In order to have a discussion about electricity in general and solar in particular, we need to define a few terms. (You engineer types, this is simplified. I am trying to keep things easy to understand, so spare me the discussion about how holes move. I am also excluding things like power factor, vectors, and other things that needlessly complicate the discussion.) There is a glossary at the bottom of this article.

The Basics

Your house gets power from the grid as alternating current, delivered at 60 hertz. There are two current carrying lines and a “return” line that enter your home from the grid. They are referred to as 2 “hot” lines and a neutral. The voltage as measured between either hot and the neutral is about 117 volts. If measured between the 2 hot lines, it’s about 235 volts. In this way, we can power smaller loads like light fixtures or televisions with a hot and a neutral. We power larger loads like water heaters, stoves, clothes dryers, and the like with 2 hot wires.

Your typical house circuit has a breaker that is either 15 or 20 amps. Any more than that, and the breaker will trip to prevent fires. (They can trip for other reasons, too, but that is beyond the scope of this article) Large 235 volt circuits may deliver up to 50 amps before tripping. If your home is new and like most homes, the total of all of the circuits in your house will be 200 amps. That works out to a maximum power of about 47 kilowatts. For short periods of time, like when your air conditioning compressor first starts, some circuits can use even more power than that.

So now that we know our house can use a maximum sustained amount of power that is equal to about 47 kilowatts, we can plan for our backup power needs. Now we need to know how much power we use each day. My utility has supplied my house with a “smart” electric meter. On average, my house is using about 25 kilowatt hours per day. During the summer, when the air conditioner is running, I am guessing we will double that. Maybe more. We will wait until July comes around before we make any decision, so we have a better idea of what our cooling will cost us.

How Solar Works

How solar works is that photovoltaic (PV) panels convert the light striking them into electric current. Nowadays, each panel puts out about 400 watts when new. (Panels lose about 0.25% of their power output each year as they age. After the 25 year warranty period, they should still be producing more than 93% of their rated power.) The power produced by those cells is direct current at about 40 volts. It needs to be changed to alternating current that matches the incoming grid power, and this is accomplished by an inverter.

Your solar system can produce more power than your home is using, and that excess power can run your electric meter backwards, effectively selling that power to the utility. At night, or when it is cloudy, your solar system doesn’t produce as much, and you buy power from the grid. If you size your system correctly, you will produce at least as much as you consume, thereby making your bill as close to nothing as possible. (Sadly, the local utility won’t let you run a negative bill. In fact, the least that your bill can be is \$30 per month.)

So let’s proceed with the assumption that we consume 50 KWh per day in the summer, and about 25 KWh in the winter. With Florida being as sunny as it is, we can count on 4 hours per day on average of sun year round, and 6 hours per day in the summer. (This accounts for nighttime, cloudy days, etc. Days in the winter are both shorter and cloudier.) I got the figure of average hours per day of sun from the solar companies. That’s the numbers that they use.

If we want to produce 55 KWh per day during the 6 hours of summer daylight, we need to have a system that produces around 9 KW. That means we need about 23 PV panels in our system, making it a 9KW system. That will give us the 50 KWh that we consume, plus 5KWh additional in case we need it. As you can imagine, some days will be extra sunny and we will have lots of power, and other days, not so much. More on how we deal with that later.

What about grid failures? When the grid goes down, the National Electric Code says that our solar system must disconnect from the line so as not to endanger linemen who may be working on the system. That’s where batteries come in. If your system has solar panels and a battery for storing power, you can get a relay installed that will disconnect your system from the grid, thereby allowing your system to act as its own backup power source. This sort of solar system is known as a hybrid system.

With the system disconnected from the grid, instead of running the meter backwards, it sends 100% of its excess to the batteries. Then at night, our house uses that stored power to keep the lights on. As you can imagine, the battery that does that is large and somewhat expensive. In fact, a battery is half or more of the cost of installing a solar system. Still, the system can’t be used as a whole house backup without a battery.

So our battery should be capable of doing two things: storing 11 or more KWh per day of energy, and delivering large amounts of current for short periods as our air conditioners and the like start up and use more than the PVs can deliver.

One note about battery systems: the battery is the weak spot of the system, and a major part of the expense. Batteries are only warrantied for ten years, and will need to be replaced more often than the rest of the system.

