Sizing
How to Size an Off-Grid Solar System
Sizing a system is the step most beginners dread, but it is really just five additions and a couple of divisions. Work through them in order and you will know exactly how much battery, how many panels, and how big an inverter you need.
The five steps, in order
- List every device and how long you run it.
- Add up your daily energy use in watt-hours.
- Size the battery bank from that daily number.
- Size the solar panels to refill the battery each day.
- Size the inverter for your biggest moment of demand.
Step 1: List your loads
Write down everything you want to power, its wattage, and how many hours a day you use it. Wattage is usually printed on the device or its charger. For each item, multiply watts by hours to get watt-hours per day.
Step 2: Add up daily watt-hours
Sum the watt-hours of every load. This total is the amount of energy your system has to deliver in a day. It is the most important number in the whole process, so get it as honest as you can.
Step 3: Size the battery bank
Decide how many days of backup (autonomy) you want, usually 1 to 3. Multiply your daily watt-hours by that number. LiFePO4 lets you safely use about 90 to 100 percent of its rated capacity, so divide by 0.9 to leave a small buffer. The result is your battery bank size in watt-hours.
Step 4: Size the solar panels
Take your daily watt-hours and divide by your local sun-hours (use a conservative winter figure if you live off-grid full time). Then add roughly 25 percent to cover wiring losses, controller losses, and panels rarely hitting their full rating. That gives you the total panel wattage you need.
Step 5: Size the inverter
Add up the watts of everything that might run on AC at the same moment, your peak demand. Your continuous inverter rating should comfortably exceed that. Also check the surge rating, because motors (fridges, pumps) briefly draw several times their running watts when they kick on.
Worked example: a campervan
Daily loads for a simple van:
- 12V fridge: 40W average × 24h = 960Wh (compressor cycles, so this is the realistic daily figure)
- LED lights: 15W × 4h = 60Wh
- Phone and laptop charging: 70W × 3h = 210Wh
- Roof fan: 10W × 8h = 80Wh
Daily total: about 1,310Wh. For 2 days of autonomy: 1,310 × 2 = 2,620Wh, divided by 0.9 = about 2,900Wh of battery. A common 12V 200Ah LiFePO4 bank is 2,560Wh, so one large battery or a pair gets you there.
Panels: 1,310Wh divided by 4.5 sun-hours = 291W, plus 25 percent = about 365W. Two 200W panels (400W) comfortably covers it. Inverter: the van has no big AC loads, so a 600W to 1,000W pure sine inverter is plenty. This lines up closely with our Classic 400W RV build.
Worked example: a small cabin
Daily loads for a part-time cabin:
- Apartment-size fridge: 1,200Wh per day
- Lights: 100W × 5h = 500Wh
- Laptop, phones, router: 150W × 5h = 750Wh
- Water pump: 50W × 1h = 50Wh
- Small TV and misc: 500Wh
Daily total: about 3,000Wh. For 2 days of autonomy: 3,000 × 2 = 6,000Wh, divided by 0.9 = about 6,700Wh of battery. A 48V server-rack battery (commonly around 5,000Wh each) or two would fit nicely.
Panels: 3,000Wh divided by 4 winter sun-hours = 750W, plus 25 percent = about 940W. Five 200W panels (1,000W) covers it. Inverter: a cabin runs several things at once, so a 3,000W (or 48V all-in-one) inverter gives headroom. This mirrors our off-grid cabin system.
Recommended gear and a faster way
Want to skip the arithmetic? The free off-grid solar calculator runs these same steps from a few inputs, and the System Builder turns the result into a parts list. To see how the numbers map to real hardware, browse our complete builds.
The two parts your sizing usually decides first are the battery and the panels:
Check 200Ah LiFePO4 Price on Amazon Check 200W Panel Price on Amazon
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Frequently Asked Questions
What is a watt-hour, simply?
A watt-hour (Wh) is one watt of power used for one hour. A 60W light left on for 5 hours uses 60 x 5 = 300Wh. Adding up the watt-hours of everything you run in a day is the single most important number when sizing a system.
How many sun-hours should I use?
Sun-hours are the number of hours per day your panels make near their full rated power. A safe planning figure for much of the US is 4 to 5 in summer and as low as 2 to 3 in deep winter. Use a winter number if you live off-grid year round.
Why size the battery for more than one day?
If you only store one day of power, a single cloudy day leaves you empty. Most off-grid builds aim for 1 to 3 days of autonomy so a stretch of bad weather does not knock you offline.
Should I just use a calculator instead of doing this by hand?
A calculator is faster and great for a sanity check, but doing it once by hand teaches you what every number means, so you can spot a kit that is undersized. Use our solar calculator to confirm your figures.
What size off-grid solar system do I need for a small house?
Add up your daily watt-hours first, because that drives everything else. A small house running a fridge, lights, a water pump, and devices often lands around 5,000 to 10,000Wh per day, which points to roughly 2 to 4kW of panels and a battery bank of 10kWh or more. Electric heat or air conditioning change this a lot, so size for the appliances you actually plan to run.
How do I calculate battery size for an off-grid system?
Take your daily watt-hours, multiply by the days of autonomy you want, then divide by how deeply you can safely discharge the battery. For LiFePO4 you can use about 90 percent, so 3,000Wh per day times 2 days, divided by 0.9, is about 6,700Wh of battery. Round up to the next real battery size.
How much does an off-grid solar system cost?
Cost tracks size, and the battery bank is usually the most expensive part. A small weekend setup is a modest investment, while a full-time off-grid home system with large storage runs well into the thousands. The cheapest way to lower the bill is to cut your daily power use before you buy, so always size the system first, then price the parts.