How to Size an Off-Grid Solar System for a Michigan Winter

A solar system that runs your home easily in June can come up short the week before Christmas. Days are short, the sun sits low over the tree line, gray skies settle in, and snow covers the panels just as the lights, well pump, and furnace blower are working their hardest. A grid-tied home leans on DTE Energy or Consumers Energy when that happens. An off-grid home has nothing to lean on.

That’s why we size off-grid systems for the darkest stretch of the Michigan year. Once the winter numbers work, the rest of the year takes care of itself. Here’s how that sizing works, step by step, and how the three main pieces of the system (the solar array, the battery bank, and the generator) get balanced so the system is reliable without being overbuilt.

Why Off-Grid Systems Are Sized for Winter

An off-grid system has to meet your needs every single day on its own, so it has to be designed around the hardest days it will face. Having no utility to fall back on is the core difference between grid-tied and off-grid solar systems: a grid-tied system that comes up short in December simply pulls the difference from the grid, while an off-grid shortfall means a dead battery bank.

Solar designers handle this with what’s called the critical design month: the month when your electrical load is highest compared to the sunlight available to meet it. Penn State’s solar design curriculum describes sizing a stand-alone system around that month, and even choosing the panel tilt that produces the most energy during it.

In Michigan, the critical month is usually December or January. Several things stack up at once:

• The days are at their shortest, so there are fewer hours of production.
• The sun sits low in the sky, so its light is weaker when it reaches the panels.
• Cloud cover is common for long stretches.
• Snow can cover the panels entirely.
• Your electrical load climbs, with longer nights of lighting, furnace blowers or boiler pumps running more often, and heat tape or block heaters in use.

You can look up the monthly figures for your own property in NREL’s free PVWatts calculator, which reports expected production month by month for any address.

Cloudy days aren’t zero-production days. Solar panels still produce power without direct sunlight, just less of it. Snow-covered panels are a different story, and those days are a big part of why the battery bank and generator steps below matter so much.

Step 1: Measure Your Winter Load

The first number we need is how much electricity your home uses on a typical winter day, measured in kilowatt-hours (kWh). An annual average hides the problem, because winter use is usually higher at exactly the time solar production is lowest.

On a rural Michigan property, the winter list usually includes:

• A well pump, which runs every time someone uses water
• A furnace blower or boiler circulator pump, even when the heat itself comes from propane or wood
• A refrigerator and often a chest freezer
• Lighting through winter nights that run around 15 hours
• A sump pump during thaws
• Heat tape on water lines and engine block heaters

Every kilowatt-hour you remove from that list is one you don’t have to generate in December, which makes load reduction the cheapest way to shrink an off-grid system. The biggest wins come from keeping heat-producing loads off the electrical system. Electric resistance space heaters, electric water heaters, electric ranges, and electric dryers are among the largest loads a home can have. Running those on propane, natural gas where it’s available, or wood keeps the solar and battery system a manageable size. Efficient appliances and LED lighting help too.

Surge Demand Is a Separate Number

Daily energy use isn’t the only electrical number that matters. A well pump or a freezer compressor draws a short burst of power when it starts that’s well above what it uses while running. That surge doesn’t change how many panels or batteries you need, but it does determine how large the inverter has to be. An inverter that can’t handle the surge will shut down every time the pump kicks on, no matter how full the batteries are.

Step 2: Size the Array for the Darkest Month

With your winter load in hand, the array calculation is straightforward. Divide your winter daily load by the peak sun hours your site gets in the critical month, then add an allowance for system losses. Peak sun hours measure a day’s sunlight as the equivalent number of hours at full strength, so a short, gray December day might deliver only a fraction of what a long June day does.

Losses come from wiring, inverter conversion, dirt and snow on the panels, and panel temperature. Penn State’s design course cites system losses of up to about 25%, which is why the raw calculation gets increased before it becomes an array size.

Tilt matters more off grid than it does on most grid-tied homes. A steeper tilt points the panels more directly at the low winter sun and helps snow slide off sooner after a storm. That’s one reason ground mounts are popular for off-grid Michigan properties: they can be set at a winter-friendly angle and are easy to reach with a soft roof rake, while roof pitch decides the angle on a roof mount.

The array you end up with will usually look oversized for summer. That’s expected. The extra summer production is the cost of keeping the lights on in December.

Step 3: Size the Battery Bank for Days Without Sun

The battery bank is what carries your home through nights and through stretches when the array produces little or nothing. Sizing it comes down to three inputs.

Days of autonomy. This is how many days the bank can run your home with no help from the panels. Penn State’s course shows how the number climbs as sunlight drops and reliability needs rise: its examples range from about 3 days for a site with strong sun and modest reliability needs to about 10 days for critical loads at a site with less sun. A Michigan winter design sits toward the higher-need end, which is where the generator in Step 4 comes in.

