Inverter size

squarebob

Long time lurker, first time poster. I need some help on sizing an Inverter. I have a 450 AH 24V battery bank that is for emergency backup. It would get recharged from a small diesel engine that is turning an alternator with a Balmar 3 stage controller, or from the grid with a 3 stage charger, so recharging is not a concern at this time. I have a trimetric 2025 on the battery bank so I know when to recharge the bank ( 50% DOD ). My plan would be to charge from 50% to 80% a max of ten times, then do a full charge to 100% for the best battery life. Some solar will be added in the future to keep the batteries topped off during the on grid times so I don't need to use the charger or the genset. I want to keep my freezer and fridge and a couple CFL or LED lights and a laptop going during a power outage. Please check my math......... The freezer has a full load amp rating of 5.0 Amps. The fridge has a full load amp rating of 6.5 Amps. So.... if the lights and laptop are on and pulling lets say 250 watts, then the fridge and freezer both want to turn on at the same time, (worst case), I need 11.5amps * 120v = 1380 watts start up draw. Total load during start up would be 250+1380 = 1630 watts. Will a 2000 watt pure sine wave inverter be sufficient for this load. Am I missing anything here? Thanks for your time.

Bob

RCinFLA

Re: Inverter size

A 2000 watt sinewave inverter will likely start your fridge and freezer.

Beyond that you need to find kWH's consumed over period of time to determine how much battery capacity will be used.

A typical refrig draws between 1 kWH and 1.5 kWH per day. A freezer, a little less. Your amperage numbers are way off. Startup on the compressors are typically 1 kW or close to 10 amp @ 120vac surge, or higher. Your inverter must handle this startup surge. Run current is likely about 1 to 1.5 amps @ 120vac. There is also defrost cycle which can be between several hundred to 1000 watts for the defrost heater. The defrost duty cycle is low so it does not impact the overall kWH too much but the inverter must supply the peak power.

Just on those two items assume they draw 2 kWH per day. That would be about 20% of your 450 AH @ 24 vdc battery per 24 hour period.

You can buy a Kill-A-Watt meter for under $25 to get a better handle on your average consumption for 120 vac devices.

Cariboocoot

Re: Inverter size

The problem with refrigeration equipment is that it is nearly impossible to get accurate start-up figures. You need a fairly expensive meter to get the exact number. What's more, the draw will vary depending on how hard the compressor has to work.

On the whole though the 2kW pure sine inverter will probably handle it. If not, it will fault; you shut stuff off, turn it back on, and start things up one at a time.

This could be a problem if you have to leave the system unattended. So could the fact that each of those refrigeration units could consume 2kW hours per day: your battery bank would just cover that (about 5.4 kW hours +/-).

waynefromnscanada

Re: Inverter size

My first impression - - WOW! That's altogether a huge load for your battery size. Second thought - - you state that for example, your freezer draws 5 amps. Is that 5 amps @ 24 volts, or 5 amps 120 VAC? If it's @ 120 VAC that will be about 25 amps from your 24 volt battery.
Finally, the idea of only bringing your battery up to full charge every 10 cycles, and the rest of the time leave them between 50% and 80%? How long will your batteries go between full charges? That is, how many days between full charges? As it's only for emergency backup. it could be quite a while. Also, only bringing them up to 80% reduces what's available to run your appliances.
Is the generator the only way you have of charging?

squarebob

Re: Inverter size

The amp ratings are full load amps at 120v off the appliance stickers. Not the running amps. I have a kill a watt meter and the units draw about 110 and 145 watts while running. I realize the battery bank is smallish and don't mind running the genset every day or twice a day for a few hours, as needed to recharge the bank. This is for emergency backup only and I am trying to get away from running the genset 24/7 with a small load during a power outage. That would be a waste of fuel to not keep the engine loaded.

Bob

Cariboocoot

Re: Inverter size

A word of caution about "appliance stickers": they're not so much accurate power measurements as they are a work of science fiction. :roll:

I recently tested an "Energy Star rated" refrigerator that should, according to its rating, have used 2 kW hours per day. It consumed 50% more than that.