During the day when the grid is still running, your system’s inverter does something smart. It powers your house from the PV cells, then sends some energy to charge your batteries, and the rest gets sent to the grid to run the meter backwards. When the power goes out, the batteries either get charged or supply power to your house, depending on the needs of the moment.

Wouldn’t it be a good idea to shut off non-essential power drains when the gird is down, so as to conserve battery power? You can do that by turning off circuit breakers to nonessential loads, or you can use a smart breaker box to do it for you. That’s where the SPAN panel comes in. This panel allows you to designate loads as being essential, nice to have, or nonessential. When the grid goes down and your PVs aren’t making enough power to supply everything, the SPAN panel will disconnect the nonessential loads from the system, sacrificing their operation to save battery power. When your batteries have less than half of their charge remaining, the SPAN panel then shuts off the “nice to have” circuits to preserve the remaining battery for things that need it, like refrigerators.

So I think that is enough to get the basics down. Ask questions in the comments.

Glossary

• Alternating current: The electricity reverses course in a cyclic fashion. The number of cycles per second is measured in Hertz. The electricity delivered by the grid is 60 hertz alternating current.
• Current is the number of electrons moving past a fixed point. It’s measured in Amps or Amperes.
• Direct current: All of the electrons move in a fixed direction. DC is the current supplied by batteries.
• Electricity is simply a measurement of the movement of electrical charges, mostly as carried by electrons.
• Kilowatts: 1 kilowatt is equal to 1000 watts
• Kilowatt hour: A measure of how much power is being used over a period of time. 1000 watts for one hour.
• Voltage (or what is called potential) is the equivalent to water pressure. The higher the voltage, the more “pressure” there is pushing the electrons through.
• Watts: A measure of power. It is calculated by Volts times Amps= Watts.
Categories: Electric and Power

Hightecrebel · April 3, 2024 at 7:21 am

As far as batteries are concerned, check out EG4’s PowerPro batteries and/or Ruixu’s server rack batteries. Better bang for the buck than a Powerwall, and uses LiFePo4 that after 10 years is still expected to have 80+% original capacity.

oldvet50 · April 3, 2024 at 7:38 am

Good article. In my opinion, this is the only type of solar power that should be allowed – personal use. Most people do not realize how polluting wholesale renewable energy is. An electrical system, such as your local utility, must ensure that the supply of power equals the demand for power at all times. If not, bad things happen, like the voltage will drop, or 60 hertz becomes 50 hertz, etc. We all have seen a diesel truck, like a semi, accelerate and witnessed the amount of smoke being pumped into the air. Coal and gas generators your utility uses to produce electricity do the same. When the wind stops blowing or the sun goes behind a cloud, your local power plant MUST immediately make up for that loss of renewable generation. Since this happens all throughout the day, your once steady-state generators are constantly being ramped up and down to make up the fluctuations of the renewables. Just like the diesel truck, these generators are pumping more pollutants than necessary into the air to make up for the “clean” renewable energy.

Henry · April 3, 2024 at 8:51 am

Nice summary – I hope you are able to get a good system for the new house at a reasonable cost. One concern I have with SPAN panels (and those of competitors) is whether the company will actually be around in the long term. E.g. spare breakers, software maintenance, and App updates all come to mind as something that matters with a system that’s absolutely core to the functioning of your house for decades to come.

Divemedic · April 3, 2024 at 2:37 pm

The panel uses Square D breakers. The software support is a valid concern.

Don W Curton · April 3, 2024 at 9:44 am

There’s also a significant safety factor in having a large battery (or bank of batteries) in your house. Remember, that’s stored energy. I would assume catastrophic failures are rare, but they do happen.

Also, since we are presumably planning for some kind of weather event, I’m guessing there’s a hurricane code for how the solar panels are attached to the roof? Do you happen to know the wind rating for yours? I’ve never looked into it, so I can only hope the panels would still be in place and working after a hurricane comes by.

Divemedic · April 3, 2024 at 2:38 pm

No less safe than storing hundreds of gallons of fuel.

Shelley · April 4, 2024 at 12:34 am

I understand that fire departments aren’t wasting their time to put out lithium fires. It isn’t practical to try. And who stores hundreds of gallons of fuel INSIDE their house?

Divemedic · April 4, 2024 at 8:40 am

That would be misinformation.

