Pipe dream or plausible?

[stacks]

Im going from no solar experience to completely off-grid solar with generator backup this year.
my concern is that i live in central michigan, and im trying to figure how much ill actually need to run the generator during the dreaded winters.

Our house pretty consistently uses about 400kWh each month from april to october, then it fluctuates up to a max of about 1000kwh during the winter months.

The solar setup i am thinking of going with is a 14.4kw bifacial array, facing south with 60° tilt, and about 92kwh of battery storage.
I ran the info through the pvwatts calculator, and it came out with having the lowest producing month being december at 970kwh.

i know that weather is never the same, but im wondering if pvwatt's estimations have been close for other people in michigan, or if theyve been pretty far off.
also, im wondering if production happens in a few small windows that are far apart, or if production happens generally pretty regularly where the 92kwh storage would carry me pretty well.

 

[FilterGuy]

The first thing to do is to reduce the load in any way you can.... It is often cheaper to conserve power than to add more solar power. You probably have already done some of the easy things but I will list them:

1) All LED light bulbs
2) Find all the constant-on loads and see if you can get rid of them. This includes things like
- Power-warts,
- networking geat,
- cable modems (These are typically big hogs)
- stereo/entertainment gear

The bigger items include

1. Better insulation
2. Heat pumps instead of electric heat or AC
3. Heat pump or gas water heaters
4. Induction cooktop instead of electric cooktop
5. Modern (more efficient) refrigerators, dishwashers, dryers, etc.

also, im wondering if production happens in a few small windows that are far apart, or if production happens generally pretty regularly where the 92kwh storage would carry me pretty well.

Just doing some math: If you have 1000KWh in a month, that is 1000/30 = 33KWh/day. At 92KWh of storage that is 92/33=2.79 days of storage.

I am not familiar with Michigan weather patterns, but my guess is you have a lot of grey days in the winter. You can still get production on grey days but I would expect to have to fire up the generator a few times during the winter. Of course, if snow builds up on the panels it will be worse.

What size generator are you considering? When sizing it, be sure to consider the load for recharging the batteries. The ideal scenario is that it only runs long enough to charge up the batteries and then shuts back down. However, if it is only sized to run the house, you won't be able to charge the batteries at the same time.

 

[Orrid]

Il tuo sogno è ambizioso e costoso, quindi gia che ci sei perché non pensi di acquistare un micro cogeneratore al posto del generatore, produrrebbe energia elettrica e ti riscalderebbe la casa durante l'inverno.

 

[Orrid]

Oh sorry.
Your dream is ambitious and expensive, so while you're at it, why not think about buying a micro cogenerator instead of the generator, it would produce electricity and heat your house during the winter.

 

[OzSolar]

i know that weather is never the same, but im wondering if pvwatt's estimations have been close for other people in michigan, or if theyve been pretty far off.
also, im wondering if production happens in a few small windows that are far apart, or if production happens generally pretty regularly where the 92kwh storage would carry me pretty well.

It's usually spot on for a grid tied system in my region. (southern Missouri)

But battery based systems aren't quite as efficient so I'd take another 5% to 10% off of that for starters.

I'd also mention that 92 kWh translates to something closer 75 usable kWh by the time you stay between the knees, factor in DC to AC losses and the inverter tare load.

Perhaps it's 80 usable kWh but maybe it's 65 kWh. My point is that in practice it will be something noticeably less than 92 kWh for planning purposes.

Also notice I'm not using 73.57 kWh. The reason is that you can't predict either daily production or usage to the decimal point for a specific day. Sure you can average it out over the month but that doesn't do you much good after 6 days of cloudy weather and record low temps for two of them.

Generator: Remember that your peak energy need will opposite your solar's production. Long cold nights followed by back to back short cloudy days means you will likely go from no generator run time during April and October to a lot more than that. Wish I could nail it down better.

Have you thought about how fast or often you want to be able to recharge the batteries? 6 hours every other day? 92/6 = 15 kW which means a 20kW generator.

Or maybe you just want to be able to replenish most of your daily usage each morning? 25 kWh/3 hrs = ~10kW generator just for battery charging.

Back when I did a few off grid systems a year we would size the generator for the (critical loads + max battery charge rate) x 120%.
So something like (3 kW + 10kW) x 120% = ~16kW. This way you can simultaneously charge batteries as fast as possible, keep your house going without having to throttle the battery charger way down plus keep the generator in it's happy place of ~80% of it's full load rating.

Your mileage may vary!

 

[stacks]

The first thing to do is to reduce the load in any way you can....

What size generator are you considering?

we've done pretty good about reducing power. all leds, heat pump ac/heat in the house, etc etc. the only exception and the reason my heat uses so much is my electric heater in my garage is not a heat pump and it CHUGS energy. i am switching that to a heat pump this summer, but id still like to prepare my system to cover myself now, then enjoy the headroom as i improve more appliances.

we are eyeing somewhere around a 10kw generator, which should be able to supply around 7kw to the batteries while also supplying the house. would probably just top up daily depending on weather forecast.

 

[stacks]

Oh sorry.
Your dream is ambitious and expensive, so while you're at it, why not think about buying a micro cogenerator instead of the generator, it would produce electricity and heat your house during the winter.

shouldnt be too expensive, relatively. ballparking be around $26k for panels, ground mount racking, cells, bms's, all-in-one's, and generator (if ordered today).
just looked into cogenerators, which seem to be just a regular generator but its liquid cooled and you use to supply heat via radiators in the targeted room? seems like a good concept, but limited options from what im seeing. most of the info/ad is about 12 years old? not adverse to it, but id rather focus on the system, then maybe adapt it as an upgrade later. definitely interesting.