If you have the units you want to run, spend about $30 on a Kill-A-Watt and get the real numbers. It won't catch the start-up surge peak, but it will tell you how many Watt hours per day the thing actually uses. A very valuable tool for off or on grid, with or without solar.

2manytoyz

Re: Inverter size

I'm using a Xantrex 1800W Prosine inverter to run two fridges during outages. I have performed a few 24 hour off-the-grid tests to determine how well the system performs. The inverter is rated at 2900 watt surge capability. The sizing of the battery bank, and wiring, is just as crucial as the inverter. In my case, I'm using a 900AH battery bank.

Either of my fridges draw about 150W during a cooling cycle. During a defrost, they both run about 600W. The energy star rated one is a power hog, despite the rating. 2.1KWH vs 1.1KWH.

Before I put in a 900W solar array, my goal was to be able to run the critical loads from the battery bank/inverter during a storm (hurricane prone FL), and replenish the energy via a Yamaha EF2400iS generator + 75A Iota charger, during the day.

But if the weather is cooporating, the solar array just about makes enough power per day to keep up with these loads. Even if the weather is hit/miss, some solar will help off-set the use of the generator, greatly conserving fuel.

FWIW, our last major outage here was in 2004. Two hurricanes in a month knocked out power for 18 days. Wish I had my setup back then!

One neat thing about my setup is I put the inverter in the standby mode, and have single circuit automatic transfer switches (homemade) on the critical loads. When the grid goes down, relays drop out, and the loads transfer to the inverter. When the inverter detects a 20W or greater load, it powers up and runs the appliances. When the grid returns, the relay re-engage, taking the loads off the inverter, and the inverter goes back into standby.

mike95490

Re: Inverter size

2manytoyz wrote: »

...

One neat thing about my setup is I put the inverter in the standby mode, and have single circuit automatic transfer switches (homemade) on the critical loads. When the grid goes down, relays drop out, and the loads transfer to the inverter. When the inverter detects a 20W or greater load, it powers up and runs the appliances. When the grid returns, the relay re-engage, taking the loads off the inverter, and the inverter goes back into standby.

Does that happen fast enough, that the inverter will keep a compressor running, or will the motor stall and choke the inverter ?

2manytoyz

Re: Inverter size

mike90045 wrote: »

Does that happen fast enough, that the inverter will keep a compressor running, or will the motor stall and choke the inverter ?

There is a delay. While in the standby mode, the inverter consumes only 1.5W. It produces no power, but puts out a pulse every 2 seconds. If it detects a 20W+ load, it will power up and start inverting. So there will up a 2 second, plus 'x' miliseconds for the relay to drop out, for the load to be transferred. But when I've pulled the power manually, the transfer is typically a second or less. I must have been lucky and time the pulse pretty close. A pic of the pulse capture on an o-scope:



Both of my fridges have a 1/4 HP motor. Pretty small. With fat wire, 8 golf cart batteries, and a decent inverter, there's no issue of a stalled compressor overloading the inverter in the past few years of having this system operational.

If I did have a large motor I was concerned about, I'd add a time delay circuit, allowing several minutes before the appliance is supplied power from the inverter.

My sensitive electronics are connected to UPS units, so they don't blink during the transition from the grid, to the inverter, and back again.

I don't mind flipping switches, moving cables, blah blah. My wife, not so much. Her one request is that all my house mods be automatic. If I'm not at home, she doesn't want to be bothered with a checklist of things to do. Not a problem. Ultimately, easier for me too. Power goes off, things continue working. No need to drag out extension cords and hunt for a flashlight.

Plucka

I would only use an inverter that is sufficient to power your needs -oversizing consumes and wastes power but if one could have a set up that would automatically switch from a smaller inverter to turn on a larger one as needed that could be interesting.Example if my fridge and freezer are running at the same time and I turn on the kettle the inverter trips out and I have to turn something off which is a pain especially at night.