Don W Curton · April 4, 2024 at 7:10 am

Most people understand that storing hundreds of gallons of gas in their basement is a bad idea. (Most, not all. The world is full of idiots.) A lot of those people also wouldn’t think twice of storing the same amount of energy in batteries in their basement. However, every new Tesla fire video is waking more people up to that fact.

Granted that properly maintained and monitored batteries can be safe, we also have to factor in the average stupidity of people in that “properly maintained and monitored” caveat.

Divemedic · April 4, 2024 at 8:40 am

Tomorrow’s post goes into that.

C · April 3, 2024 at 9:58 am

Kudos on mentioning load shedding. Most people don’t consider that.

Bigus Macus · April 3, 2024 at 10:04 am

Thanks very informative.

Clayton W. · April 3, 2024 at 10:17 am

It HAS been a while since I looked, but ISTR that PV cells wore out the same as batteries: 80% after 10 years. The answer is probably really somewhere between my number, engineering pessimism, and the sellers number, Best case scenario. Buyer bware

Divemedic · April 3, 2024 at 3:03 pm

I used actual manufacturer’s information. There is a link in the post.

Max Wiley · April 3, 2024 at 10:35 am

Based on the equations that I have been taught to use in the industry, there is a mistake in how you are calculating your output.
The back of napkin rough calculation is Solar Output (kWh) = Total Array Wattage * Peak Sun Hours * .75. It is 75% of your total array for the peak sun hours, not 100%.
Based on this, you need an 11kW system to make sure you can hit 50kWh in the summer with 6 peak hours, not 9k. Note this is the array size, not necessarily the inverter size.
This squares with the calculations our engineers use to design systems.
https://thegreenwatt.com/how-to-calculate-solar-panel-output/
https://footprinthero.com/peak-sun-hours-calculator#peak-sun-hours-map
Putting an 11kW system into the the NREL.gov calculator for a Tampa zip code and fixed roof mount returns an average of 47.6 kWh per day.
https://pvwatts.nrel.gov/index.php

Divemedic · April 3, 2024 at 2:34 pm

That already factored in. Remember when i said at the beginning of the post that i was simplifying? The actual number of hours per day at my location is 5.67 hours per day. Since I am using an average of 4 hours per day, there is a built in 30% adjustment.

Typical solar panel systems require only a minimum of four hours of sunlight per day to operate at full efficiency.

Max Wiley · April 3, 2024 at 8:40 pm

I’m not going to argue with you at your own blog, it is impolite.
I will only make a couple of statements of facts, in an attempt to be legitimately helpful to you and your readers:
1) I do this for a living.
2) Multiplying the entire array size by the peak hours of sunlight without an adjustment factor is an incorrect formula.
3) If I put a 9kW system with ideal mounting into the NREL calculator for Tampa it only makes 36ish kWh per average day in any summer month. If the entire array isn’t at the ideal tilt angle and azimuth it will be less than that.

Divemedic · April 4, 2024 at 9:05 am

Remember when I said I wanted to keep it simple?
My area actually averages 5.6 peak hours per day, on average, throughout the year.
Your way would see 5.6 peak hours times your .75 correction factor times system rating to equal power created. Let’s say that the system is rated for 10KW, just to keep the math easy. The formula would be:
5.6hrs*0.75*10KW= 42KWh.
What I did, to make the math easier to understand, was I downgraded peak hours to only 4 hours, in order to eliminate that extra step. It looks like this:
4hrs*10KW=40KWh.
Your way uses a 0.75 factor. In my way, I reduced the number of peak hours by a corresponding amount to arrive at the same answer that a 0.71 factor would give you. I am actually downgrading the system even more than you are.

Using the calculator at the link that you provided, a 9.66kw system will generate 14,903 KWh per year. That calculator even says:

System output may range from 14,252 to 15,304 kWh per year near this location

Using the calculations that I have, I calculated 14,700 KWh per year for a 9.66KW system, which is near the middle of the range predicted by that calculator.

If I put a 9kW system with ideal mounting into the NREL calculator for Tampa it only makes 36ish kWh per average day in any summer month.

Since I am using 4 hours per day as an annual average

we can count on 4 hours per day on average of sun year round

Then a 9kw system for four hours per day equals 36KWh, which is exactly what I said. You have needlessly complicated this math problem to arrive at the same exact numbers as I did. I would also point out that using the NREL site for a 9kw install in Tampa actually resulted in 14,182 KWH per year, which divided by 365 (the number of days in a year) gives an answer of 38KWH, not 36.

