Somebody posts a cabin build on a forum: four 400 W panels, one 5 kWh battery, works great. What they skip: the cabin sits in Arizona at 6.2 peak sun hours and the only real load is a mini fridge. Copy that setup to Vermont at 3.4 winter PSH with a well pump and you are dead by mid-January. Every off-grid solar sizing problem boils down to three mismatches — energy versus power, nameplate versus usable capacity, and summer averages versus winter minimums.
This estimator takes your daily kilowatt-hour load, local peak sun hours, desired autonomy days, and battery chemistry to produce a panel array size and battery bank capacity. Treat the numbers as a feasibility checkpoint: enough to price equipment and compare chemistries before paying an electrician for a stamped load calculation.
Daily kWh Load Audit: What Actually Runs
Grab a clipboard. Walk through the cabin and write down every device, its nameplate wattage, and the hours it runs per day. A 60 W porch light on a timer for 6 hours is 0.36 kWh. A pressure pump that cycles 40 minutes total at 750 W is 0.5 kWh. People consistently underestimate refrigeration — a chest freezer rated at 100 W can pull 250 W on compressor start and cycle eight or nine hours a day, racking up 1.5–2 kWh that never shows up in a casual guess.
Once you total the column, add 20%. Not 5, not 10 — twenty. Guests leave lights on, you add an electric kettle next winter, and the router you forgot about draws 12 W around the clock (that alone is 0.29 kWh/day). A 7 kWh audit becomes an 8.4 kWh design load, and that margin is the difference between coasting through a cloudy stretch and draining the bank to cutoff.
Peak Sun Hours and Panel Array Sizing
Peak sun hours (PSH) compresses a full day of variable irradiance into an equivalent number of hours at 1,000 W/m². Phoenix in December: about 5.2 PSH. Upstate New York in December: around 2.5. The gap is enormous, and it determines whether your cabin needs six panels or fourteen.
Always size to worst-month PSH unless a backup generator covers the deficit. The panel formula: Array kW = daily kWh ÷ (PSH × combined efficiency). Efficiency bundles wiring loss, charge-controller conversion, soiling, and mismatch — typically 0.72–0.78. For an 8.4 kWh design load, 3.5 winter PSH, 0.75 efficiency: 8.4 ÷ (3.5 × 0.75) = 3.2 kW, which is eight 400 W panels.
Verify your PSH with the NREL PVWatts tool — it pulls TMY data for any U.S. address and gives monthly expected output, which beats a forum post claiming “you get about 5 hours.”
Battery Bank: Capacity, Depth of Discharge, Chemistry
A battery labelled 10 kWh does not hand you 10 kWh. Depth of discharge (DoD) caps how far you can drain before cycle life tanks. Flooded lead-acid: 50% DoD. LiFePO4: 80–90%. That single variable means a 10 kWh lead-acid bank gives you 5 kWh usable while the same-rated lithium bank gives 8–9.
| Chemistry | Usable DoD | Cycle Life | Cold Penalty |
|---|---|---|---|
| Flooded lead-acid | 50% | 800–1,200 cycles | Loses ~1%/°C below 25 °C |
| AGM / Gel | 50% | 500–800 cycles | Similar to flooded |
| LiFePO4 | 80–90% | 3,000–5,000+ cycles | Must not charge below 0 °C |
Bank sizing formula: (daily kWh × autonomy days) ÷ DoD = nameplate kWh. At 8.4 kWh/day, 3 days autonomy, 80% DoD: (8.4 × 3) ÷ 0.80 = 31.5 kWh nameplate. That is the sticker number you shop for — the usable share is 25.2 kWh.
System Losses You Must Not Ignore
Between the panel face and the wall outlet, energy leaks at every junction. Most people lump everything into “85% efficient” and move on. That one number hides the component where chemistry choice actually swings the outcome:
- Wiring — 2–4%; longer runs or thinner gauge cost more.
- MPPT controller — 3–5% conversion overhead.
- Battery round-trip — 10–15% lead-acid, 5–8% LiFePO4. This is the big lever.
- Inverter — 5–10%; worse at light loads where fixed standby draw dominates.
- Soiling and snow — 2–5%, seasonal.
Over a year of daily cycling, the lead-acid round-trip penalty burns an extra 400–600 kWh compared to lithium on an 8 kWh/day system — often justifying the higher LiFePO4 price within three to four years.
Surge is a separate axis. A well pump drawing 700 W continuous may spike to 2,100 W on start. The inverter must handle that peak or it faults, regardless of how much energy the bank holds.
Design Assumptions You Can Defend to an Inspector
A permit reviewer does not care about your spreadsheet — they want traceable sources behind every number:
- PSH source: link to your PVWatts report or local TMY file. A “5 PSH” claim with no citation gets flagged.
- DoD and cycle spec: attach the manufacturer datasheet page showing the recommended discharge depth and warranted cycle count.
- Load schedule: present a device-by-device table with wattage, runtime, and daily kWh — not a single round estimate.
- Temperature envelope: note the expected low in the battery enclosure. If it drops below freezing, document the heating strategy or BMS cutoff setting.
The calculator produces a starting size. These four documentation items turn that size into a submittal package an inspector can approve.
When This Calculator Breaks Down
- Wildly variable loads. A woodshop that idles at 3 kWh but hits 30 kWh on milling days cannot be sized to a single daily average. You need a generator-hybrid model with dispatch logic.
- Extreme cold with no battery shelter. At −20 °C, lead-acid capacity drops 30%+, and LiFePO4 BMS boards block charging entirely. The calculator does not model temperature curves.
- Multi-source hybrids. Combining solar, wind, and propane generator requires load-sharing rules and state-of-charge dispatch that a single-source tool cannot capture.
In each case, use the number here as a floor, then bring in a system designer who can run hourly simulation against a full year of weather data.
Oversights that cost money: treating summer PSH as a year-round number, quoting battery capacity without specifying DoD, and skipping inverter surge rating when a well pump or compressor is on the load list.
Related tools: Solar Land Requirement Calculator when a ground-mounted array needs acreage estimates, Wind Turbine Spacing Calculator if you are evaluating a hybrid solar-wind site, Rainwater Harvesting Tank Size Calculator for pairing water self-sufficiency with energy autonomy, and Rural Utility Cost Estimator to weigh off-grid capex against a utility line extension.
Sizes shown are concept-level planning figures — not a substitute for a licensed electrician’s NEC load calculation, a manufacturer-approved battery configuration, or a stamped electrical permit drawing.