Am I missing a decimal point?

[SteveInIL]

Please help. I'm looking in to adding solar to my home. I have a modest approx 1500 sq ft home in the Midwest. Looking at data from my utility, I have a summer peak (worst single day) of around 48kWh per day. Assuming 6 charging hours per day I calculate needing 80 ea 100W panels and 40 ea 12V-100AmpHr batteries. This was just a very rough calculation as a starting point, to see if whole-house solar is feasible. I know I could use fewer but bigger panels and a 24V-48V battery bank, but the cost and space requirements wouldn't change significantly.

Do these numbers seem at all realistic, just as a starting point?

Thanks for any help on this!

 

[time2roll]

Seems like a lot of power for 1500' but not unreasonable if you have mostly electric appliances. Assuming this is a grid tie system you don't necessarily need to produce all the power to zero out your utility bill. Sometimes just 50% to 80% net grid reduction will be a better return on the investment. For net energy metering look also at your annual consumption and estimated annual production.

Not sure what the real goal is here. Off grid? Reduce the energy bill? Power outage for a few days/week?

 

[JoeHam]

Of course you would be better off using some 400w panels and building a 48v system but it’s not easy or cheap to go fully off grid.

And you didn’t even factor in 2-3 days of bad weather yet.

Grid support or generator support would help for your peak power use days.

 

[svetz]

Welcome to the forums!

Please help. ...Do these numbers seem at all realistic, just as a starting point?

Math Check​

From your forum name, let's guess you live in Pekin Illinois. First, visit an insolation Calculator and get your insolation number. To the right are the values for Pekin, but use your actual location to get real numbers. ... I have a summer peak (worst single day) of around 48kWh per day. Most places in the U.S. have some sort of net metering agreement. So, you don't need to build out for the worst day, just the average and let the grid supply the rest. Look into what your local power company's net metering agreement is before you make a lot of decisions. Typically if there's grid outage, you can reduce your power consumption for the duration. If you're off-grid, that's another can of worms.

​

How many panels do I need?

Needed: 48 kWh/d
When: July
Insolation: 5.44 (assuming tilt optimized for year-round performance)
Inverter Efficiency: 90% (use your actual number, they vary widely)
Battery Round Trip Efficiency: 94%

48 kWh/d needed / 94% Battery RTE / 90% Inverter Eff = 57 kWh of solar generated
57 kWh / 5.44 = 10.5 kW array

... needing 80 ea 100W panels ...

The 100 watt panels are pricey... I wouldn't use them unless I needed to work around a weird roof geometry. I'd go for the ~330 watt, not too heavy (although still a handful for balancing on a steep roof and can catch the wind) and probably the best price points. You can probably get slightly better prices on the 390/400 watt panels, but they're too big/heavy for me (panels are mostly all the same efficiency, so when buying watts, more just means bigger/heavier, but also lower costs in attachment points and connectors).

I calculate needing 40 ea 12V-100AmpHr batteries.

How much battery you need depends on how much energy you consume at night and how much reserve you want. Let's say you just want the batteries to get you thought the night and that's 60% (e.g., solar gets you through the day) then 48 kWh/d x 60% = 28.8 kWh. As 12V, that's 2400 Ah. But with a 90% efficient inverter that's =2660 Ah.

Here are some FAQ threads that might interest you:

• Energy Audit
• How much Solar Energy should I expect where I live?
• What does it mean to have solar panels in parallel and series?
• How to figure out how much Solar and inverter wattage is needed to charge a battery
• Figuring out how many panels in series and parallel based on your MPPT
• What wire gauge should I use? (video, calculator)

Hope this helps!

 

[GXMnow]

I have to agree on a few points here. 48 KWH in a day is a lot of power. I have a larger home in So Cal and my normal days are around 20 KWHs, a lot of days that I don't need A/C can fall as low at 15 KWHs. My worst case peak when it is over 100F all day has hit 50 KWH's, but that was like 4 days in a whole year. Unless you are planning to go completely off rid, it is certainly not cost effective to build for the worst case day ever. Grid power is still cheaper than a fully off grid system. Batteries are still expensive and only last about 10 years.

What most people do is put up enough solar to cover normal days. And if you want backup power and maybe some self consumption time shifting, you put in a batter system that can run your essential loads during a power outage. Do you have long and frequent power outages, or is it a rare thing you just want to cover?

