Most off-grid solar systems don’t fail because someone bought bad panels. They fail because of bad math — or no math at all.
I’ve seen it play out both ways. Someone spends $30,000 on a system with more battery capacity than a small cabin will ever use in a week, because a salesperson told them “bigger is safer.” And I’ve seen the opposite: someone builds a lean, budget-friendly setup, runs it for ten days, and their batteries are dead by day six because nobody sat down and actually calculated the daily load.
If you’re here, you’re probably trying to avoid both of those outcomes. Good. That’s exactly what this guide is for — a real sizing process you can walk through with your own numbers, not vague advice to “calculate your energy needs” without ever showing you how.
Quick answer: To size an off-grid solar system, you add up your daily energy use in watt-hours, divide by your location’s peak sun hours to get panel wattage, then size a battery bank based on how many days of backup power (autonomy) you want, adjusted for your battery’s usable depth of discharge.
That’s the whole framework. Now let’s actually run the numbers.
Step 1: Figure Out Your Daily Energy Use
This is the step almost everyone rushes, and it’s the one that determines everything else. Get it wrong here and your panel count, battery bank, and budget are all wrong too.
List every device you’ll run, its wattage, and how many hours a day it actually runs — not how many hours it’s plugged in, but how many hours it’s drawing power.
Here’s a worked example for a 400 sq ft cabin running a fridge, some LED lighting, a laptop, and a water pump:
| Appliance | Watts | Hours/Day | Watt-Hours/Day |
| Fridge (compressor cycles ~30%) | 150W | 8 hrs equivalent | 1,200 Wh |
| LED lighting (6 bulbs) | 60W total | 5 hrs | 300 Wh |
| Laptop + charging | 65W | 4 hrs | 260 Wh |
| Water pump | 400W | 0.5 hrs | 200 Wh |
| Misc (router, phone charging) | 20W | 6 hrs | 120 Wh |
| Total | ~2,080 Wh/day |
Round that up to 2.5 kWh/day with a bit of headroom, because real life is messier than a spreadsheet — someone leaves a light on, a friend visits with a laptop, whatever.
For context, a typical small cabin runs somewhere between 2–4 kWh/day, while a full-time off-grid house with a well pump, washer, and more appliances can easily hit 8–12 kWh/day. If you’re not sure where you land, this step is worth doing properly before you buy a single panel.
Step 2: Choose Your Solar Panels
Once you know your daily watt-hour need, you divide it by your region’s peak sun hours — the number of hours per day the sun delivers roughly 1,000 W/m² of usable intensity, not just daylight hours. This varies a lot by latitude and season, which is exactly why generic sizing guides get this wrong so often.
If you want the deeper explanation of what peak sun hours actually measures and how to look up your local number, it’s worth a quick read on peak sun hours before you commit to a panel count.
Using our cabin example at 2.5 kWh/day and, say, 4.5 peak sun hours (typical for a lot of the continental US in shoulder seasons):
2,500 Wh ÷ 4.5 hours = ~556W of solar needed, before accounting for system losses.
Add 20–25% for real-world inefficiencies — wiring loss, dust, panel angle not being perfect, inverter conversion loss — and you land around 700W of panel capacity. That’s two 350W panels or three smaller ones, depending on your roof or mounting space.
A quick comparison of panel types:
| Type | Efficiency | Cost | Best For |
| Monocrystalline | 18–22% | Higher | Limited roof space, best long-term output |
| Polycrystalline | 15–17% | Lower | Budget builds, flat land with room to spread out |
| Thin-film | 10–13% | Lowest, most flexible | RVs, curved surfaces, portable setups |
Monocrystalline gets recommended constantly, and for good reason — it’s the most efficient per square foot. But if you’ve got open land and space isn’t the constraint, polycrystalline can genuinely save you money without sacrificing much. Thin-film makes sense almost exclusively for RVs and boats where flexibility and weight matter more than raw output.
Also worth deciding early: whether you’re going with a DC-coupled or AC-coupled setup, since that affects how your panels, batteries, and inverter talk to each other. If you’re not sure which fits your situation, this breakdown of DC vs AC coupled solar is a good next stop.
Step 3: Size Your Battery Bank
This is where most guides get vague, so let’s not do that.
The formula: Daily kWh usage × desired days of autonomy ÷ usable depth of discharge = battery capacity needed.
“Days of autonomy” just means how many cloudy or low-sun days you want to survive without recharging. Most off-grid setups target 2–3 days.
Let’s use our 2.5 kWh/day cabin example with 3 days of autonomy:
2.5 kWh × 3 days = 7.5 kWh of total storage needed — before adjusting for depth of discharge.
Here’s where battery chemistry actually matters:
- Lead-acid batteries are cheaper upfront but you can only safely use about 50% of their capacity (50% DoD) without shortening their lifespan badly. So you’d need 15 kWh of lead-acid capacity to get 7.5 kWh usable.
- LiFePO4 (lithium iron phosphate) batteries can handle 80–90% DoD safely. At 85%, you’d need roughly 8.8 kWh of LiFePO4 capacity for that same 7.5 kWh usable.
LiFePO4 costs more per kWh upfront — often close to double. But lead-acid batteries typically need replacing every 3–5 years, while a decent LiFePO4 bank can run 8–10+ years with proper care. Run the math over a decade and LiFePO4 usually wins for anyone using the system regularly, not just occasionally. For a weekend cabin used a handful of times a year, lead-acid can still make financial sense — you’re just not cycling it enough for the lithium premium to pay off.
If you want the full side-by-side on lifespan, charging behavior, and cold-weather performance, that’s covered in more depth on our <a solar system types comparison.
