Help with home battery backup system

[bcfromfl]

Thank you so much, svetz! OK, whoops! Looks like I've inadvertently switched terminologies. I guess it's correct to say I need 6kW per day...not 6kWh? Three 5.12kWh EG4 batteries would cover us for two days?

Gosh, this is embarrassing. I'm glad I posted this on the Beginner's forum!

 

[Mattb4]

Thank you so much, svetz! OK, whoops! Looks like I've inadvertently switched terminologies. I guess it's correct to say I need 6kW per day...not 6kWh? Three 5.12kWh EG4 batteries would cover us for two days?

Gosh, this is embarrassing. I'm glad I posted this on the Beginner's forum!

People get these confused.So don't think you are alone. Loads have ratings for watts or amps. Watts= Volts X Amps

If your load is 10 amps at 120v it means you need 1200 watts to serve it. Now time comes into play to get power used. Say you run that 1200 watt item for 10 minutes during a day. 1200 watts at 10 mins would be 1/6th of an hour. So you divide 1200 by 6 to get 200 watt-hrs of power used for the day.

You are sold power by the electrical utility in kilowatt-hours. A kwh is 1000 watt-hrs.

 

[bcfromfl]

Now I'm wondering if I have to go back to all my original calculations. I basically just took the wattage ratings for various things, estimated how long they would run (or took estimates off the web), and added them up. Like, a microwave might be rated at 1,000 watts, but if it only runs for two minutes, that's about 35 watts. Or, if we watch TV for four hours, 4 x 200 = 800...that sort of thing. Isn't this correct?

 

[Mattb4]

Now I'm wondering if I have to go back to all my original calculations. I basically just took the wattage ratings for various things, estimated how long they would run (or took estimates off the web), and added them up. Like, a microwave might be rated at 1,000 watts, but if it only runs for two minutes, that's about 35 watts. Or, if we watch TV for four hours, 4 x 200 = 800...that sort of thing. Isn't this correct?

That gives you a Ballpark figure. A better way to test is to get a meter like a Kill-A -Watt ( https://www.amazon.com/P3-P4400-Electricity-Usage-Monitor/dp/B00009MDBU) and plug it in between the item and the house receptacle. For bigger loads you can use clamp on ammeters to get amperage and then do the math.

BTW your microwave would use ~35wh not 35 watts.

 

[Mattb4]

Sorry if about what I am about to write if it is basic knowledge to you.

Batteries are chemical devices that can act loosely as electrical storage. They are a finite amount of power. They will not generate power beyond what they are rated to store. They can be rated in amp-hr (ah) or by watt-hr (wh). Amp hour ratings are based on delivering current for a period of 20 hours (though some have a 10 hour rating). Thus a 100ah battery at 12 volts is rated to give a 5 amp load at 12 volts for 20 hours time before the battery is dead. It can also give a much higher amount of current faster. Say you draw at 20 amps. 20 amps will last for less than 5 hours (due to battery chemistry faster discharge means less over all capacity; a 100 amp hour battery likely will only deliver 1050wh at a 1 hour discharge rate.).

Batteries rated in watt-hrs are simply taking the ah rating and multiplying volts. W=VA

So you want to power a load with a battery. Take your 1000 watt microwave. You run it 2 minutes for a usage of ~35wh. In order to do so you need an inverter that converts the battery power to AC voltage. That inverter must be of sufficient output watts to cover the start current and the running current for the microwave. It also uses power in the conversion process of going from battery dc voltage to AC voltage. In this case likely that would mean a 2000 watt inverter to cover starting surge as well as running amps.

What your battery sees is the demand for current of perhaps 1200 watts (100 amps if it is 12vDC) lasting 2 minutes. Your battery is depleted by 100ah/100a 2min = 3.3ah

Hope this helps in your calculations.

 

[svetz]

Now I'm wondering if I have to go back to all my original calculations. I basically just took the wattage ratings for various things, estimated how long they would run (or took estimates off the web), and added them up. Like, a microwave might be rated at 1,000 watts, but if it only runs for two minutes, that's about 35 watts. Or, if we watch TV for four hours, 4 x 200 = 800...that sort of thing. Isn't this correct?

You need both the total power and the total energy.

For your microwave example (1000W for 2 min), the energy is 2 min / 60 (min/hour) x 1000 watts = 34 wh of energy. The energy number is important for battery and solar panel calculations.

But the power needed is still 1000 Watts. The inverter needs to output enough power for everything that is on simultaneously (plus any inrush current). So, you'd need at least a 1000 watt inverter to run the microwave even though you might only consume 34 wh (don't forget to add in the losses mentioned in #8 if you do redo the calculations).

