Day three of gray skies. The fridge is warm, the laptop is dead, and the battery monitor reads eleven percent. Undersized solar does not fail dramatically; it fails on a random Tuesday in November, right when you need it most.
The load audit is the whole game, and here is how to run one without an engineering degree. You will list every device, multiply watts by hours, and size the battery bank to bridge the cloudy stretches. You will also learn how sun hours change the math and which sizing mistakes leave builders running a generator at midnight. Do the arithmetic once and sleep through the storms.
Energy Independence Is a Math Problem
Solar for a tiny house is not “put panels on the roof and hope.” It is a load audit, a sun-hours calculation, and a battery bank sized to bridge the cloudy days. Get the math right and the lights stay on through a week of rain; get it wrong and you are running the generator at midnight. This guide walks the full sizing process.
1. The Load Audit: Know Your Appetite
List every electrical device with watts and daily hours: LED lights (60W total × 5h = 300Wh), laptop (65W × 6h = 390Wh), phone charging (20Wh), mini-split (800W × 8h = 6,400Wh — the elephant), fridge (100W average × 24h = 2,400Wh — the other elephant), water pump (100W × 1h = 100Wh), miscellaneous (300Wh). Total the watt-hours. A modest tiny house: 3,000–5,000Wh/day. A comfortable one with mini-split and full fridge: 8,000–12,000Wh/day. Be honest — the audit is the foundation; every downstream number multiplies its errors. Measure real usage with a Kill-A-Watt meter ($30) for a week rather than guessing.
2. Sun Hours: Your Location’s Fuel Supply
Peak sun hours (not daylight hours): the equivalent hours of 1,000W/m² sun. Phoenix: ~6.5 summer, ~4.5 winter. Seattle: ~5 summer, ~1.5 winter. Design for the worst month you will occupy the house — usually December/January. Look up your location (NREL PVWatts calculator, free). The rule: size the array for winter sun hours, not the annual average. A system that sings in June and starves in January is a summer system.
3. Sizing the Array: Panels
Formula: Array watts = daily Wh ÷ sun hours ÷ 0.75 (the 0.75 accounts for real-world losses: heat, wiring, inverter, dust, panel aging). Example: 5,000Wh/day ÷ 4 winter sun hours ÷ 0.75 = 1,667W → a 1,600–2,000W array (4–5 × 400W panels). Panel choice: monocrystalline (highest efficiency — the tiny house standard; roof space is limited so efficiency matters), 400W+ panels (fewer panels, fewer mounts, less wiring). Roof vs ground: roof mounts are clean but fixed-angle; ground mounts tilt seasonally (+15–25% winter yield) and stay clean — if you have the land, ground-mount wins.
4. Sizing the Battery Bank: The Heart of the System
Formula: Battery Wh = daily Wh × autonomy days ÷ depth of discharge. Autonomy days: how many cloudy days to ride through — 2–3 for most (more = more batteries = more cost). Depth of discharge (DoD): lithium (LiFePO4) 80–90%, lead-acid 50%. Example: 5,000Wh × 2.5 days ÷ 0.85 = 14,700Wh → a ~15kWh lithium bank. Battery choice: LiFePO4 is the tiny house standard (3,000–6,000 cycles, light, maintenance-free — $300–500/kWh). A 15kWh bank: $4,500–7,500. Lead-acid is cheaper upfront ($150–250/kWh) but heavier, shorter-lived, and needs maintenance — false economy for full-time living. Voltage: 48V for systems over 2,000W (lower current, thinner wires, more efficient); 24V or 12V for small systems.
5. The Charge Controller and Inverter
Charge controller: MPPT (not PWM — 20–30% more harvest, mandatory for larger arrays). Size: array watts ÷ battery voltage × 1.25. Example: 2,000W ÷ 48V × 1.25 = 52A → a 60A MPPT controller ($400–600, Victron SmartSolar is the standard). Inverter: pure sine wave (not modified — electronics and motors need clean power), sized to the maximum simultaneous load + 25% (2,000–3,000W covers most tiny houses; 5,000W for electric cooking). Inverter/charger combo (Victron MultiPlus — charges from shore power/generator, inverts off-grid, auto-transfers — $1,000–1,500): the tiny house all-in-one that simplifies the whole system.
6. Kits vs DIY: The Buying Decision
Solar kits (Renogy, Eco-Worthy, Rich Solar — 400W to 3,000W+ kits, $500–4,000): panels + controller + mounting + wiring in one box — good value, matched components, but batteries often sold separately and kit inverters can be weak. DIY component selection: pick each piece (panels, Victron controller, LiFePO4 batteries, MultiPlus) — 10–20% more cost, significantly better system, full control. The verdict: kits for small/simple systems (under 1,500W); component DIY for full-time off-grid houses. Either way, buy the battery bank from a reputable LiFePO4 brand (Battle Born, SOK, EG4) — cheap no-name lithium is the most dangerous corner to cut.
7. Wiring, Fusing, and Safety
Solar is low-voltage DC — which means HIGH current and real fire risk if wired wrong: fuse everything (battery positive terminal fuse — Class T or ANL, sized to the wire; every circuit fused), wire gauge (size for current with <3% voltage drop — 48V systems use dramatically thinner wire than 12V), disconnects (battery disconnect + PV disconnect — labeled, accessible), and grounding (panel frames and mounts grounded; lightning arrestor in storm country). Battery placement: ventilated, temperature-controlled space (LiFePO4 hates charging below freezing — heated batteries or an insulated compartment in cold climates), secured against movement (the house moves — batteries must not).
8. The Generator Question
Even great solar needs backup: a 2,000–3,000W inverter generator (Honda EU2200i or equivalent — quiet, efficient, $1,000–1,500) covers the darkest weeks and equalizes the batteries. Size the generator to the battery charger’s draw, run it during the day (charges batteries + powers loads simultaneously), and treat it as the 5% solution — if the generator runs more than 5% of the time, the array or bank is undersized. The generator is not failure; it is the insurance policy.
9. Costs: The Full Budget
A complete 2kW/15kWh off-grid system: panels $1,500–2,000, mounting $300–600, MPPT controller $400–600, LiFePO4 batteries $4,500–7,500, inverter/charger $1,000–1,500, wiring/fuses/disconnects $500–800, generator $1,000–1,500. Total: $9,000–15,000 for genuine energy independence. Smaller systems (weekend use, propane appliances): $3,000–6,000. The 25-year math: at $0.15/kWh grid power, a 5,000Wh/day system offsets ~$275/year — solar is not about beating grid prices; it is about independence where the grid does not reach.
Final Considerations
Size from the load audit, design for winter sun, buy LiFePO4, fuse everything, and keep the generator as backup. Solar rewards the careful planner and punishes the optimist. Do the math once, buy once, and the sun — free, silent, and maintenance-free — powers the house for decades.