Step 4: Charge Controllers and Inverters
Your charge controller regulates how power flows from panels to batteries, and the choice here materially affects your system’s efficiency.
MPPT (Maximum Power Point Tracking) controllers are more expensive but roughly 20–30% more efficient than PWM, especially when your panel voltage is higher than your battery bank voltage. For most off-grid builds today, MPPT is worth the extra cost.
PWM (Pulse Width Modulation) controllers are cheaper and simpler, but they waste more power converting voltage and generally only make sense for very small systems — think a single panel charging a small battery for a shed or a basic RV setup.
For your inverter, buy a pure sine wave inverter, not modified sine wave. Modified sine wave is cheaper but can damage or poorly power sensitive electronics like laptop chargers, some fridges, and anything with a motor.
One thing people consistently forget: inverter sizing isn’t just about your total wattage — it’s about surge load. A fridge compressor or water pump can draw 2–3x its running wattage for a second or two on startup. If your inverter can’t handle that surge, it’ll trip or shut down. Size your inverter’s surge rating with that in mind, not just its continuous rating.
Step 5: Do You Need a Backup Generator?
Honestly, for most cabins a generator is overkill. A slightly bigger battery bank is usually the better dollar-per-comfort trade, especially since generators mean fuel storage, maintenance, and noise.
Where a generator does make sense: full-time off-grid living in a low-sun region, or running high-draw equipment occasionally — power tools, a well pump, a workshop — where sizing your whole battery bank around rare peak loads would be wildly expensive. In that case, a generator sized to your peak surge load, used sparingly, is the more sensible fix.
DIY or Professional Install?
If you’re comfortable with basic electrical work, low-voltage DC wiring for a small system is genuinely learnable. Panel mounting, running conduit, and basic wiring aren’t beyond a capable DIYer.
Where I’d draw the line: anything involving AC wiring into a home’s panel, battery bank installations above 48V, or work in your local jurisdiction that requires permitting. Get a licensed electrician involved there — not because DIYers can’t learn it, but because mistakes at that voltage and scale are the kind that start fires, not just trip a breaker.
Common Mistakes People Make
- Undersizing wire gauge. Thin wire over long runs causes voltage drop and heat — a real fire risk, not just an efficiency loss.
- Ignoring winter sun-hour drop. Your summer peak sun hours can be double your winter number depending on latitude. Size for your worst realistic month, not your average.
- Forgetting inverter surge ratings. Covered above, but it’s the single most common reason a “properly sized” system trips constantly.
- Skipping the 20–25% buffer. That margin isn’t padding for its own sake — it covers real losses from wiring, dust, temperature derating, and imperfect panel angles.
- Buying batteries before finishing the load calculation. Batteries are the most expensive part of the system. Get Step 1 right before you spend money here.
Real Cost Ranges
For a small cabin system like our example (700W panels, ~9 kWh LiFePO4 battery bank, MPPT controller, pure sine inverter), expect somewhere in the $4,000–$7,000 range for components, more if you pay for professional installation. A larger full-time off-grid home system with 8–12 kWh/day usage can run $15,000–$30,000+.
Compare that to running grid power lines to remote property — utilities often quote $15,000–$50,000+ per mile depending on terrain. If you’re more than a quarter mile from the nearest connection point, off-grid solar frequently comes out cheaper, and you skip the monthly bill entirely. If you’re close to existing lines, grid extension or a hybrid solar system might genuinely make more financial sense — it’s worth running both numbers before deciding.
It’s also worth understanding how off-grid differs from grid-tied setups with battery backup, since people sometimes conflate the two. If that’s you, this comparison clears it up quickly, as does a look at how on-grid solar systems work by contrast.
FAQs
How do you size an off-grid solar system?
Add up your daily energy use in watt-hours, divide by your local peak sun hours to get required panel wattage, then add 20–25% for system losses. Size your battery bank by multiplying daily usage by your desired days of autonomy, then dividing by your battery’s usable depth of discharge.
How many solar panels do I need to run a house off-grid?
It depends entirely on your daily energy use and local sun hours, but a typical small cabin using 2–3 kWh/day needs roughly 600–800W of panels, while a full-time home using 8–12 kWh/day may need 2,500–3,500W or more.
What’s the difference between MPPT and PWM charge controllers?
MPPT controllers are more efficient, especially when panel voltage exceeds battery voltage, and typically deliver 20–30% more usable power than PWM. PWM controllers are cheaper and simpler, suited mainly to small, low-power systems.
How long do off-grid solar batteries last?
LiFePO4 batteries typically last 8–10+ years with proper care, while lead-acid batteries usually need replacing every 3–5 years due to their lower safe depth of discharge and shorter cycle life.
Can I install off-grid solar myself?
Basic DC wiring and panel mounting for a small system is realistic for a capable DIYer. AC integration, higher-voltage battery banks, and anything requiring local permitting is worth handing to a licensed electrician.
Is off-grid solar cheaper than running power lines?
Often, yes — especially past a quarter mile from the nearest grid connection, where utility line extension costs can run $15,000–$50,000+ per mile. Closer to existing infrastructure, grid extension or a hybrid setup may be more cost-effective.
If you’re still deciding between system types altogether, it’s worth stepping back and reading through how off-grid, on-grid, and hybrid solar systems actually compare before you lock in your design — and if you’re weighing solar against other renewable options for a remote property, solar vs wind energy is worth a look too.
Once you’ve got your numbers — daily load, panel wattage, battery capacity — you’re not guessing anymore. You’re building a system sized for the life you actually run, not the one a sales quote assumed you’d run.