Cigarillo Pancho Hat · April 3, 2024 at 10:36 am

The offramp for Pineland has the solar panel eyesore and last year someone crashed through the guardrail and took out a row.
Texas had the hail storm wipe out way more than just a row and now nothing will grow in that place due to hazardous materials in the soil.
Wind turbines would work great out at the Pineland Safehouse but they are so ugly and the birds get sliced and diced.
You also don’t want to live close to one due to the humming sound at all times.
Harold the Brain has an LNG tank to power the generator and it would probably last a month before needing a refill.
This society will go bat guano when there is no Barryflix and social media hive or sailfawn phone to prattle on.
Good, good, let the cray cray flow through you.

Divemedic · April 3, 2024 at 3:15 pm

I dont see how a car accident or a hailstormhas anything to do with this post.

Floyd Pendergraft · April 3, 2024 at 10:52 am

Having had my solar for 7yrs here in Cali.. I haven’t paid 1 dime to my utility. I have negative bills. I over-generate to the tune of 5000kwh each year after use. PS: My PG&E summer bills were +- \$1400 per month.

Divemedic · April 3, 2024 at 3:15 pm

Sadly, Florida power companies charge you even if you don’t use any power.

Don W Curton · April 4, 2024 at 12:02 pm

Texas does too, or at least it used to. We had a camp house on some inherited land. We’d go out for the weekends or sometimes a full week or two in the summers when I was young. Routine was always the same. Get there, go to the breaker box, and flip the main to “ON” before going inside. As we left, last thing to do was flip the main to “OFF”. There’d be entire months with zero electrical usage and still a ~\$20 charge. Reasoning was taxes, fees, etc. and the privilege of being connected to the grid, even if you used zero power.

Botan · April 3, 2024 at 11:38 am

Good discussion for the non engineers but most of the technical illiterates being turned out by our current education systems will find even this beyond their grasp.

OldGuy · April 3, 2024 at 1:50 pm

The posting of this topic is most timely. I recently started looking at “portable” solar systems such as the Goal Zero 4000 Pro. That was before I read your comment a few days ago. I have absolutely NO background in this type of thing and my biggest fear is spending \$10k or more and not getting what I need. In short, here was my plan, after a “visit from the good idea fairy”:
I have a duel fuel generator now. I recently started thinking about longer term power, as I’ll run out of gas within 3-4 days. I live I hurricane country, and in addition to short outages, this neighborhood has been without power for 14 days straight, twice since 2000. So my good idea fairy plan was to get something like the 4000 Pro, get enough battery tanks to make it a 20 kWh unit, and with the additional help of some solar panels, recharge the batteries whenever they get low by plugging them into the wall whenever I’m running the fuel generator anyway to keep my critical items powered up. By doing that, I was hoping to greatly extend the time I have some version of power. Hopefully you are tracking that. That might be the dumbest idea ever, which is why your post is very timely. I’m trying to sort all this out, and like you said, there is a lot of bad information out there. Thanks for what you do.

Alvin from Maine · April 3, 2024 at 4:38 pm

Good article on solar. Well written. I’ve had solar since 2000. I have a “Triplex” system. Dump on grid, dedicated loads, battery/Inverter A PLC controls how its done and which mode is used.

Anonymous · April 3, 2024 at 6:53 pm

Your statement on minimum utility bill and not having a negative bill, although correct for your location, is not universal.The utility rules for compensation of residential rooftop solar is governed at the state level and will therefore vary. Check the rules that apply to your electric utility.

Divemedic · April 3, 2024 at 7:11 pm

Read it again. I can’t type it any more slowly. Here is the exact thing I typed:

(Sadly, the local utility won’t let you run a negative bill. In fact, the least that your bill can be is \$30 per month.)

Noway2 · April 4, 2024 at 12:10 am

In all honesty, though I haven’t looked, I’d be interested in seeing a 1 line diagram of how these solar systems work that tie back to the electric grid and can feed power to it, yet island or isolate (the relay you mention) in the ad ent of a grid down scenario. I actually have trouble envisioning it, and I come from a background of being an EE who worked for several years in power quality, e.g. industrial UPS systems and static transfer switches.