Of course, things are rarely that simple and you'd have multiple devices turning themselves on and off like a refrigerator does and anything with a motor (e.g., refrigerator, pump) is going to have an in-rush current that the inverter must satisfy too. Say the pump draws 25 amps @ 240V during inrush, that's 25x240 = 6 kW and 5 amps running (5x240=1200 W). If you want to turn the 1K microwave on and have the pump startup, you need an inverter that could provide a continuous current of 1000W + 1200W = 2200W and a maximum of 1000 + 6000 = 7000 W.

Hope that helps!

 

[svetz]

Just an update, earlier you asked about a soft-starter for the well pump, these guys say it's beneficial. As I mentioned earlier, I have a Microair easy-start on my AC and it's been working great, reducing the starting amps to about 1/3rd the original value. Although, there's a lot to be said with the other idea of 100 gallon water tank in the attic so you can avoid it altogether.

 

[bcfromfl]

Thank you very much, Mattb4 and svetz, for your additional help to get my calculations closer. I'm trying my best to wrap my head around all this, and what's described is the reason I added about 25% overhead "slop" to account for stuff I just don't understand! I've watched Will's videos for a couple of years, and I just marvel at how well he gets everything, and it's just second nature! Just like you guys! Like I mentioned earlier, I think I understand enough to ask the right questions, but there's just so much to all this, I can get lost.

I think the calculators I've seen online just use the simplified method of calculation, but not the more precise method you describe.

Thanks, too, svetz, for the link about the soft-starter. I watched a couple videos yesterday about someone installing one on an a/c unit, and the reduction in starting amps was astronomical! I can't remember the numbers precisely, but it was a 4-ton unit that went from 130 starting amps to 28, or something like that. If I go with the Sol-Ark unit, I'm not sure it would be necessary for my well pump, but at least it's another "trick in the bag!" Apparently the Sol-Ark is beefy enough to be installed in front of the main, but I would have it put between the main and the subpanel, and just do without the 240v appliances.

The only thing yet I haven't been able to resolve (or understand) to my satisfaction about the Sol-Ark, are the criticisms about the 12k unit. I don't know if this is also an issue with the 15k unit. But some were saying that it's possible for the phases to get unbalanced, and the unit shuts down in protection mode. I may be mistaken, but I think one video mentioned it had to do with the "transformerless design." This is a concern, and perhaps a reason to get a soft-starter for the well pump even though the Sol-Ark can handle the amperage.

 

[Mattb4]

...

I think the calculators I've seen online just use the simplified method of calculation, but not the more precise method you describe.

....

Many of the calculators found online are based on trying to sell a customer a product. They gloss over real world needs and results. One of the big issues in my mind is ratings that are misleading. It could take a book to explain what ratings mean.

Suffice to say your 25% overhead is not unreasonable and may not be enough. But at some point you have to shoot the engineers and go ahead with production.

 

[bcfromfl]

Haha...thanks for the laugh Mattb4! It's easy to forget to stay positive when you're struggling to understand something!

 

[wattmatters]

If you're consuming 6 kWh of energy per day, a Sol-Ark 15kW sounds like serious overkill

I agree it's an overkill unit. However he has a deep well pump for water needs, which is the main concern. It's a power issue rather than an energy one. It would help if the OP got themselves a good clamp meter and measured the inrush current demand for their pump.

I wondered whether it would be possible to get through an outage / emergency scenario without the well pump (e.g. keeping an emergency water supply on hand) but it depends on the duration of outage one is planning for (in this case a storm induced emergency). Short outages (<1 day) is no big deal but if water is required for days/week+ for sanitation, then more than some drinking water stores are going to be required.

 

[bcfromfl]

I agree it's an overkill unit. However he has a deep well pump for water needs, which is the main concern. It's a power issue rather than an energy one. It would help if the OP got themselves a good clamp meter and measured the inrush current demand for their pump.

I wondered whether it would be possible to get through an outage / emergency scenario without the well pump (e.g. keeping an emergency water supply on hand) but it depends on the duration of outage one is planning for (in this case a storm induced emergency). Short outages (<1 day) is no big deal but if water is required for days/week+ for sanitation, then more than some drinking water stores are going to be required.

Actually, it's not really a drinking water problem, or even a shower problem. It's our septic. Yes, we can store some water in buckets to flush the toilets, which we have done in the past, but the problem is compounded by the fact that our septic has a lift station. The lift station pump is small, but without power, our septic will start backing up.

Only flushing toilets infrequently in a relatively small home can make "roughing it" even more unpleasant!

