how many solar panels to run a house off grid — the real math

Every solar installer website has a version of this article and every one of them makes it sound simpler than it is. "Just divide your daily usage by panel output and that's how many panels you need!" Great. Except that calculation works perfectly if you live on the equator, never have clouds, and don't care about having electricity in December.

I've been running a solar-and-battery hybrid system at my place for over two years now. I've lived through the winter months when my panels produced about half of what the spec sheets promised. I've watched my battery bank drain to nothing during a five-day stretch of overcast skies in January. And I've fired up my generator more times than I care to admit in the first year before I sized things properly.

So here's the actual math. The uncomfortable math. The kind that makes solar salespeople change the subject.

start with how much power your house actually uses

The average American home uses about 30 kWh per day. That's roughly 900 kWh per month, or about 10,800 kWh per year. If you've never looked at your electric bill closely, go pull it up. The number you want is your monthly kWh usage.

But "average" is doing a lot of heavy lifting in that sentence. A 1,200 sq ft house in Oregon with gas heat and two people might use 15 kWh/day. A 3,500 sq ft house in Texas with electric everything and a family of five might use 50-60 kWh/day. Your number is your number, and it's the only one that matters.

Here's what drives residential electricity consumption:

• Heating and cooling: 40-50% of total usage for most homes. Electric heat pumps, central AC, and space heaters are the biggest draws. If you heat with gas or propane, your electrical load drops significantly.
• Water heating: 15-20% if electric, near zero if gas.
• Appliances: Refrigerator, washer, dryer, dishwasher, oven. Another 15-20%.
• Lighting, electronics, miscellaneous: 10-15%. TVs, computers, phone chargers, that one grow light in the basement, whatever.
• Well pump: If you're off-grid, you probably have a well. A well pump can use 1-3 kWh/day depending on depth and frequency.

For this article, I'm going to use 30 kWh/day as the baseline because it's the national average and it makes the math clean. If your usage is different, scale accordingly.

the simple math (and why it's wrong)

Here's the calculation you'll see on every solar blog and installer website:

Step 1: Daily usage = 30 kWh
Step 2: A 400W panel produces about 1.6 kWh per day (400W x 4 peak sun hours)
Step 3: 30 kWh / 1.6 kWh = 18.75 panels
Step 4: Round up to 19 panels

Nineteen panels. Done. Easy. Go buy 19 panels and live off the grid.

Except you can't. And here's why.

peak sun hours are an average, not a guarantee

The "4 peak sun hours" number is a national annual average. It means that on average, across the entire year, across the entire country, you get the equivalent of 4 hours of direct, unobstructed sunlight hitting your panels at full intensity.

But averages are liars. In Phoenix, you get 6-7 peak sun hours. In Seattle, you get 3-4. And those are annual averages — in winter, Seattle drops to about 1.5 peak sun hours per day. Phoenix drops to about 5.

If you're sizing your system for the annual average, you'll overproduce in summer and come up drastically short in winter. For off-grid, you have to size for your worst month, not your average month. Nobody cares that your panels made plenty of power in July if you're sitting in the dark in January.

panel degradation is real

Solar panels lose about 0.5% of their output per year. That doesn't sound like much, but after 10 years your 400W panel is producing 380W. After 20 years, 360W. The spec sheet number is what you get on day one. Every day after that, it gets a little worse.

Good system design accounts for at least 10 years of degradation in the sizing calculation. If you're building a system to last 25 years (and you should be), you either oversize now or accept declining performance later.

inverter and system losses eat 15-25%

The electricity your panels produce has to go through several conversions before it powers your toaster. DC from the panels gets converted to AC by the inverter. If you're charging batteries, that's another conversion (AC to DC going in, DC to AC coming out). Every conversion loses energy as heat.

Typical system losses:

• Inverter efficiency: 93-97% (lose 3-7%)
• Battery round-trip efficiency: 85-95% for lithium, 75-85% for lead-acid (lose 5-25%)
• Wiring losses: 1-3%
• Soiling (dust, pollen, bird droppings): 2-5%
• Temperature derating: 5-15% on hot days (panels lose efficiency as they heat up — ironic, I know)
• Shading: Varies wildly. Even partial shading on one panel can drag down an entire string.

Stack those up and your real-world system output is 75-85% of the theoretical maximum on a good day. On a bad day — hot, hazy, partially shaded — you might be at 60%.

the real math: 25-35 panels

Let's redo this calculation honestly.

Daily usage: 30 kWh
System losses: 20% (conservative estimate for a well-designed lithium battery system)
Adjusted daily need: 30 / 0.80 = 37.5 kWh
Design for winter: Assume 3 peak sun hours instead of 4 (realistic for most of the U.S. in December/January)
Panel output in winter: 400W x 3 hours = 1.2 kWh/day per panel
Panels needed: 37.5 / 1.2 = 31.25 panels

That's 32 panels to get through winter without regularly draining your batteries to zero. In summer, those same 32 panels will overproduce substantially — which is fine, you can use the excess to top off batteries, run the AC harder, or just let it go.