In short, I can see your system being like a UPS (or similar to a VFD) in that it takes grid power in, rectifies it, and applies the incoming power to a DC bus, to which your batteries are applied. This in turn feeds an inverter which supplies you with clean AC power regardless of the “grid” status. As I envision it, the rectifier portion of the circuit, sans a shorted SCR or diode would naturally prevent you from putting power on the grid, and if you did, it would be via connection to the DC bus. I understand if this is hard to explain without a block diagram.

I realize technology has changed A LOT in the last couple of decades, but to me who has been in the business some of what they supposedly do today seems like black magic. Just as I understand the concern about people back feeding generators and it taking out a lineman working on the system. In theory, I understand, but in practice I don’t see it. If your 7kw generator were to suddenly see the load (reflected impedance) of just your neighborhood, it would stall out in seconds. Likewise, an inverter suddenly subject to an extreme load is going to self protect and say, “I’m out of here” and I’m thinking data center level UPSes / inverters, far more sophisticated than the residential solar ones sold today.

Then again, I realize things have changed and it’s been going on 15 years since I left that industry. It’s fascinating stuff and just recognizing the difference that the combination of DSP technology, coupled with advances in semiconductors (e.g. using fast MOSFETS instead is SCRs – the power loss, and hence heat, in the switching device occurs when it’s turning on and off and that’s gotten A LOT faster) and how it allows the use of high switching frequencies (V=L*di/dt) that were impossible not all that long ago. Fascinating stuff indeed.

Divemedic · April 4, 2024 at 8:45 am

There is a relay that is wired between the incoming grid and the home. When the grid goes down, the relay opens, which disconnects the system from the grid.
As far as how the system does it- there is an inverter that converts the DC to AC. This can happen in a few spots- at the PV itself, or at the battery. The two main types of inverters are string inverters, which make the conversion for a string of PVs, and micro inverters, which do the conversion for each individual PV panel.

Rick T · April 4, 2024 at 2:56 pm

The big difference between a datacenter UPS and a hybrid PV/Battery system is in the inverters… Datacenters use double-conversion (AC to DC for the battery bus then back to AC for the loads) so they have extremely clean power and no power issues if external power is lost. The efficiency losses of doing double conversion is just a cost of having clean power.

With my SolarEdge system the solar cells and battery were on the DC side of the inverter and it was one-way (DC-AC) with no ability to charge the battery via the grid, so backfeeding from the solar arrays (or via commanded discharge of my battery) was relatively simple. The system is alreadly paralleled so just bump up the output voltage or push the phase forward a degree or two and power goes back to the grid.

If the grid drops there is a momentary disconnect as the isolating relay trips then the Battery-connected grid-forming inverter energizes and picks up the internal loads.

Dave Waterman · April 4, 2024 at 2:21 pm

Well written and very informative. Thanks and much appreciated.

D · April 5, 2024 at 2:12 pm

That’s a good write-up.
One slight nit-pick is that the voltages should be 120 between the leg and ground and 240 between both legs. The local utility usually has guidelines on how far out of spec they can be before they need to adjust stuff. But most people will see between 117 and 123 per leg. Right now I’m doing 121.2 on both legs.

A few tools that might help people:

The Sense Energy Monitor does a pretty good job of tracking electricity usage. It can be integrated into Home Assistant easily. It does a fairly terrible job of identifying “devices” based on their electrical signature, but I mainly use it for monitoring. You can also get an additional set of CT clamps and monitor a specific circuit or something like your inverter and how much power it’s putting out.

There’s a tool called “Solar Assistant” that comes on a small Raspberry Pi style credit-card computer. It talks to a lot of different inverters and basically sits there generating graphs and stats so you better optimize your system.

If you have panels on your roof in the US, you need somethig that will do rapid shutdown…Tigo makes some great ~\$50 RSD devices…but one of their lines also has “optimization” in it. It does a spectacular job of squeezing more power out of the system if a panel gets partial shading or when clouds roll past. I’m not sure it’s worth the \$50/panel cost…but it seems to be pretty close.

Divemedic · April 6, 2024 at 9:15 am

The national standard for utility voltage tolerance in North America is ANSI C84.1. This standard establishes nominal voltage ratings and operating tolerances for 60Hz electric power systems above 100 volts. Here is a table of common voltages for common services. Note that the center of the ranges below are 117 and 235 VAC. I would assume that we call them 120 and 240 for the same reason that we call it a 57 Magnum, even though the bullet isn’t that size.

Again, I was trying very hard not to get tied down in minutiae.