Usable capacity. No battery should be drained to zero. Each battery chemistry has a recommended depth of discharge, the share of its rated capacity you can use regularly without shortening its life, and the bank has to be sized so your daily needs fit within that share.

Temperature. Cold batteries deliver less of their rated capacity, so a bank sized at room-temperature ratings will come up short in January unless the design accounts for it.

Where the Batteries Live Matters

That makes battery placement a core part of the design. The bank belongs in conditioned space or in an enclosure built to keep it within its temperature range through a Michigan winter. Panels can work well on an outbuilding, and solar panels on a pole barn roof are a common choice on rural properties. The batteries, though, usually belong somewhere warmer than an unheated barn. Equipment selection matters here too, since some batteries include built-in heating designed for cold climates. We help homeowners compare those options as part of our solar battery storage work, recommending whatever fits the property rather than a single brand.

Step 4: Decide What the Generator Covers

Nearly every off-grid system in Michigan includes a generator, and for good reason. Building a battery bank big enough to carry a home through a two-week January overcast would mean a very large and very expensive bank that sits mostly unused the rest of the year. A generator that starts automatically when the batteries run low covers those rare stretches for a fraction of the cost.

The real design question is how often you’re comfortable running it. A system designed for the generator to run only a handful of times each winter needs more array and more battery. A system that leans on the generator more often can be smaller and less expensive up front, with more fuel and maintenance over time. Neither is wrong. It depends on your budget, your fuel access, and how much generator noise you’re willing to live with.

The three levers work together:

• More array shortens the recovery time after a cloudy stretch and covers more winter days directly.
• More battery rides out more sunless days before the generator starts.
• More generator runtime lets both of the others be smaller.

A Worked Example

Here’s how the math comes together for one hypothetical home. Every number in this example is illustrative. It shows how the calculation works and is not a quote, a recommendation, or a typical Michigan system size. Your load, your site’s sun hours, and your equipment will change every line.

Input	Illustrative value	What it means
Winter daily load	12 kWh per day	Measured from the home’s winter appliance list
Critical month peak sun hours	1.5 hours per day	A placeholder; use your site’s December value from PVWatts
Loss allowance	25%	Wiring, inverter, snow, dirt, and temperature
Array size	10 kW	12 ÷ 1.5 = 8 kW, increased 25% for losses
Days of autonomy	3 days	Sunless days covered before the generator starts
Usable depth of discharge	80%	Share of rated capacity used regularly
Cold temperature factor	90%	Capacity the bank can deliver in winter conditions
Battery bank size	50 kWh	12 × 3 = 36 kWh, divided by 0.8 and by 0.9

Now watch what the levers do. If the same household moves its clothes dryer and water heater to propane and cuts winter use to 9 kWh a day, the array drops to 7.5 kW and the battery bank drops to 37.5 kWh, with the same reliability. If the household instead keeps the 12 kWh load but lets the generator start after two sunless days instead of three, the battery bank drops to about 33 kWh, and the generator runs more often. Load reduction shrinks everything at once, which is why it comes first.

Michigan Factors That Change the Answer

The general method works anywhere. These are the Michigan-specific details that change the inputs.

Cloud Cover Varies Across the Lower Peninsula

Lake-effect clouds form when cold air crosses the relatively warm Great Lakes, and they’re a regular part of a Michigan winter. A NOAA study of 17 cold seasons found lake-effect clouds somewhere in the Great Lakes region on at least 60% of days in every month from October through March, with cloud bands over Lake Michigan especially common from December through February. Two properties a couple of hours apart can face very different winters, which is why we model each site individually instead of using one number for the whole state.

Snow and Wind Loads

Michigan arrays have to carry the weight of heavy snow and stand up to winter wind. Racking for an off-grid system is engineered for those loads, and the winter-friendly steeper tilt that helps with production also helps snow clear on its own.

Permits Still Apply Off Grid

Off grid doesn’t mean outside the building code. Off-grid solar and battery systems still go through permits and inspections, and the requirements vary from one Michigan municipality to the next. Going off grid in Michigan comes with its own legal and code requirements, and we handle the full permitting process on behalf of our customers.

Plan for Loads You’ll Add Later

Winter sizing is based on the loads you have today, but most homes add more over time. An electric vehicle, a heated workshop, or a second freezer can change the winter math significantly. A good design leaves room to add panels and batteries later, so a new load doesn’t mean starting over.

What a Professional Off-Grid Design Adds

The rough calculation above is a useful way to understand the tradeoffs, but a design you can build and live with goes further. A professional off-grid design includes a full inventory of your loads, including the ones people forget, along with surge analysis for pumps and compressors, winter production modeling for your exact site including tree shading, battery siting for the cold, and generator integration so the system starts and stops it automatically. It also produces the equipment specifications and documentation your municipality will want to see at permit time.

That’s the work we do for off-grid solar design and installation on Michigan properties. We’ve been designing and installing solar systems since 2010, and because we’re manufacturer-agnostic, every recommendation is based on what your property and your winter actually need.