Also look at the electric rate structure when you add solar. This one caught me off guard. We had a tiered system before where the first 400 KWH's in a month was at a low 15 cents/KWH but once you go net metering, that rate is no longer available. The cheapest rate for a grid tie system without an electric car or other qualifying green energy system is 22 cents / KWH now. And from 4 pm to 9 pm, it jumps to 43 cents, even if it is the only power you use. So my savings off my electric bill is about half what I had originally calculated. I am using far less grid power, but every little bit I do use is at a much higher rate. But even with the bad rates, my initial grid tie only solar power system will still pay off in just 8 years.

Going with a hybrid system with storage is goin g to cost more than grid tie alone. The batteries add a lot to the cost. There are also losses when power is charged into the battery and then pulled out again. I am now cycling about 8 KWH's into a battery when my solar is making power, and then use that power in the evening when the time of use rate is at it's highest, so I am not buying any of that expensive power. But I always have the grid available if I do want to run my A/C and cool down the house. The losses from the charger, the battery, and then the inverter amounts to losing almost a KWH per day. So that does cost me 2 cents per day, but then I have 5 hours where I am now running for free on my stored battery power. 8 KWH at 43 cents - 22 cents = saving about $1.68 each day. That may not sound like much, but in 10 years, that works out to over $5,000 to help pay for having backup battery power.

 

[upnorthandpersonal]

Step one when considering going off-grid or less reliant on the grid, is to reduce consumption. Insulation is cheaper than solar. More efficient devices is cheaper than building infrastructure to generate power.

 

[Steve_S]

The general unwritten rule is to not draw more than 250A from a battery bank (excepting surge handling). Stackable inverters and such are another matter handled differently.
48V@250A=12,000W (100A AC)
-- for quick reference. The (100A AC) can be 120V/100A out to panel or 240V/50A split phase to panel.
In simple terms. -- 240V/15A=3600W uncorrected
? 250A. Because it's recommended to be the MAX draw limit for a standard"single" system. Single, meaning non-stacked inverters and similar.
! Inverter Wattage has to support the watts required plus surge capacity. Should never be run at the limit for an extended period of time (that's undersized).

If Grid-Tied there are fees, inspections and certifications and that varies depending on your specific location. That can easily add up to 35% ON TOP of the cost of equipment so do remember to calculate those cost differences separately when comparing system design types.

Grid-Tied with no battery. Permits etc on top
Grid Connected with battery, (no permits or minimal)
- drawing power from grid to Charge Batteries when required as "backup" or to continue providing primary power with battery as backup.
- Can be used in the interim as the battery bank can be increased over time, to take over full house support.
- Can be used to run certain circuits strictly from Solar System, while also allowing grid power to recharge batteries IF short.
Off-Grid, which is well the full meal deal.

MOST IMPORTANT RULE OF SOLAR ! Conservation is Way Cheaper than Generation and/or Storage.
Look at energy saving ap[pliances & devices as well as home energy improvements.

 

[SteveInIL]

Seems like a lot of power for 1500' but not unreasonable if you have mostly electric appliances. Assuming this is a grid tie system you don't necessarily need to produce all the power to zero out your utility bill. Sometimes just 50% to 80% net grid reduction will be a better return on the investment. For net energy metering look also at your annual consumption and estimated annual production.

Not sure what the real goal is here. Off grid? Reduce the energy bill? Power outage for a few days/week?

Thanks for the sanity check. I wasn't prepared for size and expense necessary.

I didn't know what my final goal should be until I got through the order-of-magnitude calculations. I guess you could say I wanted as much cost reduction as possible depending on whatever my limiting factor would be.

My grid service is actually split into an incoming 200 Amp panel, which feeds the home's original 100 Amp panel, and I've added a 3rd 60 Amp panel being fed from the original. When I bought the home, the 100A panel was horribly overloaded with 2 and even 3 circuits per breaker. Not overloaded with power but by the sheer number of circuits. So I added the 3rd panel to reduce the clutter. The wiring is ideally set up to transfer part of my load to solar.

Sad to say, my home's orientation and being surrounded by mature trees will prohibit anything except a very small system, if anything at all. :-( But it was worth looking into. Thanks for the reply!

 

[SteveInIL]

I've been thinking since my first reply, and you've put me onto a better line of thinking. That is, aim for a system that will be used to its full capacity every day. The only drawback to installing just a couple panels on my roof is opening up the possibility of water leaks etc, for a relatively small payoff.

