Solar backup

Insolated · September 2015

Hello! I've read these forums the past few months, trying to learn. I'm not as far along in my research as I'd like, but am considering getting in on the host's Labor Day sale -- so I apologize for the many beginner's mistakes I'm about to make. Thank you in advance for any help you're willing to provide.

My grid power has become less reliable lately, and I'm interested in solar backup for a small subset of my usual loads. I'm not interested in a generator (I know, first thing you'd recommend).

For solar insolation, consider me as being in Columbia, SC: per PVWatts, 3.32 (December) - 6.26 (June) kWh/m^2/day.
Ideally the batteries should be maintenance-free, so that means AGM. (I actually favor LiFePO4, but I know this forum considers it unproven. I'll look into it separately; let's talk lead acid here to keep things simple.)
Since this is backup, I'm fine using it only during daylight hours. I know I need batteries, but is it possible to reduce their size (for cost savings) with that approach?
I already have a 12 V inverter I could re-purpose for this, though if a higher voltage battery bank is recommended I don't mind purchasing e.g. a 24 V inverter instead.
I have one heavy load I'd like to support that dwarfs any others: 1 kW continuous (no surge). For sizing, assume it would be in use each day as long as the backup would permit.

More details on loads: The 1 kW load is actually electric vehicle charging (120 V 8 A). Yes, I'm crazy. I know I wouldn't get many miles each day, but some is better than none. When not charging the vehicle, I might use the solar for (data gathered from Kill-a-Watt testing, except the last two):

Small electric stove: 767 W average, 1.5 kW surge; 230 Wh per use (1-2 uses per day)
Not very important, but possible: (plasma) TV (including e.g. DVD player): 400 W average, 510 W surge
Fridge: 60 W average (125 W surge?)
Laptop charging: 45 W
Cell phone charging: 5 W

What are your thoughts and suggestions? What all do I need to purchase? Whatever my mistakes above, at least I haven't fallen victim to the most famous classic blunder. Thanks again for your help!

lkruper · September 2015

Insolated wrote: »

Hello! I've read these forums the past few months, trying to learn. I'm not as far along in my research as I'd like, but am considering getting in on the host's Labor Day sale -- so I apologize for the many beginner's mistakes I'm about to make. Thank you in advance for any help you're willing to provide.

My grid power has become less reliable lately, and I'm interested in solar backup for a small subset of my usual loads. I'm not interested in a generator (I know, first thing you'd recommend).
For solar insolation, consider me as being in Columbia, SC: per PVWatts, 3.32 (December) - 6.26 (June) kWh/m^2/day.

Ideally the batteries should be maintenance-free, so that means AGM. (I actually favor LiFePO4, but I know this forum considers it unproven. I'll look into it separately; let's talk lead acid here to keep things simple.)

Since this is backup, I'm fine using it only during daylight hours. I know I need batteries, but is it possible to reduce their size (for cost savings) with that approach?

I already have a 12 V inverter I could re-purpose for this, though if a higher voltage battery bank is recommended I don't mind purchasing e.g. a 24 V inverter instead.

I have one heavy load I'd like to support that dwarfs any others: 1 kW continuous (no surge). For sizing, assume it would be in use each day as long as the backup would permit.

More details on loads: The 1 kW load is actually electric vehicle charging (120 V 8 A). Yes, I'm crazy. I know I wouldn't get many miles each day, but some is better than none. When not charging the vehicle, I might use the solar for (data gathered from Kill-a-Watt testing, except the last two):
Small electric stove: 767 W average, 1.5 kW surge; 230 Wh per use (1-2 uses per day)

Not very important, but possible: (plasma) TV (including e.g. DVD player): 400 W average, 510 W surge

Fridge: 60 W average (125 W surge?)

Laptop charging: 45 W

Cell phone charging: 5 W

What are your thoughts and suggestions? What all do I need to purchase? Whatever my mistakes above, at least I haven't fallen victim to the most famous classic blunder. Thanks again for your help!