 

[stacks]

It's usually spot on for a grid tied system in my region. (southern Missouri)

But battery based systems aren't quite as efficient so I'd take another 5% to 10% off of that for starters.

I'd also mention that 92 kWh translates to something closer 75 usable kWh by the time you stay between the knees, factor in DC to AC losses and the inverter tare load.

Perhaps it's 80 usable kWh but maybe it's 65 kWh. My point is that in practice it will be something noticeably less than 92 kWh for planning purposes.

Also notice I'm not using 73.57 kWh. The reason is that you can't predict either daily production or usage to the decimal point for a specific day. Sure you can average it out over the month but that doesn't do you much good after 6 days of cloudy weather and record low temps for two of them.

Generator: Remember that your peak energy need will opposite your solar's production. Long cold nights followed by back to back short cloudy days means you will likely go from no generator run time during April and October to a lot more than that. Wish I could nail it down better.

Have you thought about how fast or often you want to be able to recharge the batteries? 6 hours every other day? 92/6 = 15 kW which means a 20kW generator.

Or maybe you just want to be able to replenish most of your daily usage each morning? 25 kWh/3 hrs = ~10kW generator just for battery charging.

Back when I did off grid systems a year we would size the generator for the (critical loads + max battery charge rate) x 120%.
So something like (3 kW + 10kW) x 120% = ~16kW. This way you can simultaneously charge batteries as fast as possible, keep your house going without have to throttle the battery charger way down plus keep the generator in it's happy place of ~80% of it's full load rating.

Your mileage may vary!

ive been taking the usable power into consideration, but i didnt think there would be that much loss, ive been using 80kwh as the number. maybe ill rethink it through and be ultra pessimistic on the numbers.
ive been getting more paranoid as i talk to more people about it and was about adding another pallet, bringing it up to about 140kwh of storage

we were thinking of going with a 10kw generator. figured itd take about 3-5 hours a day to top up (on a 93kwh system), depending. we'd probably top up daily vs all at once, so if there was an issue with generation, we'd have a day or two of power to patch up the issue.

 

[shadowmaker]

With snow make sure tilt can be adjusted close to vertical. Ground clearance is also important. Panels are so cheap (at least in here) you should consider bigger array but dividing it to different azimuths if you have room for it. This way you can utilize your inverter better (inverter restricted ~100% production from sunrise to sunset instead of normal parabolic curve). This will reduce generator demand with minimal cost.

I'm grid tied so certainly not an expert on the matter.

 

[MajicDiver]

14.4kw bifacial array, facing south with 60° tilt, and about 92kwh of battery storage.

Here in southeast Tennessee I planned for 7 days of overcast sky’s in December worst case. Where I average production of 5% of what PV watts predicts. 7 days doesn’t happen every year but it will at least 1 in 3 years. This year was 3 days, that good won’t happen often. My recollection of winters I spent in upstate New York and northern Illinois early in my work life is that it’s worse for long spans of overcast days up there in general.

Also look very close for shading in the few weeks either side of 21-December. There are things you won’t realize if you just walk around looking at things other times of the year.

 

[OffGridForGood]

North of MI, I have found that no matter what you plan for, nature will through you some curve balls, and surprise ya.
This year, we had most of December Foggy - it was too mild and instead of clear cold skys, nature gave me fog and overcast. Not for a few days, no for nearly 13 days in a row. If I had my 'dream 30kW PV array' set up, I would still not have collected enough energy in December.

Generator:
I like to use a smaller generator, and not try to size this to run loads and charge battery together.
Here is my approach:
The daily loads are 30kWh.
The ESS is (now) 99kWh or about 3 days. (a bit less really with losses and keeping 20% min SOC).
The weather forecast is pretty accurate for 2-3 days.
At the end of each day, you can see how much you used, and how much solar you collected.
You can decide if you need to top-up any shortfall with the generator, or not based on the forecast and the SOC.

Say today we have 12kWh of solar collected by 5:00PM, 30-12 is a 18kWh short-fall.
You can make up the 18 kWh by running a generator for 3 hrs x 6kW/hr, and this would hold the ESS SOC level "neutral" (zero net change) for the day.
If the forecast shows good sunshine tomorrow, you can decide to NOT add the 18kWh tonight, let the ESS drop and wait for the sun. If the forecast is all clouds and fog, run the generator to make up the shortfall.

With a chargeverter and small(er) generator you can input energy you need to the ESS without trying to run the peak loads and charge the ESS at the same time. Generator kWh's are the most expensive ones, I like the generator to run at it's most efficient power point (70-75% of rated power) and run it in batch cycles ie a few hours steady running at a time at optimal load. I recommend a dual fuel generator to keep your options open as much as possible. Good luck with your new system and place, hope to see you post your progress.

 

[seneysolar]

Over-panel!

Get a top quality generator, no matter what you will use it more than you think. I prefer diesel as fuel is easy to get on weekends and holidays (not dependent on propane delivery) Fuel consumption and parts/service availability should be the deciding factor in what you purchase.

Maybe dont quit the grid cold turkey. Get your system going and working, prove the concept while keeping the option of grid power if you need. After your first winter, you can make the call.