no fumes, no combustible liquid, portable.

What are the drawbacks besides expense?

 

[Craptastic]

Ok, but since I need probably a 750 watt inverter to cover the surge at motor start does it still draw 300 watts or does the inverter draw 750 watts the whole time it runs?

In other words is there some overhead for the inverter?

I was thinking deep cycle since I don’t want to run a vehicle the whole time. Might as well get a small generator.

My thought is a battery and inverter will be quiet, not much heavier than a generator, no fumes, no combustible liquid, portable.

What are the drawbacks besides expense?

Yes, inverters cause some overhead usage. Usually not much but your mileage may vary. That is variable by the inverter used. Good electronics usually means less overhead, and vice versa.

I would certainly be inclined to investigate a generator that can handle the starting load and that used propane. Would have to be placed in a well ventilated area, and those generators generally fall lower on the power rating scale than their gasoline fired counterparts. But it would probably be less expensive overall and at a lower weight to power ratio in the long haul than the batteries required to be a viable long term solution. No gas cans to haul around or refilling of tanks on a hot genny. And more versatile if portable.

Seeing Neal's below response about using Jackery's line of products reminded me that we use a Ecoflow battery gen for camping. Love it, and also love the Jackery stuff. Essentially the same concept. We have 2 solar panels at 320w that can actively recharge the unit even during use. The Ecoflow does run about 7-9% overhead on consumption (which are not bad numbers). Like Neal says, they do run a bit expensive but you can get some quite hefty units that would do what you need. Basically self contained battery and inverter/converter combinations that have multiple charging methods from AC to solar to DC.

My $0.02

 

[bowman]

Look at the Jackery line of products. Not cheap, but they are quiet.

 

[JNCarr]

I think explanation of the specific use case might be needed to better narrow this down. But...
Your 300 W invertor will indeed run a 2.5A x 120v motor - BUT electrical ratings are typically done at 20C (68F) ambient and need to be derated with temperature. I would probably use at least a 400-450W invertor - again depending on use case.

If you want 8 hrs of run time you would need 300W x 8 / efficiency of invertor.
Assuming an invertor efficiency is 85% you would need 300 x 8 . .85 = 2823 W-hr.
At 12v that's 235 A-hr of capacity (2823/12)
A deep discharge battery is probably what you want.
Check the specs of the battery you intend to use and divide 235 by the recommended discharge depth.
For example if the recommended discharge depth is 40% then you need 235 / .4 = 587 A-hr of storage.
If you want to parallel wire batteries, make sure they are the same type and capacity. Different capacities can potentially cause some inefficiencies reducing effective capacity.

 

[Wilsoncb]

Ultimately, I think you will find a generator to be the least expensive solution. I recently set up a Transit van as a camper and the power system was the most expensive part of it. You don't necessarily need solar charging, but there will be a significant expense in wiring components (fuses, heavy wire, connectors, bus bars) in addition to the inverter and batteries.

Check out this site. It has a ton of information including comparison of different battery types, component costs and event generator information.

Electrical System Guide for DIY Van Conversion - FarOutRide

Complete guide to design and install your own DIY electrical system in your camper van conversion. Wiring diagram and tutorial inside!

faroutride.com

 

[tvrgeek]

A small generator might be a better option.

 

[Echd]

A small inverter generator is the proper and no drama way to do this.

Those "solar generators" and giant battery packs, while they continuously get better and better and cheaper, are still very deficient for a use like you have in mind compared to even an inexpensive inverter. Predator from harbor freight makes a quite good one for not a lot of money. A lot less than a bunch of 12 V batteries and the pain of dealing with them!

I'd personally spend a bit more and sacrifice a bit of fuel efficiency to get something more generally useful around the house and shop, but this is very inexpensive, will run over 6 hours at 50% load (close enough to what you're planning) and needs only a half gallon of gas. It's not a whole lot bigger than a 12v battery and weighs about 20 lbs.

 

[JimD]

Instead of a generator that burns propane or gasoline, you could also consider a "generator" powered by a large battery pack. I have a couple large Milwaukee units that deliver 1800W for a few hours. I have one hooked to a transfer switch to cover the circuit the refrigerator is plugged into in a power outage. I have used it and it works. A smaller unit would power a sander for hours and could then be recharged. My Milwaukee unit seems to handle a startup surge well too. Probably wouldn't need one much over 300W. My Milwaukee units could definitely do this but they aren't cheap and I think you could find a less expensive way. I have an inverter that is powered by 40V Ryobi lawn equipment batters, for instance. If you happen to have some of those batteries the inverter was only about $100. I use it to power the electronics during a power outage. It will run a 65 inch TV + modem and wifi router. It doesn't last real long with that sort of load (an hour or so) but I have several batteries and power outages are typically short here.

 

[Wilsoncb]

If you really want to do it right, just get an F-150 hybrid, or full electric Lightning.

I copied the following from this Motor Trend article. https://www.motortrend.com/features/2022-ford-f-150-lightning-home-power/

In a stroke of genius, Ford added an option called Pro Power Onboard to the 2021 F-150. A built-in inverter puts out 2.0, 2.4, or 7.2 kW through one 240-volt outlet and either two or four 120-volt outlets based on spec. The inverter is powered by either running the engine or, on hybrid models, drawing off the hybrid battery. The brilliance of this feature was on full display in the winter of 2021 when terrible ice storms knocked out power to much of Texas and F-150 owners ran extension cords from their trucks to vital HVAC systems and appliances.