Based on your energy requirements, and in large part due to the 24X7 need to charge at 1kW, I calculate a need for 1672AH @ 48 volts. This is for 50% DOD, 40% for inefficiencies from inverter, controller, charging losses, and wiring, etc. If you decide you want more autonomy, (ie days when it is cloudy) you will need to factor that in.

I would recommend Rolls Surrette 2v batteries at 1766 AH each. You will need 24 of them to make a 48v battery bank. This will cost $20,232 and will account for most of your expense. These are flooded lead acid batteries that should last 10 years. I know you asked for low maintenance, but AGMs will cost 2X as much and last 1/2 as long.

You will also need to be able to charge at a rate of 200 amps @ 48 volts with both generator and solar, so you will need MPPT charge controller , a manual charger to use with a generator, and enough solar panels to provide the amps you need.

I am new to solar, so I look forward to seeing what others say and also being corrected, if necessary

Insolated · September 2015

lkruper wrote: »

in large part due to the 24X7 need to charge at 1kW,

Thank you for your response! Looks like I already need to clarify what I was trying to say. It's not a 24x7 need to charge at 1 kW; I'm targeting charging 1 kW during daylight hours.

What I'm trying to say is I need to support a 1 kW load, but the kWh per day is what's negotiable. If I can only charge a few miles per day, I'll live with that; but I don't have the ability to reduce the rate lower than 1 kW.

oil pan 4 · September 2015

Get a portable propane or white gas stove for cooking.
Using PV to power a resistive heater element is ridiculously wasteful and can be accomplished though much simpler means.
Remove the cooking load and everything else becomes that much more possible.

What kind of fridge do you have? 60w run and 125 watt starting surge just sounds really low.

lkruper · September 2015

Insolated wrote: »

Thank you for your response! Looks like I already need to clarify what I was trying to say. It's not a 24x7 need to charge at 1 kW; I'm targeting charging 1 kW during daylight hours.

What I'm trying to say is I need to support a 1 kW load, but the kWh per day is what's negotiable. If I can only charge a few miles per day, I'll live with that; but I don't have the ability to reduce the rate lower than 1 kW.

That makes a big difference! So now I calculate a need with the same parameters of 9800 ah for a 24 hour period. You could use 8 6v batteries like the Sun XTender AGMs 3050T at $418 each to get 305AH @ 48v. You can now charge faster if you want, from 0.2C to a whopping 5C. 0.2C is 61 amps which falls under the limit of 80A for many MPPT controllers, take your pick.

oil pan 4 · September 2015

$418 x 8 = get a generator

Insolated · September 2015

oil pan 4 wrote: »

Get a portable propane or white gas stove for cooking.
Using PV to power a resistive heater element is ridiculously wasteful

Yeah, you're right. I suppose I was looking at it as being a secondary load if I wasn't charging the vehicle, since the power would already be there. All things considered, yes, fuel-based cooking is a better option.

oil pan 4 wrote: »

What kind of fridge do you have? 60w run and 125 watt starting surge just sounds really low.

I thought the same, but don't have any comparison points. I measured it with a Kill-a-Watt, but I took those measurements a while ago and for all I know wrote it down wrong. I'll look up its specs and maybe measure again. Admittedly, those numbers are just for keeping the fridge going after it's already at its set temperature, not starting from room temperature (i.e. a power failure scenario). I believe I left the door closed the entire time, so those end up being the minimum power draw.

oil pan 4 wrote: »

$418 x 8 = get a generator

:-D That's certainly a valid option. I'll admit I simply have a fascination with solar, and ultimately I may need to accept I'm being unrealistic. But I'm going to dig into this a little further first.

lkruper wrote: »

That makes a big difference! ... 9800 ah for a 24 hour period. You could use 8 6v batteries like the Sun XTender AGMs 3050T at $418 each to get 305AH @ 48v.

We're making progress -- that cut a lot of the cost! I've been busy since posting yesterday, so haven't had a chance to fully explore your suggestion yet. I'll take a look, but I wanted to at least post and say thank you to both of you for responding. Thank you!

