Quick answer
An off-grid solar system gets built in a fixed order, and the order is the roadmap:
- Audit your loads — total daily watt-hours, honestly counted.
- Size the battery bank — daily Wh × days of autonomy ÷ usable depth of discharge.
- Size the array — daily Wh ÷ worst-month peak sun hours ÷ 0.75–0.85 for real-world losses.
- Size the inverter — simultaneous running watts × 1.25, then verify the worst motor-start surge.
- Pick the charge controller and wire every circuit — MPPT amps ≈ array watts ÷ battery volts × 1.25, fuses sized to conductor and code.
- Permit and inspect — off-grid permitting is lighter than grid-tied but rarely zero; call your AHJ before buying hardware.
Panels are the fourth thing you size, not the first. A DIY off-grid build runs roughly $1,100–$2,900 in parts for a 1 kWh/day weekend cabin and $5,400–$16,000 for a 7 kWh/day full-time home — batteries dominate either budget. Each step below carries worked math and links to the deep-dive guides, and the solar maintenance guide covers what keeps a build healthy after commissioning.
Key takeaways
- The sizing chain is loads → battery → array → inverter → controller/wiring → permits. Reversing it is how systems end up undersized.
- Battery bank Wh = daily Wh × days of autonomy ÷ DoD. LiFePO4 delivers ~80–90% usable; lead-acid ~50%.
- Size the array on your worst sun month, not the annual average — winter output can drop by nearly half in northern states.
- Inverters fail on surge, not running watts: motors need 2–3× their running watts to start, well pumps 3–7×.
- MPPT beats PWM in cold weather and low light; sizing rule: array watts ÷ battery volts × 1.25.
- Off-grid skips the utility interconnection agreement but usually still needs an electrical permit under NEC Article 690.
- The 30% federal ITC expired December 31, 2025 (P.L. 119-21) — 2026 installs get no federal credit, so DIY is the biggest cost lever.
Step 1: Audit your loads — the number everything else flows from
Every component is downstream of one number: daily watt-hours. The formula is watts × hours = Wh per day, per device. Walk the space room by room, list everything that plugs in or runs on a thermostat, and be honest about run times — a forgotten device is a daily deficit you’ll feel every winter.
Watch the three classic traps:
- Cycling loads. A 150W fridge doesn’t draw 150W × 24h. The compressor runs 30–50% of the time, so a full-size unit averages roughly 1,000–1,440 Wh/day. A $20 plug-in wattmeter over 24 hours captures this automatically.
- Phantom loads. Routers, chargers, and standby electronics add 50–150W of continuous draw — up to 1.2–3.6 kWh/day if ignored.
- Seasonal loads. Size for the worse season. Winter usually loses: more heating and lighting demand and less sun.
Worked example — a small full-time cabin:
| Device | Watts | Hours/day | Wh/day |
|---|---|---|---|
| Fridge (150W, 40% duty cycle) | 150 | 9.6 | 1,440 |
| LED lighting (whole cabin) | 40 | 5 | 200 |
| Water pump | 300 | 0.5 | 150 |
| Laptop + router + phones | 90 | 8 | 720 |
| TV | 100 | 3 | 300 |
| Phantom/misc standby | 25 | 24 | 600 |
| Total | 3,410 |
Call it 3,400 Wh/day and carry that number through every step below. Full worksheet method: how to calculate your solar load.
Step 2: Size the battery bank (days of autonomy × DoD)
The battery bank is sized from the load audit, not from panel output:
Battery Wh = daily Wh × days of autonomy ÷ usable DoD
Days of autonomy is how long the system runs on stored energy with no meaningful solar input. Most builds plan 1–2 days; beyond that, a backup generator is usually cheaper than a third day of lithium.
DoD depends on chemistry:
| Chemistry | Usable DoD | Practical trade-off |
|---|---|---|
| LiFePO4 (lithium) | ~80–90% | Higher upfront cost, 3,000+ cycle ratings, light, maintenance-free |
| Flooded/AGM lead-acid | ~50% | Cheaper per nameplate kWh, but you buy roughly double the capacity and replace it sooner |
Worked example (our 3,400 Wh/day cabin, 2 days of autonomy):
- LiFePO4: 3,400 × 2 ÷ 0.85 = 8,000 Wh ≈ 8 kWh bank — at 48V that’s 8,000 ÷ 48 ≈ 167 Ah (four 48V 100Ah modules gets you close; four 12V 200Ah units in series/parallel also works).
