Solar System Sizing - Capacity Factor

I don’t have wintertime stats as I haven’t yet utilized my solar outfit in wintertime. There are summer stats. I have 800W of panels on the roof, and ~6 average solar hours per day summertime. I have a 6700 WH AGM battery bank. I utilize an MPPT controller and a series-parallel panel configuration to maximize solar power absorption into my battery array.

If you’re interested in my figures here is more detail.

ColoradoMinuteMan said:

It’s amazing how much of a difference clouds make. If a lone cloud temporarily covers the direct sunlight my production will go down to 20% during coverage. On broken cloud days I’m getting 50%. I haven’t found that I get a lot of low light benefit from the MPPT, but I find that it helps generally increasing W output due to Lack of loss incurred in PWM due to lack of voltage tracking/conversion.

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With my 640 watts of solar panel I see readings of 20 to 40 watts a lot when it is totally overcast using a MPPT controller.

With PWM I would be getting zero.

Over the course of a day that is probably a 200 watt hour difference.

Depending on how you look at it that can be a huge difference, or not worth the hassle.

In a cabin or camper situation that 200 watt hours means you can run as many LED lights as you want, watch a couple hours of TV and charge your phone and still have powere to spare. In that case it I view it as a lot of power.

In a grid tied home situation that 200 watt hours that it took ALL day to produce amounts to 1 or 2 CENTS off your monthly electric bill. In that case it is an insignificant amount of power.

In both cases it is 200 watt hours, but in one case it can totally change the way you live. In the other it is a joke.

lasers said:

It probably does help in full sun as well, I have no way to measure that. A PWM wont charge the battery until the panel is producing about 13.1 volts or higher to get above the voltage of the charged battery. A 12 volt panel wont make that unless it is in nearly complete sun. So if it is cloudy it wont put any power into the battery.

A MPPT will start charging the battery as soon as it is producing any voltage then convert the voltage to be just a bit higher than whatever the battery needs to accept a charge at that moment.

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In theory yes, but I don’t really know of any actual 12V or panels. Even the smallest panels generally produce at least greater than 18V in full sunlight and are producing over 13V the moment light is upon all of the cells. Really the additional MPPT value comes in being able to consume the full power (minus overhead) of the panels. MPPT actually slightly looses efficiency in low light conditions because they take a greater amount of power to conduct voltage tracking because PWM doesn’t conduct tracking. Of course the benefit of gaining the otherwise “wasted” voltage from a PWM control by converting it to the battery/appliance VA far outweighs to MPPT power overhead. I guess my point is though, I don’t believe there is any inherent low-light gain from an MPPT controller. I pretty obsessively analyze my solar performance thanks to the wonderful instrumentation available these days. A friend of mine has a similar setup to mine w/same panels in a PWM system and I regularly produce 10-15% more than he does while we’re both camping at the same place at the same time.

Are you trying to cover most of the conveniences that electricity offers? When power is down, you cut back on anything that is not necessary to get thru the day.

A larger bank is good to focus on. This way it covers night time and the off days when sun is not producing.

You can add panels until at a comfortable level of power to recharge. Securing alternative heating sources would seem to be paramount that far north.

Coffee maker and microwave should be considered luxuries when grid is down. Focus on keeping food from going bad and learn to see in the dark. Re-learn how to read real books to pass your time if everything else is under control.

After that last arctic blast before Christmas, I had enough of relying on propane to keep warm. It gets expensive fast. I finally installed a pot belly stove and worked a way to have quasi central heating so the upstairs and downstairs is relatively at the same temperature.

All of my lighting is LED and most of the standard bulbs only draw 4 watts. I carefully watch what is drawing current later in the afternoon until I loose all sunlight for panels. During the day, not so much. The batteries need some load so they get a full charge, instead of in float.

During milder weather and longer days, I might not draw enough from battery bank to take the control charger out of float. Sometimes this has been a inconvenience at night if I dont want to let the voltage drop too much.

There are enough things from your list that will be a constant drain on your battery bank. So at night, your batteries have to be in full charge or you drop below a save margin for longevity.

Hopefully you are not in a large populated area or inner city. Having some acreage or at least access to wood to burn if necessary sometimes. At least some deadfall during emergencies.

My solar array is fairly small, but runs everything I need for now. I have south and west facing panels on the roof. Around 3 pm during the winter months, my panel output drops off substantially for a half hour as the sun transitions between the two rood angles.

It took me a while to even figure this out. During the summer months, not even noticeable. Even with the leaves blocking a lot of the panels. But the winter angle of sun really cuts down on output. It does not take too many branches or limps to block a few cells to drop voltage down a bunch.

fordtrucksforever said:

Are you trying to cover most of the conveniences that electricity offers? When power is down, you cut back on anything that is not necessary to get thru the day.

A larger bank is good to focus on. This way it covers night time and the off days when sun is not producing.

You can add panels until at a comfortable level of power to recharge. Securing alternative heating sources would seem to be paramount that far north.

Coffee maker and microwave should be considered luxuries when grid is down. Focus on keeping food from going bad and learn to see in the dark. Re-learn how to read real books to pass your time if everything else is under control.

