If you’ve ever stood in the middle of a remote cabin at dusk, with no grid power and a fridge full of perishables, or planned an off-grid van trip where you can’t rely on gas stations for small appliances, you’ve probably asked this question: “What size off-grid solar system do I actually need?” As someone who’s been selling off-grid setups for years, I’ve lost count of how many customers show up thinking they just need to slap a panel on the roof and call it a day—only to come back a month later because their AC died at 2 a.m. or their well pump couldn’t draw enough power. The truth is, there’s no one-size-fits-all answer here, but I’ll break it down simply, no jargon, so you don’t waste cash on too much (or too little) gear. Off Grid Solar System
First, let’s start with the basics: an off-grid system isn’t just solar panels. It’s panels, batteries, inverters, and a charge controller—all working together to store power for when the sun’s not shining. So sizing it means calculating two key numbers: how much power you use in a day (that’s your daily load) and how much sun your location actually gets (that’s your peak sun hours, or PSH). Skipping either part is a recipe for frustration.
Let’s talk daily load first, because that’s the number you control most. I always tell customers to list every single device they’ll use, how long they run it each day, and then add them up. But here’s the trick: don’t just plug numbers from the back of your devices into a calculator. Most people forget that things like fridges or well pumps cycle on and off—your fridge only runs about 30% of the time, not nonstop. For example, a 100W laptop used 4 hours a day is 400Wh, a 150W TV run 3 hours is 450Wh, a 200W well pump that runs 1 hour is 200Wh, and a 100W fridge (average for a 12V model) that runs 8 hours total a day is 800Wh. Add those up, and your daily load is 1850Wh, or ~1.85 kWh. That’s a small off-grid setup—perfect for a tiny cabin or a van. If you add a washing machine, a space heater, and a chest freezer, that number jumps to 5-7 kWh for a small home. I’ve had customers with big family cabins hit 12 kWh a day, which is a whole different ballgame.
Next, peak sun hours. This is not the number of hours the sun is up—like 10 hours in summer—this is the actual sunlight intensity that turns into power. For example, a place like Arizona gets ~5.5 PSH all year round, while the Pacific Northwest only gets ~3.5, and Alaska gets just 1.5 in winter. If you ignore this, you’ll size your panels wrong. Let’s go back to that 1850Wh daily load. If you’re in Arizona (5.5 PSH), you don’t need big panels—you just need enough to cover 1850Wh divided by 5.5, which is ~336W of panels. But if you’re in Oregon (3.5 PSH), that same load needs ~529W of panels. That’s a $200 difference just because of location—easy mistake if you don’t check your area’s PSH. Pro tip: I tell all customers to use the lowest winter PSH for their area, because that’s when power is tightest. A lot of people size for summer, then wonder why their lights dim in December.
Now, the batteries. This is the part where people cut corners and regret it. Batteries store power for nights or cloudy days, so you never want to run them completely dead—deep cycling them (draining below 50%) shortens their life a lot. The rule of thumb here is: your battery bank should be able to store at least 2-3 days of your daily load, and you should only use 50% of their capacity on average. Let’s stick with that 1850Wh daily load. Multiply that by 2 days of backup: 3700Wh. Divide by 0.5 (since you only use half the capacity) and you get 7400Wh of battery capacity. Most 12V lithium batteries are 100Ah (which is 1200Wh), so that’s a 7400Wh bank, which would be six 100Ah lithium batteries. If you go lead-acid (which are cheaper upfront but shorter-lived), you actually need a bigger bank because they can only handle 30% depth of discharge, so you’d need ~12,300Wh of lead-acid batteries. I usually push lithium for off-grid because even though they cost more, they last 10+ years vs 3-5 for lead-acid—so it’s cheaper in the long run.
Then there’s the inverter. The inverter converts DC power from your panels/batteries to AC power for regular devices. You need an inverter that can handle the peak power of your biggest device, not just your total daily load. For example, if your well pump is 200W, but it has a 1000W startup surge (common for pumps, AC, and power tools), your inverter needs to be at least 1000W, not 200W. I see people buy a 500W inverter and wonder why it can’t run their well pump—because inverters have to handle that initial kick. Rule of thumb: pick an inverter rated for 2x your biggest device’s startup power, minimum. For small cabins, 1000W inverters work; for homes with AC or electric ranges, you’ll need 3000W or more.