Here's how the number changes by location:

Look at Portland. Sixty-three panels to run a house off-grid through winter using solar alone. That's not a rooftop system — that's a small solar farm. Which is exactly why almost nobody in the Pacific Northwest goes fully off-grid without a generator.

battery storage: the expensive half nobody mentions

Here's the thing about solar panels: they're actually the cheap part. The expensive part — the part that makes or breaks your off-grid system — is the batteries.

Solar panels produce power for about 6-8 hours a day. You use power for 24 hours a day. So you need enough battery storage to get you from sunset to sunrise, plus enough reserve to survive cloudy days without running a generator.

The standard design target for off-grid systems is 2-3 days of autonomy. That means your battery bank can power your home for 2-3 full days with zero solar input. Less than that and you'll be firing up the generator every time a weather system moves through.

Battery sizing math:

Daily usage: 30 kWh
Days of autonomy: 2-3
Usable capacity needed: 60-90 kWh
Depth of discharge (lithium): 80-90%
Total battery capacity: 67-112 kWh

For context, a Tesla Powerwall 3 has 13.5 kWh of usable capacity. You'd need 5-7 Powerwalls. At roughly $10,000-$12,000 installed per unit, that's $50,000-$84,000 in batteries alone.

Other battery options exist — and some are better for off-grid than the Powerwall — but the cost per kWh of usable storage is in the same ballpark across lithium iron phosphate (LFP) batteries: $400-$700 per kWh installed. Check my breakdown of the best home battery backup systems for specific recommendations.

Lead-acid batteries are cheaper upfront ($150-$300/kWh) but only give you 50% depth of discharge, degrade faster, and need regular maintenance. For a new off-grid system in 2026, lithium is the only serious option unless you have a very compelling reason to go lead-acid.

full off-grid solar system cost breakdown

Let's put the whole picture together for a 30 kWh/day home going fully off-grid. I'm using mid-range pricing — not the cheapest, not the most expensive.

Yes, those are big numbers. A mid-range full off-grid system for an average American home is roughly the price of a new car. A nice car. And that's after the tax credit.

Compare that to a whole-house generator at $10,000-$25,000 installed and you can see why most people don't go fully off-grid with solar. The economics only make sense if you're in a location with no grid access (the cost to run utility lines can exceed $50,000 for remote properties) or if energy independence is a priority you're willing to pay a premium for.

why most "off-grid solar" homes still have a generator

Walk around any off-grid community and you'll hear generators running. Not every day — but regularly. And this isn't a failure. It's good system design.

The math works like this: sizing your battery bank for a worst-case scenario — say, a week of heavy cloud cover in December — means buying enough batteries to store 150-210 kWh. That's an additional $30,000-$50,000 in batteries beyond what you'd need for 2-3 days of autonomy. Or you can buy a $5,000-$15,000 generator that burns $30-$50/day in fuel for the 50-100 hours a year you actually need it.

The generator is cheaper. Dramatically cheaper. And it eliminates the single biggest risk of off-grid solar: the extended cloudy period that your battery bank wasn't quite big enough to handle.

This isn't a compromise. It's the smartest way to design an off-grid power system. Even the most committed solar advocates I know — people who've been off-grid for a decade — keep a generator. They run it 30-80 hours a year, spend $500-$1,500 on fuel, and sleep well knowing they won't wake up to dead batteries in February.

If you're shopping for a generator to pair with solar, a portable solar generator might seem appealing but for true whole-home backup you want a real fuel-burning generator. Something in the 8-12kW range is usually enough since the generator only needs to cover your base load while giving the batteries a partial charge — it doesn't need to power the whole house solo.

slim's hybrid approach (what I actually run)

My system isn't purely solar and it isn't grid-tied. It's what I call a "solar-primary hybrid" and it's the approach I recommend for most people who want to be off-grid or mostly off-grid.

Here's what I'm running:

• Solar array: 28 panels, 400W each = 11.2 kW nameplate capacity
• Battery bank: 40 kWh usable (lithium iron phosphate)
• Inverter: Two 5kW hybrid inverters, 10kW total
• Backup generator: 10kW propane, auto-start when batteries hit 20%

From April through October, the generator basically doesn't run. The solar array produces 40-55 kWh/day and my house uses 25-30 kWh. The excess tops off the batteries and I'm usually at 100% state of charge by early afternoon.

November through February is a different story. Solar output drops to 15-25 kWh/day. On cloudy days, I might only get 8-12 kWh. The batteries cover the gap most of the time, but during extended cloudy stretches — three or more days of heavy overcast — the generator kicks in. It usually runs 4-8 hours, charges the batteries back up to 80%, and shuts off.

Over the past year, my generator ran about 60 hours total. That's roughly $300 in propane. My system produces about 85% of my annual electricity from solar, and the generator covers the other 15%.