After that last arctic blast before Christmas, I had enough of relying on propane to keep warm. It gets expensive fast. I finally installed a pot belly stove and worked a way to have quasi central heating so the upstairs and downstairs is relatively at the same temperature.

All of my lighting is LED and most of the standard bulbs only draw 4 watts. I carefully watch what is drawing current later in the afternoon until I loose all sunlight for panels. During the day, not so much. The batteries need some load so they get a full charge, instead of in float.

During milder weather and longer days, I might not draw enough from battery bank to take the control charger out of float. Sometimes this has been a inconvenience at night if I dont want to let the voltage drop too much.

There are enough things from your list that will be a constant drain on your battery bank. So at night, your batteries have to be in full charge or you drop below a save margin for longevity.

Hopefully you are not in a large populated area or inner city. Having some acreage or at least access to wood to burn if necessary sometimes. At least some deadfall during emergencies.

My solar array is fairly small, but runs everything I need for now. I have south and west facing panels on the roof. Around 3 pm during the winter months, my panel output drops off substantially for a half hour as the sun transitions between the two rood angles.

It took me a while to even figure this out. During the summer months, not even noticeable. Even with the leaves blocking a lot of the panels. But the winter angle of sun really cuts down on output. It does not take too many branches or limps to block a few cells to drop voltage down a bunch.

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Thanks to all who responded so far.

I spent the day at my friend's house looking at his set up and a few hours with a colleague who does solar installs. Of course the solar guy wanted to sell me a system, but he realized quickly I am no where near what his typical customer wants to spend. He does everything from 5KW arrays to 35KW arrays, off grid, hybrid, grid tied and big battery storage systems. He uses higher end stuff (see Sol Ark inverter/charge controllers). He was able to access the inverter/charge controllers and show me the production day to day and month to month for a number of his clients. He has access so he could troubleshoot or answer questions when they call.

Anyway, I learned a lot about solar.....everything from panels with micro-inverters for 2 leg-240vAC grid tied systems, to what I want to do - simple/basic battery back up, not grid tied, for critical loads (power to outlets, fridge, freezer, and lighting for when we lose power).

The major take away is that for my AO, for the worst "sun-hour" months (November-February), I would need about 5000-6000 watts of solar to be able to provide the energy for the loads. That isn't gonna happen right now.

But I can get an inverter/charge controller unit that is big enough for my loads and can handle 5K watts solar for the final build out, if I go there. They are from reputable companies with a good warranty, serviced in the USA, have replacement components available, and aren't much more than cobbling together parts from amazon.

The inverter/charger units can take power from solar panels and from normal grid power for charging the batteries. So if my panels don't produce enough, I can top off batteries overnight via normal grid power. I'll put the charger on a timer.

The most important thing is battery back up tied to an inverter/charger that can handle the critical loads for about 3 days. This assumes having 800-1200 watts of panels producing about 30% due to cloudy conditions for the three days in a row. And during "good" months (June-September) we should get about 5-6 sun hours per day, so it might be able to provide the necessary power.

I'll probably go with a set up from EG4. Their EG4 3000 watt inverter/charger and three EG4 Lifepower 48V batteries.

Notes for people's replies.....correct, NW PA it not a very good place for solar compared to out west like New Mexico. But I like the no noise aspect. If SHTF I can take the stuff to the BoL. and not draw attention when I'm using. (of course light discipline will be used at nighttime).

Since the HVAC will be all electric, the house will get a 500 gallon propane tank for heat back up. I already have a ProComm vent less propane fireplace to install. The house will also get an antique wood burning parlor stove. That's why on my list in the OP I have box fans. I figure I'd have to move that heat around in a power outage.

I have a couple 4k gasoline generators that can tie into a transfer panel if we lose power for longer than the batteries can supply. Or just use the genny to charge the batteries.

And if we lose power, managing power usage from the batteries will be priority.....No microwave popcorn at that point! But coffee is a must.

Thank you colorado and lasers...you gave the real time info and lit a fire for me to go seek more.

ColoradoMinuteMan said:

It’s amazing how much of a difference clouds make.

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This is something I wish I could put data up against. I have heard some suggest that mono panels will outproduce poly panels as the clouds roll in. And then there is one post that comes up when I try to search it that suggests that the mono panels do great down to a certain level of cloudiness where they fall flat and the poly panels just keep on producing.

I know Will Prowse did a video where he covered a part of each of several panels with a piece of paper and noted the results, which really weren't anything very dramatic. But I'm not sure that was really an accurate representation of "cloudy". Direct sunlight is the best but clouds do more than drop the intensity, they also change the characteristics, primarily dispersing the light, not just directly blocking it.

I get a lot of cloudy days here (north-central TN) in wintertime, too, so it's something I'm interested in. Without buying a few panels and actually trying it out for myself, I'm not sure I'll get an answer... unless someone has already done it and wants to share, like real panels in real cloudy / overcast weather, like side by side. Big numbers for maximum output on sunny days is great! But the cloudy days are the ones that throw the turd in the punch bowl. I'd sacrifice some of the high end to add to the low end, if that's what it took. (?)