Also, I didn't really explain my thinking in the original post, so for further discussion let me say this. I can make my loads minimal except for the car charging. If I have batteries sufficient for the other loads each day and enough solar panels to bring them to float each morning, then try to use the excess solar for opportunity car charging, do the batteries still need to be spec'ed for the 1 kW load? I imagine this doesn't work in practice due to the power curve of the panels as the sun moves throughout the day, but perhaps there's a way to balance battery size with when during the day I allow the car to charge. lkruper mentions AGMs that can take up to 5C, so I imagine I could overpanel for the purpose.

lkruper · September 2015

Insolated wrote: »

Yeah, you're right. I suppose I was looking at it as being a secondary load if I wasn't charging the vehicle, since the power would already be there. All things considered, yes, fuel-based cooking is a better option.

I thought the same, but don't have any comparison points. I measured it with a Kill-a-Watt, but I took those measurements a while ago and for all I know wrote it down wrong. I'll look up its specs and maybe measure again. Admittedly, those numbers are just for keeping the fridge going after it's already at its set temperature, not starting from room temperature (i.e. a power failure scenario). I believe I left the door closed the entire time, so those end up being the minimum power draw.

:-D That's certainly a valid option. I'll admit I simply have a fascination with solar, and ultimately I may need to accept I'm being unrealistic. But I'm going to dig into this a little further first.

We're making progress -- that cut a lot of the cost! I've been busy since posting yesterday, so haven't had a chance to fully explore your suggestion yet. I'll take a look, but I wanted to at least post and say thank you to both of you for responding. Thank you!

Also, I didn't really explain my thinking in the original post, so for further discussion let me say this. I can make my loads minimal except for the car charging. If I have batteries sufficient for the other loads each day and enough solar panels to bring them to float each morning, then try to use the excess solar for opportunity car charging, do the batteries still need to be spec'ed for the 1 kW load? I imagine this doesn't work in practice due to the power curve of the panels as the sun moves throughout the day, but perhaps there's a way to balance battery size with when during the day I allow the car to charge. lkruper mentions AGMs that can take up to 5C, so I imagine I could overpanel for the purpose.

I have seen it discussed that an excess of power at the batteries will be applied to the inverter and also there are some charge controllers that have an output where they split power between a load and charging. I don't know how car charging works and hopefully someone who does can provide that sort of guidance. As for the 5C, it is only certain brands like LifeLine AGM that have this in their technical manual. So make sure you follow the guidelines of your vender. I will also say that some on these forums are skeptical about that high of a charge rate, but Lifeline does support it.

oil pan 4 · September 2015

Have you considered doing a grid tie that could quickly switched over to off grid, stand alone?
That way the solar panels would get max utilization.
Over panel enough solar to run your needs during the day and have a adjustable charge controller to go with the minimum number of big batteries and not fry them. Also get a small used generator.
Just leave the battery on a float charger till you need it.
Use the generator at night after the battery gets low or on over cast days to power everything and charge your battery all the way back up quickly.

DanS26 · September 2015

oil pan 4 wrote: »

$418 x 8 = get a generator

LP is selling for $.89 per gallon. Store as much as you want for as long as you want. LP does not go bad in storage. Large excess LP tanks are stacked in tank farms across the Midwest just waiting for the cutting torch.....maybe you can rescue a couple.

bill von novak · September 2015

Insolated wrote: »

[*]Since this is backup, I'm fine using it only during daylight hours. I know I need batteries, but is it possible to reduce their size (for cost savings) with that approach?

Sure. Go with a standard grid-tie installation and use an SMA inverter with the US option (secure power.) It provides 120VAC at up to 1500 watts during sunlight hours, provided the array provides more than that. If you want backup get a $100 UPS from Best Buy and charge during the day.

KenZ71 · September 2015

If the original poster has an electric vehicle why not use that as the backup generator?

Insolated · September 2015

lkruper wrote: »

As for the 5C, it is only certain brands.... I will also say that some on these forums are skeptical about that high of a charge rate....

Okay. I don't know much about AGM, and I was surprised to hear you mention 5C. That some are skeptical makes sense; few batteries can tolerate that. I'll be cautious if I go that route.

oil pan 4 wrote: »

Have you considered doing a grid tie that could quickly switched over to off grid, stand alone?