 

[wattmatters]

It's our septic.

So it's a second pump, but yes it's still a sanitation issue and that's really important!

Generally with a septic you can get away with a short period without pumping but as you say, not for long! I'm with you, if you can manage to build a system to keep the water online, then that's ideal. Knowing the AC in-rush current demand of the motor would be helpful (as it would for the well pump).

Our septics are both gravity fed. One plumber suggested we might change our main home to use the newer system which was installed for the second home on our block but it was going to require pumping and I just didn't want to be reliant on that. Gravity never fails. So I kept the old one as is.

 

[bcfromfl]

They screwed up when they built our home, and didn't grade the property properly. The first owners completely ruined the septic and drain field because it was originally set up to be gravity-feed, which didn't work. The builder warranted the septic, completely tore out the old one, and replaced with a brand new system and drain field with a pump station, to get around the gravity problem.

During the hurricane, I didn't know at what level the float was in the pump station. I had been told that the amount of wastewater moved by the lift station was somewhere between 50-75 gallons per cycle. What happened is we were pretty close to the limit of the float, so when we finally did get power (via a generator) a few days later, the alarm went off, and it ran for perhaps ten minutes or so before shutting off.

I'm still trying to get a response from the well company. It takes so long to get simple questions answered, and that holds up the whole investigation process...

 

[labeeman]

Thanks, svetz. Yes, I actually figured in quite a bit of "slop," to account for efficiency losses, and, for things like, if we're out of power during the winter and freezing, we could run a space heater for a couple of hours. Our must-haves are thus: side-by-side refrig/freezer, chest freezer, a few lights sparingly, maybe a fan, a small lift-station pump for our septic, and the well pump. Enough to keep us in the house and protect our food. Probably on the order of 3-4kWh per day, 1.5kW per hour if we add a heater.

Panels on the roof are a no-go. We paid extra for a 50-year transferable warranty for the roof replacement, which would be voided by panels. If I decide to add panels in the future, they would be stored in the garage until needed. My wife wouldn't be strong enough to horse them around, though...

I hate to bust your bubble on roof warranties but they are worthless as most companies will be out of business not even halfway through the warranty period.

 

[Danke]

I hate to bust your bubble on roof warranties but they are worthless as most companies will be out of business not even halfway through the warranty period.

My dad found that out. Flat roof started to leak less than four years after install. Company out of business.

 

[bcfromfl]

I hate to bust your bubble on roof warranties but they are worthless as most companies will be out of business not even halfway through the warranty period.

I understand what you're saying, but we used the leading shingle manufacturer (can't remember which one at the moment) for their manufacturer's warranty, and as part of the warranty, also paid extra for the Peel 'n Seal underlayment...I think that was required for the warranty. The shingles are the 50-year architecturals. I hear what you're saying, but I certainly don't want a bunch of holes drilled through the shingles and underlayment, even if we didn't have a warranty. We had leaks everywhere during the storm, and I don't want to gamble with that.

 

[labeeman]

I understand what you're saying, but we used the leading shingle manufacturer (can't remember which one at the moment) for their manufacturer's warranty, and as part of the warranty, also paid extra for the Peel 'n Seal underlayment...I think that was required for the warranty. The shingles are the 50-year architecturals. I hear what you're saying, but I certainly don't want a bunch of holes drilled through the shingles and underlayment, even if we didn't have a warranty. We had leaks everywhere during the storm, and I don't want to gamble with that.

I am with you on that I do not want a bunch of holes in my roof either that is why I put mine on a ground mount warranty or no warranty.

 

[400bird]

I just did the math for the first time and I am curious how well you're going to do emergency mode?

15kWh is roughly 300 watts per hour. That's lower than many homes base load.

kWh is a unit of storage, like the size of the fuel tank on your car.
kw is an instantaneous power unit, like the horse power rating on the engine to said car.

In a car you can't really convert between the two. But here, it's just a simple math problem, multiply or divide by time (in hours)

 

[bcfromfl]

I just did the math for the first time and I am curious how well you're going to do emergency mode?

15kWh is roughly 300 watts per hour. That's lower than many homes base load.

kWh is a unit of storage, like the size of the fuel tank on your car.
kw is an instantaneous power unit, like the horse power rating on the engine to said car.

In a car you can't really convert between the two. But here, it's just a simple math problem, multiply or divide by time (in hours)

Yeah, that was my mistake earlier. Other participants in this thread corrected me, and what I meant to say was 15kW. I took all our "must-haves," estimated the amount of time each would have to run, added them together, then added another 25% on top for overhead to last two days.