- Lead-acid: 3,400 × 2 ÷ 0.50 = 13,600 Wh ≈ 13.6 kWh bank — 71% more nameplate capacity for the same usable energy.
That asymmetry is why daily-cycling off-grid systems are almost always lithium today. Compare chemistries in lithium-ion vs lead-acid; run your numbers with the battery capacity calculator.
Step 3: Size the array to refill the bank in your worst month
The array’s job is to replace a full day’s usage plus system losses, on a bad-sun day, in the season you actually use the system:
Array watts = daily Wh ÷ peak sun hours ÷ system efficiency
Peak sun hours are not daylight hours — most US locations average 3.5–6.5 per day annually, but the worst-month figure can be half the annual average in northern states. Use 0.75–0.85 as the efficiency factor (heat, wiring, controller and inverter losses); drop to 0.75 for a conservative off-grid estimate.
Worked example (3,400 Wh/day):
- Annual-average planning at 4.5 sun hours: 3,400 ÷ 4.5 ÷ 0.8 = 944W → ~1,000W of panels
- Worst-month planning at 2.5 sun hours: 3,400 ÷ 2.5 ÷ 0.8 = 1,700W of panels
If the cabin is used year-round, the honest answer sits between those numbers — or you accept generator assist in deep winter. A 400W panel produces roughly 1,100–1,600 Wh/day in average US sun, so the 1,000–1,700W range means 3–5 large panels. Location-specific numbers: solar panel output calculator.
Step 4: Size the inverter for running watts and surge
Add up everything that can run at the same time, multiply by 1.25 for headroom, then check the worst startup moment — your biggest motor starting while everything else runs. Motors need roughly 2–3× their running watts to start; hard-start loads like well pumps can briefly demand 3–7×.
Worked example (our cabin): evening simultaneous load = fridge 150W + lights 40W + TV 100W + laptop/router 90W + water pump 300W = 680W; add a 1,000W microwave → 1,680W peak running.
- With 25% headroom: 1,680 × 1.25 = 2,100W → pick a 2,500–3,000W inverter
- Worst-case startup: the pump (300W running, ~1,800W surge) starting while everything else runs: 1,680 − 300 + 1,800 = 3,480W surge — a 3,000W pure-sine inverter with a typical 2× surge rating (6,000W) covers it
Buy pure sine output (modified sine runs motors hot and can damage electronics), and verify the surge rating on the datasheet, not the marketing page. Run your own load list through the inverter sizing calculator; see pure sine vs modified sine for why the waveform matters.
Step 5: Charge controller, wiring, and protection
Controller sizing. MPPT controllers are rated by output amps at battery voltage:
Controller amps ≈ array watts ÷ battery volts × 1.25
Our 1,000W array on a 48V bank: 1,000 ÷ 48 × 1.25 = 26A → a 30A MPPT. Sized up to the 1,700W winter array: 1,700 ÷ 48 × 1.25 = 44A → a 60A class unit or two 30A controllers. MPPT beats PWM in cold weather and partial sun — MPPT vs PWM explains why; charge controller cost covers pricing tiers.
Wiring and protection. Off-grid systems concentrate the highest currents of any solar build — the battery-to-inverter run especially — so protection is not optional:
- Array side: fuse/breaker sized to panel short-circuit current × 1.56
- Battery side: fuse sized to the inverter’s max draw (3,000W ÷ 48V ÷ ~0.85 efficiency ≈ 74A → 100A class fuse and appropriately heavy cable)
- Disconnects: every current path gets one you can reach without touching a live conductor
Details in the wiring cluster: wire size and voltage drop, fuse and breaker sizing by circuit, battery cable size for inverters, series vs parallel array wiring, combiner boxes and disconnects.
Step 6: Permits and inspection reality for off-grid structures
Off-grid systems skip the utility interconnection agreement — no grid export, no utility sign-off — so permitting is lighter than grid-tied. But it is rarely zero:
- Most AHJs (Authorities Having Jurisdiction) still require an electrical permit for permanently installed PV — NEC Article 690 applies whether or not the grid is involved.
- A building permit may apply to ground and roof mounts — wind and snow loads don’t care that you’re off-grid.
- Insurance is the sleeper issue. Many insurers won’t pay a fire or electrical claim on an unpermitted system, and unpermitted installs can complicate a home sale.
Call your local building department before buying hardware: ask which NEC edition they enforce and what the inspection checklist covers. RV and mobile systems usually skip building permits entirely, but the electrical work should still meet NEC for insurance. Full walkthrough: solar permits and building codes.