Could I add more panels and more batteries to eliminate the generator entirely? Yes. It would cost about $35,000-$45,000 and save me $300/year in propane. That's a 117-year payback. I'll keep the generator.

reducing your panel count: the cheaper option nobody wants to hear

The most cost-effective way to reduce the number of solar panels you need is to use less electricity. I know. Revolutionary. But hear me out because the leverage here is enormous.

Every kWh you eliminate from your daily usage removes roughly one panel from your array AND reduces your battery needs AND reduces your inverter sizing. The savings cascade.

Real changes that make a real difference:

• Switch to a propane or wood stove for primary heat. Electric heating is the single biggest drain on an off-grid solar system in winter — exactly when solar production is lowest. A high-efficiency propane heater or wood stove takes your winter electrical load from 40-50 kWh/day down to 20-25 kWh/day. That alone can cut your panel count by a third.
• Gas water heater instead of electric. Saves 4-6 kWh/day. That's 3-4 fewer panels.
• Gas dryer or clothesline. An electric dryer uses 3-5 kWh per load. If you're doing laundry daily, that's meaningful.
• LED everything. You probably already have, but if you haven't, this is low-hanging fruit. An old house with incandescent bulbs might use 3-5 kWh/day just on lighting. LEDs cut that to 0.5-1 kWh.
• Right-size your refrigerator. That big side-by-side uses 2-3 kWh/day. A well-insulated chest-style fridge or a smaller modern unit uses 0.5-1 kWh.
• Insulation and air sealing. The less energy your house leaks, the less your HVAC has to work. This doesn't reduce panel count directly but it reduces battery drain during peak demand hours.

I've seen people go from a 35-panel design to a 20-panel design just by switching heating and water heating to propane and being intentional about large appliance usage. The money saved on panels and batteries far exceeds the cost of propane.

the roof space problem

Even if you can afford 30+ panels, you need somewhere to put them. A 400W panel is roughly 21 sq ft. Thirty panels need about 630 sq ft of south-facing, unshaded roof space. That's a lot of roof.

Most homes have 500-1,000 sq ft of usable roof space after you account for orientation, pitch, vents, chimneys, dormers, and the setbacks required by fire code (most jurisdictions require 3-foot pathways for firefighter access). A complex roof with multiple angles and obstructions might only have 300-400 usable sq ft.

If your roof can't fit enough panels, you have three options:

• Ground-mounted array: No roof limitations. You can aim them perfectly south, set the optimal tilt angle, and scale as large as you want. Costs about 10-20% more per watt than roof-mounted due to racking and trenching, but gives you complete flexibility. This is what most serious off-grid setups use.
• Higher-wattage panels: 500W+ panels are available and shrink the footprint by 20-25% compared to 400W panels. They cost more per watt but might make a tight roof work.
• Combination: Max out the roof and put the rest on a ground mount. More complex but sometimes the best use of available space.

If you're building a new off-grid home, design the roof for solar from day one. Simple roof lines, south-facing pitch, no dormers or chimneys on the south face. It's dramatically easier to plan around solar than to retrofit it.

what about grid-tied with battery backup?

If you're not going fully off-grid but want serious backup power, a grid-tied solar system with battery backup is a fraction of the cost and covers 90% of the same use cases.

A grid-tied system with 10-15 panels (4-6 kW), a single battery unit (10-15 kWh), and an automatic transfer switch gives you 8-12 hours of backup power during outages while reducing your grid bill by 30-60% the rest of the time. Total cost: $20,000-$40,000 before the tax credit.

That's not off-grid. But if your goal is resilience — keeping the lights on when the grid fails — rather than full independence, it's the sweet spot for most homeowners. You get solar benefits every day, battery backup when you need it, and the grid as your infinitely large backup generator.

I wrote a full comparison of home battery backup options if you're leaning this direction.

the bottom line on going off-grid with solar

Going fully off-grid with solar is possible, proven, and expensive. The technology works. People do it every day. But the honest numbers are bigger than what you'll see on most websites because most websites are trying to sell you panels, not prepare you for reality.

Here's what I'd tell a friend:

• If you're building on a remote property with no grid access: Off-grid solar is almost certainly your best option. The cost to run utility lines to a remote site can exceed $50,000-$100,000, which makes a solar system look reasonable by comparison. Budget $80,000-$130,000 for a properly sized system.
• If you're on-grid but want independence: A hybrid grid-tied system with battery backup gives you 90% of the benefit at 30% of the cost. Start there.
• If you're somewhere in between: Start with a solar assessment, run the numbers with a sizing calculator, and be honest about your budget and your tolerance for generator usage.

The worst thing you can do is undersize the system because the real numbers scared you. An undersized off-grid solar system means running your generator constantly, which defeats the purpose and costs more in fuel than you saved by buying fewer panels. Size it right or don't do it.