Wait, but let’s test this with real customer examples, not just numbers. Last month, a guy came in wanting a system for his 200 sq ft off-grid van. His daily load: laptop (400Wh), phone chargers (50Wh), mini fridge (200Wh), and a 100W cooler for camping. That’s 650Wh total. He lives in Colorado, which gets ~4.5 PSH year round. So panels: 650 / 4.5 = ~145W. But wait, he sometimes runs a portable space heater for 2 hours a night in winter—add that 200W x 2h = 400Wh, so now daily load is 1050Wh. Panes jump to 1050 / 4.5 = 233W. Then batteries: 1050Wh x 2 days = 2100Wh, so ~2 100Ah lithium batteries. Inverter: his space heater is 200W, startup 600W, so a 1000W inverter. That set him back ~$1,800 total. He was worried that was too expensive, but when he told me he tried a smaller system last year that died after 6 months, he got it.
Another example: a family of four with a 1,200 sq ft cabin in northern Minnesota. Their daily load: fridge (800Wh), well pump (500Wh), TV/lights (400Wh), electric stove oven (1,000Wh for 1 hour a day), and a small electric water heater (1,500Wh for 1 hour a day). That’s 4,200Wh total daily load. Minnesota winter PSH is ~2.8, so panels: 4200 / 2.8 = 1500W. Batteries: 4200 x 2 = 8400Wh, so 7 100Ah lithium batteries. Inverter: oven and water heater have a 3,000W startup each, so a 6,000W inverter. That’s a $6,500 system, but they said their old generator cost $150 a month in gas and broke every other winter. So it paid for itself in a year and a half.
Now, common mistakes to avoid here. First, not accounting for growth. I had a customer a few years ago who sized his system just for his initial load, then decided to add a chest freezer and a portable AC a year later. He had to add $1,200 worth of panels and batteries because his original system was too small. Always add 10-15% extra capacity for future upgrades—it’s way cheaper upfront than retrofitting later. Second, ignoring shading. If your panels are under a big tree most of the day, their actual PSH drops by half. If that’s your setup, add extra panels to compensate. Third, buying cheap panels. A lot of people go for the $0.50/watt panels from random brands, but those have 25-year performance guarantees vs $0.70/watt panels with 10-year guarantees. The cheap ones will lose power fast, so you’ll end up needing more panels anyway.
Wait, what if you’re not sure how to calculate your load? I tell customers to track their power use for a week. Grab a cheap watt meter (they’re $10 at hardware stores) and plug each device into it for a day. Write down how much power each uses, how long, and add it up. No guessing, no mistakes. That’s way more accurate than using average numbers.
Now, let’s wrap this up: the key steps are 1) calculate your daily load exactly, 2) find your location’s average PSH (winter minimum), 3) size panels to cover that, 4) size batteries for 2-3 days backup at 50% depth, 5) get an inverter that handles your biggest device’s startup surge. If you follow these, you won’t end up with a system that can’t power your coffee maker or a system that costs twice as much as you need.
If you’re still confused, or you have a specific setup (van, cabin, tiny home, remote workshop), don’t guess. We’ve helped hundreds of customers size their off-grid systems correctly, so they don’t waste money or deal with power outages. Send us your details, we’ll walk you through it, no sales pressure, just real advice from folks who’ve been in the off-grid game for years. We’ll make sure you get exactly what you need, not a one-size-fits-all kit that doesn’t work for your space.
Lithium Battery References:
- National Renewable Energy Laboratory (NREL) — Peak Sun Hour Data by U.S. Location, 2023
- Off Grid Solar Association — Deep Cycle Battery Depth of Discharge Guidelines, 2022
- International Electrotechnical Commission (IEC) — Solar Panel Sizing for Off-Grid Applications, 2021
- U.S. Department of Energy — Inverter Startup Surge Requirements for Off-Grid Systems, 2023
Hebei Mutian Solar Energy Technology Development Co., Ltd.
Hebei Mutian Solar Energy Technology Development Co., Ltd. is one of the most professional off grid solar system manufacturers and suppliers in China, featured by quality products and good service. Please rest assured to buy off grid solar system in stock from our factory. Contact us for customized service.
Address: Room 3120, Block A, Zhili Building, 77 Fuxing West Road, Baoding City, Hebei Province, China
E-mail: mona@solarmt.com
WebSite: https://www.solar-systemkit.com/
