Sizing Your Off-Grid Solar System: How to Actually Calculate What You Need (Not Guess) ⚡🔋

Every off-grid solar post online tells you the same three things: “figure out your loads,” “size your battery bank,” “get enough panels.” Cool. Thanks. None of them show you the actual math with real numbers, so you’re left guessing — and guessing on a solar system means either wasting thousands of dollars overbuilding, or getting stranded in the dark because you underbuilt.

I’m not going to do that to you. I’m going to walk you through exactly how I sized my own system out here in Valle Vista — real loads, real battery specs, real panel counts — so you can run the same math on your own numbers and land on a system that actually fits how you live instead of how a spec sheet assumes you live. 🧮

Step 1: Figure Out Your Real Daily Load (Not Your Guessed Load)

Before you buy a single panel or battery, you need to know how many watt-hours you actually burn in a day. This is the step everybody skips, and it’s the one that determines literally everything downstream.

Walk through every single thing that draws power and multiply its wattage by how many hours a day it actually runs:

• 2x 12K BTU window AC units at 1,200W each = 2,400W while running
• 2x gaming PCs at roughly 500W each = 1,000W while running
• That’s a running load of around 3,400W just from those items alone

That number matters two ways: your peak load (can your inverter handle everything running at once without tripping?) and your daily energy use (how many watt-hours total do you burn across a full day, which determines battery size). I actually hit this exact wall — my system kept shutting itself off whenever both window ACs kicked on together, because the combined draw exceeded my inverter’s 3K continuous rating. Symptom: random shutdowns under heavy load. Solution: either upsize the inverter or cut the load — I’m doing both, upgrading the inverter and replacing those window units with inverter mini-splits, which pull far less on startup and while running.

The lesson: your peak load has to fit under your inverter’s continuous rating, with headroom. Don’t just add up your appliances and assume it’ll work — check what actually happens when they all kick on together.

Step 2: Size Your Battery Bank Around Your Real Load, Not a Guess

Once you know your daily watt-hour use, you can size a battery bank that’ll actually get you through a night — or a stretch of bad weather — without leaving you stranded.

Here’s my actual bank: 6x lithium packs (8S configuration), 29V nominal / 33.6V full charge, 67Ah each, wired all in parallel. That works out to 402Ah total, right around 11.6kWh of usable capacity.

That’s not a number I picked out of thin air — it’s built to comfortably cover my running loads with room for cloudy days, without me babysitting the state of charge constantly. If you’re running less than I am — no gaming PCs, more efficient cooling — you can size down. If you’re running more, size up. The math scales, but it has to start from your real numbers, not mine or some generic “average household” figure that doesn’t reflect your actual life.

One thing that trips people up: battery specs aren’t just about total capacity. My boost charge setting needed adjustment from 31.6V up to 33.2V to match how these specific packs actually want to charge. Get the right capacity, then dial in the charging parameters to match your specific batteries — don’t just set it and forget it.

Step 3: Size Your Panel Array to Actually Feed That Battery Bank

Panels are where people either wildly overspend or leave themselves short, because panel math involves more than just “how many watts total.”

My current string: 9 panels in series, using a mix of 320W and 250W polycrystalline panels, totaling 2,880W. That string runs around 305V open-circuit / 324V Vmp — numbers that matter because they have to land inside your inverter’s MPPT voltage window, or the whole string is useless no matter how many watts it’s rated for on paper.

Here’s the part nobody tells beginners: you will end up with unused panels in reserve, and that’s not a failure, that’s normal. I’ve got 11 more 320W panels and 7 more 250W panels sitting in reserve right now, waiting on an inverter upgrade before I bring a second string online. Solar systems get built in phases as your loads grow and your budget allows — very few people nail the “final” system on the first install.

Panel mount angle matters more than people think, too. Mine sit at 15-20° right now for summer, oriented 171° true south, with a target of around 35° for a better year-round average. A few degrees of tilt is free extra output — don’t skip dialing that in once your panels are up.

Step 4: Match Your Inverter to Everything Above It

Your inverter is the piece that ties the whole system together, and it needs headroom on every spec — not just enough to handle today’s loads, but tomorrow’s.

I’m currently running a 24V, 3,000W-continuous inverter, and upgrading to a 24V hybrid unit rated for 4200W continuous, 8400W surge, a 6200W max PV array rating, and a 60-500V MPPT range that gives my panel strings real room to work in. The upgrade isn’t because the current one is broken — it’s because my loads have grown past what a 3K inverter comfortably handles, and I’d rather upgrade proactively than keep running into overload shutdowns.

Sizing rule of thumb: don’t buy the inverter that exactly matches your loads today. Buy the one that matches where your loads are headed once you add that mini-split, that second fridge, that workshop tool you know you’re getting eventually.

Putting It All Together

Here’s the actual sequence, in order, every time:

1. Add up your real daily loads — appliance by appliance, hours per day, not guesses
2. Check your peak load against what your inverter can handle running simultaneously
3. Size your battery bank to cover your daily watt-hour use plus a cushion for bad weather
4. Size your panel array to fully recharge that battery bank on an average day, keeping string voltage inside your inverter’s MPPT window
5. Size your inverter with headroom for where your loads are going, not just where they are today

Skip step 1 and everything downstream is a guess dressed up as an engineering decision.

One thing that’s genuinely made this easier for me: keeping actual records of system specs, panel counts, battery parameters, and load changes over time instead of trying to hold it all in my head. That’s part of why I built HomePlot — it’s meant for exactly this kind of homestead and property tracking, so when you’re six months into a build and can’t remember your exact MPPT range or which boost voltage you settled on, it’s logged instead of lost. If you’re doing a build like this yourself, having that record matters more than you’d think once you’re troubleshooting a problem at 11pm and need your own past notes to figure out what changed. 📱

Next up in this category: I’ll walk through the actual problems I’ve run into — what the symptoms looked like and how I solved them — so you’re not starting from zero the first time your system acts up.

See you out here. ⚡🏜️