Budget table: three system sizes, DIY parts
Using the sizing math above (2 days autonomy, LiFePO4 at $150–$300/kWh in equipment, panels at $0.40–$1.20/W, worst-month array for the year-round cases):
| Weekend cabin | Full-time cabin | Full-time home | |
|---|---|---|---|
| Daily load | 1 kWh/day | 3.4 kWh/day | 7 kWh/day |
| Array (worst month) | 300–500W | 1,000–1,700W | 2,000–3,500W |
| Battery bank (2 days) | ~2.4 kWh | ~8 kWh | ~16.5 kWh |
| Inverter | 1,000–1,500W | 3,000W | 6,000–8,000W |
| MPPT controller | 15–20A | 30–60A | 60A+ or hybrid all-in-one |
| Panels | $120–$600 | $400–$2,000 | $800–$4,200 |
| Batteries | $360–$720 | $1,200–$2,400 | $2,500–$5,000 |
| Inverter | $150–$400 | $300–$1,500 | $1,000–$3,000 |
| Controller | $120–$350 | $150–$500 | $300–$900 |
| Wiring + protection | $200–$500 | $300–$800 | $500–$1,500 |
| Mounting | $150–$400 | $200–$800 | $400–$1,500 |
| DIY parts total | ~$1,100–$2,970 | ~$2,550–$8,000 | ~$5,500–$16,100 |
Context: turnkey installed systems run far higher — installed home batteries alone bill at roughly $1,000–$1,400/kWh, versus $150–$300/kWh for DIY LiFePO4 equipment, and grid-tied installs price at $2.50–$3.50/W before batteries. DIY typically lands 40–60% below an installed quote, and since the 30% federal ITC expired December 31, 2025, no federal credit softens a 2026 quote. Component detail: off-grid cabin solar cost breakdown and solar system cost breakdown.
First 30 days after commissioning
- Install a battery monitor (shunt) before day one. Voltage alone lies about state of charge; a shunt-based monitor is the only honest fuel gauge, and it catches sizing mistakes while they’re cheap to fix.
- Log daily Wh in and Wh out. After two weeks you’ll know your real consumption versus the audit — most people underestimated by 10–20%.
- Verify charge settings against the battery datasheet. Absorption/float voltages, and for lithium, the low-temperature charge cutoff — charging LiFePO4 below freezing without a heater can permanently damage cells.
- Check every termination for heat. After the first week of charge cycles, feel lugs at the battery, controller, and inverter. A warm connection is a loose or undersized one.
- Test the limits deliberately. Run the bank to your planned DoD floor once and confirm the low-voltage cutoff behaves; stage the worst-case startup moment and confirm the inverter holds.
- Re-torque after thermal cycling. Connections loosen slightly as temperatures swing; recheck terminations around week three.
- Clean the array and re-baseline. Note output on a clear day at solar noon for a reference number in future troubleshooting.
Common startup mistakes
- Sizing the array on summer sun. The annual average flatters the math. If the system must work in January, size it on January’s peak sun hours — in much of the northern US that nearly doubles the array.
- Buying panels first. Panels are the fourth thing you size. Starting at the array end leaves a bank that never fully charges — the root cause of “solar doesn’t work” complaints, since chronically undercharged batteries die early.
- Skipping the shunt monitor. Without real amp-hour data you’re managing the most expensive component by guesswork, and a failing string goes unnoticed until the bank is damaged.
- Undersizing the battery-to-inverter circuit. It’s the highest-current run in the system. Thin cable or an undersized fuse there means voltage sag, hot terminations, and nuisance inverter shutdowns — see battery cable sizing before buying wire.
FAQ
How much does a complete off-grid solar system cost?
How many days of battery autonomy do I actually need?
Can an off-grid system run air conditioning or a well pump?
Do I need a permit for an off-grid solar system?
Should I build at 12V, 24V, or 48V?
Next logical reads
How to calculate your solar load How to size a solar system (with load planner) Cabin solar sizing, step by step Battery capacity calculator Lithium-ion vs lead-acid batteries Solar panel output calculator How to size an inverter MPPT vs PWM charge controllers Fuse and breaker sizing by circuit Solar wire size and voltage drop Solar permits and building codes Off-grid cabin solar cost breakdown DIY vs installer cost How to install solar panels yourself Solar installation safety guide Common solar installation mistakes Solar battery maintenance guide Low solar output troubleshooting