With the F-150 Lightning EV, Ford's gone a step further. Not only does the Lightning include the 2.4-kW system and eight outlets, but an optional 9.6-kW system ups the count to 11 120-volt outlets and a 240-volt outlet. With between 98 kWh and 131 kWh of power stored in a fully charged battery pack, it's like having seven to nine Tesla Powerwall home batteries sitting in your garage. In a second stroke of genius, Ford came up with a way to turn that massive truck battery into a home battery.

 

[Craptastic]

If you really want to do it right, just get an F-150 hybrid, or full electric Lightning.

I copied the following from this Motor Trend article. https://www.motortrend.com/features/2022-ford-f-150-lightning-home-power/

In a stroke of genius, Ford added an option called Pro Power Onboard to the 2021 F-150. A built-in inverter puts out 2.0, 2.4, or 7.2 kW through one 240-volt outlet and either two or four 120-volt outlets based on spec. The inverter is powered by either running the engine or, on hybrid models, drawing off the hybrid battery. The brilliance of this feature was on full display in the winter of 2021 when terrible ice storms knocked out power to much of Texas and F-150 owners ran extension cords from their trucks to vital HVAC systems and appliances.

With the F-150 Lightning EV, Ford's gone a step further. Not only does the Lightning include the 2.4-kW system and eight outlets, but an optional 9.6-kW system ups the count to 11 120-volt outlets and a 240-volt outlet. With between 98 kWh and 131 kWh of power stored in a fully charged battery pack, it's like having seven to nine Tesla Powerwall home batteries sitting in your garage. In a second stroke of genius, Ford came up with a way to turn that massive truck battery into a home battery.

I'd go with the hybrid. Words I never want to utter.

"Hi honey. Yeah. Done working for the day. I'll be home in about 5 hours after I get enough of a charge on the truck batteries that can get me there."

 

[Mike Davis]

If you really want to do it right, just get an F-150 hybrid, or full electric Lightning.

I copied the following from this Motor Trend article. https://www.motortrend.com/features/2022-ford-f-150-lightning-home-power/

In a stroke of genius, Ford added an option called Pro Power Onboard to the 2021 F-150. A built-in inverter puts out 2.0, 2.4, or 7.2 kW through one 240-volt outlet and either two or four 120-volt outlets based on spec. The inverter is powered by either running the engine or, on hybrid models, drawing off the hybrid battery. The brilliance of this feature was on full display in the winter of 2021 when terrible ice storms knocked out power to much of Texas and F-150 owners ran extension cords from their trucks to vital HVAC systems and appliances.

With the F-150 Lightning EV, Ford's gone a step further. Not only does the Lightning include the 2.4-kW system and eight outlets, but an optional 9.6-kW system ups the count to 11 120-volt outlets and a 240-volt outlet. With between 98 kWh and 131 kWh of power stored in a fully charged battery pack, it's like having seven to nine Tesla Powerwall home batteries sitting in your garage. In a second stroke of genius, Ford came up with a way to turn that massive truck battery into a home battery.

I wonder if it burns as hot as a Tesla?

 

[Gofor]

Probably as hot, but not as long!

If you do decide to get a small genset, I can recommend the Westinghouse 2500w inverter. Although it is only gas, you will get about 8-12 hours run time on a tankful. Weighs about 40 lbs and is easy to start. Has a fuel gauge as well as low oil shut-off. One thing I lake about the Westinghouse is that they have a drain on the carburetor, so you can shut off the fuel and not have to run it dry. I can run the Fortress 2 gal air compressor off of mine. (Note: I also have a 2100w inverter, and it will run the compressor up from empty, but will overload when the compressor kicks back on with pressure in the tank.. The 2100 lacks some of the niceties like fuel level, etc.)

Westinghouse has given me great support. On my larger duel fuel, I messed up the fuel selector switch. They had me send a picture of the damaged piece, and then sent me a new one, free of charge. They appear to also be a company that supports you doing your own maintenance. Instead of "take to an authorized repair facility" or limiting user maintenance, they include the procedures for setting the valve lash in the owners' manual! They also supported me doing my own repair of the selector valve.

If buying, check Sam's club on line as well as Amazon. I got both of my small ones when on sale: one from each.

edited to change kw to w .

 

[ethanferguson]

Mike,

Seasoned mechanical engineer and navy trained electrician weighing in.

I have a setup for like this to power my shop when I can’t run an extension cord due to weather. I have 4 deep cycle marine batteries (80 amp hours each) to power my inverter. It works well, but cost about $130 each for the batteries, plus the cost of the inverter my 1000W was $230, but you could get away with 500 Watt inverter assuming it can handle the motor starting current.

I agree with JNCarr’s calculations above regarding capacity, which for lead acid batteries lead me to think you would need 6x100Ah batteries at $150 each or half the number of lithium batteries at 4x the price as of my most recent searches show.

If you need to do this once, it will be much cheaper to buy a gas generator. If you need to do it daily, it might be worth it and I’m happy to help out with more details, having built multiple rigs like this before

 

[Mike Davis]

I looked at the Westinghouse 1500, it seems to be the sweet spot $349, 1000w run, CO auto shutoff, 33 lbs, 52db, full sine wave, low oil shutdown, 3 year warranay,

Thanks for all the great info and opinions. Really helps.