Off-grid is the one solar project where there's no safety net. A grid-tied homeowner who undersizes their system loses a little money. An off-grid cabin owner who undersizes loses lights on a February night — or worse, deep-discharges their batteries into an early grave.
So sizing matters more here than anywhere else in solar. The good news: it's a four-step process, and every step uses numbers you can gather in an afternoon. Here's the complete method, with worked examples for a weekend cabin and a full-time home.
Step 1: Count Your Loads (the Step Nobody Does Properly)
Everything starts with a load list. Walk through your property and write down every device you plan to run, its wattage, and hours per day. Multiply watts × hours for each, then total them up.
Example — weekend cabin:
| Appliance | Watts | Hours/Day | Daily Wh |
| LED lights (8 bulbs) | 80 | 5 | 400 |
| Refrigerator | 150 | 8 (cycling) | 1,200 |
| Water pump | 500 | 0.5 | 250 |
| Laptop + phone charging | 100 | 3 | 300 |
| TV | 80 | 3 | 240 |
| Coffee maker | 900 | 0.25 | 225 |
| Daily total | ~2.6 kWh |
Example — full-time off-grid home (no electric heat/AC): lights, fridge, pump, cooking appliances, laundry, entertainment — typically 8–15 kWh/day. Add mini-splits or electric cooking and you're in 15–30+ kWh/day territory.
The number people forget: surge
Watts on the label aren't the whole story. Motors and compressors — well pumps, fridges, power tools — draw 2–3× their rated wattage for a second or two at startup. A 750W pump can spike past 2,000W. Your system must survive the worst simultaneous moment, not the average hour. When you total your loads, also note the biggest surge: that single number will pick your inverter for you.
Step 2: Size the Inverter
The inverter converts DC battery power to AC. Two specs decide it:
- Continuous output (W or kW) — must exceed the total of everything running at once. Your coffee maker (900W) and water pump (500W) and lights (80W) on together = ~1,500W continuous.
- Surge capacity — must cover your largest startup spike, with headroom.
Rule-of-thumb mapping from real projects:
| Property type | Inverter size |
| Tiny cabin, weekend use | 3–3.5 kW |
| Standard cabin or small full-time home | 5–6 kW |
| Large full-time home, heavy loads | 8–10 kW |
The Growatt SPF 3500TL LVM-US ($799) fits the first tier: 3.5kW continuous AC output with 4.5kW of solar input and an integrated MPPT charge controller — and it's one of the few inverters in its class that runs with or without batteries, which makes it a flexible starter unit. See the SPF 3500TL specs.
For a standard full-time home, the Growatt SPE 6000 US ($1,599) delivers 6kW of 120/240V split-phase output — the same voltage as a US utility panel, so standard appliances and breaker panels just work — with 9kW of DC solar input across dual MPPTs and a 550V PV input window that gives your array design room. Check the SPE 6000 specifications.
Step 3: Size the Battery Bank
Off-grid batteries carry the house through every night and every cloudy stretch, so this is where off-grid systems differ most from grid-tied ones: the bank must be bigger.
Battery kWh = Daily use × Days of autonomy ÷ Usable depth of discharge ÷ System efficiency
- Days of autonomy: 2 for sunny climates, 2–3 where winters are grey, 3–5 if you have no backup generator
- Usable DoD: 0.8 for LiFePO4 (its BMS protects the cells; you never manage this by hand), 0.5 for lead-acid
- System efficiency: 0.9
Worked example — weekend cabin, 2.6 kWh/day, 2 autonomy days: 2.6 × 2 ÷ 0.8 ÷ 0.9 = 7.2 kWh → two 5 kWh modules
Worked example — full-time home, 12 kWh/day, 2.5 autonomy days: 12 × 2.5 ÷ 0.8 ÷ 0.9 = 41.7 kWh → nine 5 kWh modules
Go LiFePO4. The cycle-life math alone settles it: 4,000–6,000 cycles (11–16 years of daily use) versus 500–1,000 for lead-acid, and 80% usable capacity versus 50%. Our 48V home battery collection covers the 5 kWh-module building blocks, and our guide on how many batteries a home needs goes deeper on this step.
Step 4: Size the Solar Array
Your panels must refill the bank — fully — on a realistic winter day, not a summer one. A panel's rated output is a lab number; real-world conditions deliver roughly 4–5 peak sun hours per day in most of the US, less in winter.
Array size (kW) = Daily use ÷ (Sun hours × System efficiency)
Worked example — cabin, 2.6 kWh/day, 4 sun hours: 2.6 ÷ (4 × 0.8) = 0.8 kW → three to four 350W panels
Worked example — full-time home, 12 kWh/day, 4 sun hours: 12 ÷ (4 × 0.8) = 3.75 kW → ten to twelve 400W panels
Three array-design checks before you buy panels:
- PV input voltage window. Count panels per series string so the cold-morning open-circuit voltage stays under the inverter's max PV input (the SPE 6000's 550V window gives generous room for series strings).
- Number of MPPT trackers. Panels on different roof faces or orientations need separate MPPTs to perform — two roof planes minimum means you want dual MPPT.
- Generator input. For off-grid reliability, an auto-generator-start input isn't a luxury. During a week of storms, it's the difference between a managed system and a dark house.
Put It All Together: Two Reference Systems
Weekend cabin (~2.6 kWh/day): 3.5kW inverter + 10 kWh battery + ~1 kW of panels. Budget-friendly, fully self-sufficient for Friday-to-Sunday living.
Full-time home (~12 kWh/day): 6kW split-phase inverter + 40–45 kWh battery + ~4 kW of panels. This is the shape of Growatt's SPE 6000 + 20kWh complete kit — inverter, battery bank, and matched components designed to work as one system, expandable as your loads grow.
Frequently Asked Questions
What size off-grid system does a typical cabin need? A weekend cabin using 2–4 kWh/day runs comfortably on a 3.5kW inverter, 8–10 kWh of LiFePO4 battery, and about 1 kW of panels. Full-time cabins need 2–4× that.
Can an off-grid inverter run without batteries? Some can — the SPF 3500TL works with or without batteries, powering loads directly from solar during the day. Without batteries, though, you have no power at night or in clouds, so most off-grid homes treat batteries as non-negotiable.
What happens after three cloudy days? If your autonomy days were sized honestly, the bank still holds. If not, the generator auto-start kicks in — which is why we recommend keeping a generator input in any off-grid design, even if you rarely use it.
Is off-grid cheaper than paying for a grid extension? Extending utility lines commonly costs 30,000–100,000+ per mile in rural America. For remote properties beyond a few hundred meters from the line, a properly sized off-grid system usually pays for itself outright.
Do off-grid systems need permits? Requirements vary by county. Off-grid systems generally skip utility interconnection paperwork, but electrical permits and inspection still apply in most jurisdictions — hire a licensed electrician for the final hookups.
The bottom line: loads → inverter → battery → array, in that order, and always check surge before anything else. If you'd rather have someone run the numbers with you, browse Growatt's off-grid inverter collection — or contact our team for a free custom system design built around your actual load list.