I might just buy one flex panel or even one of the tiny USB charging panels and gorilla-tape it to my roof to see what I can get out of it. My first and biggest priority is to "do no harm." I could even move the panel around and see how much power I can get at various locations on the south-facing portion of my roof. The cost per Watt would be high but at least I could get some real data to consider.

 

[time2roll]

Consider having a solar installer give you an estimate. It is a big job not suited for everybody. I had 3 kW solar put on my roof by the old Solar City and it has paid for itself almost twice now. No possible way I would have done the job myself and I consider myself to be pretty good at these things. Don't get oversold. After you get the estimate ask about just 60% to 80% coverage.

 

[SteveInIL]

Step one when considering going off-grid or less reliant on the grid, is to reduce consumption. Insulation is cheaper than solar. More efficient devices is cheaper than building infrastructure to generate power.

Yes, I'm coming to realize that now! Building gadgets is more fun than boring ole insulation, but my 70-year-old house could definitely use some TLC. ?

 

[SteveInIL]

Welcome to the forums!

Math Check​

From your forum name, let's guess you live in Pekin Illinois. First, visit an insolation Calculator and get your insolation number. To the right are the values for Pekin, but use your actual location to get real numbers. Most places in the U.S. have some sort of net metering agreement. So, you don't need to build out for the worst day, just the average and let the grid supply the rest. Look into what your local power company's net metering agreement is before you make a lot of decisions. Typically if there's grid outage, you can reduce your power consumption for the duration. If you're off-grid, that's another can of worms.

View attachment 55144​

How many panels do I need?



48 kWh/d needed / 94% Battery RTE / 90% Inverter Eff = 57 kWh of solar generated
57 kWh / 5.44 = 10.5 kW array


The 100 watt panels are pricey... I wouldn't use them unless I needed to work around a weird roof geometry. I'd go for the ~330 watt, not too heavy (although still a handful for balancing on a steep roof and can catch the wind) and probably the best price points. You can probably get slightly better prices on the 390/400 watt panels, but they're too big/heavy for me (panels are mostly all the same efficiency, so when buying watts, more just means bigger/heavier, but also lower costs in attachment points and connectors).


How much battery you need depends on how much energy you consume at night and how much reserve you want. Let's say you just want the batteries to get you thought the night and that's 60% (e.g., solar gets you through the day) then 48 kWh/d x 60% = 28.8 kWh. As 12V, that's 240 Ah. But with a 90% efficient inverter that's =266 Ah.

Here are some FAQ threads that might interest you:

• Energy Audit
• How much Solar Energy should I expect where I live?
• What does it mean to have solar panels in parallel and series?
• How to figure out how much Solar and inverter wattage is needed to charge a battery
• Figuring out how many panels in series and parallel based on your MPPT
• What wire gauge should I use? (video, calculator)

Hope this helps!

Thanks! That's a lot of good resources. I know I'll have to figure in all the efficiencies if I decide to proceed. It's the first time I tried these calculations and I was worried (actually hoping) that I was off by a factor of ten or something huge. Looks like my basic math was correct, even if my decision-making wasn't.

 

[SteveInIL]

I have to agree on a few points here. 48 KWH in a day is a lot of power. I have a larger home in So Cal and my normal days are around 20 KWHs, a lot of days that I don't need A/C can fall as low at 15 KWHs. My worst case peak when it is over 100F all day has hit 50 KWH's, but that was like 4 days in a whole year. Unless you are planning to go completely off rid, it is certainly not cost effective to build for the worst case day ever. Grid power is still cheaper than a fully off grid system. Batteries are still expensive and only last about 10 years.

What most people do is put up enough solar to cover normal days. And if you want backup power and maybe some self consumption time shifting, you put in a batter system that can run your essential loads during a power outage. Do you have long and frequent power outages, or is it a rare thing you just want to cover?

Also look at the electric rate structure when you add solar. This one caught me off guard. We had a tiered system before where the first 400 KWH's in a month was at a low 15 cents/KWH but once you go net metering, that rate is no longer available. The cheapest rate for a grid tie system without an electric car or other qualifying green energy system is 22 cents / KWH now. And from 4 pm to 9 pm, it jumps to 43 cents, even if it is the only power you use. So my savings off my electric bill is about half what I had originally calculated. I am using far less grid power, but every little bit I do use is at a much higher rate. But even with the bad rates, my initial grid tie only solar power system will still pay off in just 8 years.