Do you mean grid-interactive? Originally I was considering a Radian, but it quickly got complicated and expensive. A/C is my biggest normal load, so I'd either have to size to include it to prevent overloading the inverter at the point a power outage occurs, or I guess put in a sub-panel that the inverter would power. I came to the conclusion I should look at something just for backup to keep it simple, though eventually I may go the whole-house route.

DanS26 wrote: »

LP is selling for $.89 per gallon.

I don't know anything about propane generators. Are they quieter than a gas generator? I do have neighbors close by.

bill von novak wrote: »

standard grid-tie installation .... If you want backup get a $100 UPS

Grid-tie won't help in a power outage situation, and a cheap UPS won't last long charging the car (though it would work for the other loads).

KenZ71 wrote: »

If the original poster has an electric vehicle why not use that as the backup generator?

My concern is the opposite direction -- charging the car during an outage so I can drive somewhere. The other loads are secondary.

softdown · September 2015

Don't buy expensive batteries for your first bank. There are 100 ways to kill a battery/bank. Golf cart batteries are pretty good solar batteries.

oil pan 4 · September 2015

The grid interactive ones do sound really cool but I have a sneaking suspicion that when the power grid gets taken out say by lightning, or even a tree falling on a power line could create a huge power surge damaging or killing the inverter. The reason for having gird tie and standalone inverters apart from each other.
This is also assuming the power surge doesn't damage panels by back feeding power through the inverter, I have a feeling this would be unlikely.

I have witnessed several events over the years to this effect.
In 1999 my next door neighbor had a tree branch sever their service drop in the middle of the day while they were home. Some how this created a power surge that destroyed every bit of electronics running at the time. The fridge, TV, cable box, cordless phone chargers, desk top computer that were on, all fried. Another TV that was off, their well pump and furnace that were not running were all fine.
I had a lightning strike earlier this year well with in 1 second away at the same instant my window unit air conditioner growled like a transformer about to explode, lucky for me it did not explode but kept running. Yeah, it kept running for a few more days and the compressor went out. About a week later the power supply for my rougher started over heating and cutting out. I had just went to bed so everything else was as "off" as it can be and plugged into a surge protectors.

Insolated · September 2015

softdown wrote: »

Don't buy expensive batteries for your first bank.

You're saying go with flooded instead of AGM. I considered myself as having already compromised going from expensive LiFePO4 to AGM. Since this wouldn't be for day-to-day use but rather emergencies, my thinking was that so-called "maintenance-free" would be more likely to hold up well.

oil pan 4 wrote: »

I have a sneaking suspicion that when the power grid gets taken out say by lightning, or even a tree falling on a power line could create a huge power surge damaging or killing the inverter. The reason for having gird tie and standalone inverters apart from each other.

That hadn't occurred to me -- thanks for sharing what you've seen happen.

Only a few hours left in the sale, and I'm still unsure what I want to do. Guess I'd better make up my mind quickly. I'm considering that one reason to go with solar for this may be to use it as a small system to learn from. I do eventually want to put up a system for the whole house, but given the experiences I've seen on this forum I can expect to make mistakes along the way -- better to make them with a small system first.

Thank you everyone for your help. This forum has been a great source of wisdom the past few months, and I appreciate how friendly you are to someone who has a lot left to learn.

oil pan 4 · September 2015

I have an AGM battery that I picked up in 2013 on my generator inverter rig.
I know that doesn't seem like very long but if it were a standard starting battery I think it would have died or almost be dead from age by now.
The AGM has held up well and I plan on replacing it with another AGM when this one goes bad.

bill von novak · September 2015

Insolated wrote: »

Grid-tie won't help in a power outage situation, and a cheap UPS won't last long charging the car (though it would work for the other loads).

The SMA inverters with the US option do indeed work in a power outage situation. They will provide up to 1500 watts of 120 volt AC power. They are a good option for people who want to go grid tie and some backup. The cheap UPS gets charged during the day and runs loads at night.

Solar backup

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