Going with a hybrid system with storage is goin g to cost more than grid tie alone. The batteries add a lot to the cost. There are also losses when power is charged into the battery and then pulled out again. I am now cycling about 8 KWH's into a battery when my solar is making power, and then use that power in the evening when the time of use rate is at it's highest, so I am not buying any of that expensive power. But I always have the grid available if I do want to run my A/C and cool down the house. The losses from the charger, the battery, and then the inverter amounts to losing almost a KWH per day. So that does cost me 2 cents per day, but then I have 5 hours where I am now running for free on my stored battery power. 8 KWH at 43 cents - 22 cents = saving about $1.68 each day. That may not sound like much, but in 10 years, that works out to over $5,000 to help pay for having backup battery power.

Thanks for sharing! I haven't gotten any info from my utility yet. If we have the same kind of time-of-day pricing, I might be able to make something cost-effective without any solar panels at all. I'm surrounded by trees and that's almost certainly going to be my limiting factor.

 

[12VoltInstalls]

IMHO small panels etc don’t do enough to make a difference.

If you want to dip your toes in: a 200W starter bundle like windyNation is a few bucks cheaper than separate items. An inverter and solar transfer switch, coupla batteries. You are on your way.

 

[rin67630]

Please help.

Look at the off-grid calculator in my signature, it does the math for you...

 

[digitaldoug]

Please help. I'm looking in to adding solar to my home. I have a modest approx 1500 sq ft home in the Midwest. Looking at data from my utility, I have a summer peak (worst single day) of around 48kWh per day. Assuming 6 charging hours per day I calculate needing 80 ea 100W panels and 40 ea 12V-100AmpHr batteries. This was just a very rough calculation as a starting point, to see if whole-house solar is feasible. I know I could use fewer but bigger panels and a 24V-48V battery bank, but the cost and space requirements wouldn't change significantly.

Do these numbers seem at all realistic, just as a starting point?

Thanks for any help on this!

Use 20 400W panels. Thats what I have, but only generate about 35KWH on a sunny day.

 

[digitaldoug]

Use 20 400W panels. Thats what I have, but only generate about 35KWH on a sunny day. Use a 48V inverter. I use 2 mppt with 10 panels in series.

 

[digitaldoug]

In todays technology, Lifepo4 batterys (48V) are the only smart choice for price and long lasting.

 

[rin67630]

In today's technology, Lifepo4 batteries (48V) are the only smart choice for price and long lasting.

My god, what a simplistic point of view...

 

[Hedges]

How much battery you need depends on how much energy you consume at night and how much reserve you want. Let's say you just want the batteries to get you thought the night and that's 60% (e.g., solar gets you through the day) then 48 kWh/d x 60% = 28.8 kWh. As 12V, that's 240 Ah. But with a 90% efficient inverter that's =266 Ah.

Most of the math in this thread seemed OK with a cursory check, but Svetz slipped a decimal place with this one.

28,800 Wh / 12V = 2400 Ah.

I agree with the idea of minimal battery, just enough to get through the night.
I make something between 60 kWh and 100 kWh during the day (mostly to bank with net metering) and my AGM battery is 20 kWh, 14 kWh usable. With several inefficient refrigerators/freezers, if I turn off unnecessary yard lights, rope lights, tube amps, it might keep the mechanical timer running long enough to warm up espresso machine by dawn.

You should be able to get through the night with maybe 25% what I've got (cost me $5000). The freezer will do just fine with power off for the night. What, if anything, do you need to power?

During the day, if grid is down, I can run A/C, laundry, anything I want. PV costs as little as 10% what batteries do, so I'd rather waste photons than waste my money on jugs of deteriorating chemicals.

I don't think you've said why you want batteries. Net metering or zero export can be done without batteries. Some inverters are batteries optional, so you can put in a small battery if you want, or not.

AGM like I use is a reasonable choice for backup, at most a few hundred cycles in 10 years. If you want more cycles LiFePO4 will do it, but most will cost as much as utility rates so no savings. Only lower cost batteries made from recycled cells, or DIY batteries, offer savings per kWh. DIY LiFePO4 can be lower up-front cost than AGM, so worth considering.

Right now everything is hard to come by. Some lead-acid batteries have 26 week lead time.

PV panels lightly used are a good option. I would select best brands and more watts per square meter rather than cheapest per watt.

Solar Panels | SanTan Solar

store.santansolar.com