Solar Design Guide
The complete solar design sequence: measure the load, size panels and batteries, verify string voltages, choose the controller, and check the economics — each step linked to its calculator.
Everything starts with the load
Solar design is load-first: every watt-hour the system must deliver daily cascades into panel, battery and inverter sizes. List each load, its watts and hours, and sum — the honest number, not the optimistic one. A small cabin lives on 1.5–3 kWh/day; a full off-grid house typically needs 8–15 kWh/day plus generator backup for winter.
The off-grid system calculator takes that daily figure and produces the battery bank and array sizes with every assumption (DoD, autonomy, derate) explicit. The panel count calculator answers the narrower question: how many panels for a kWh target.
Batteries: usable energy, not rated energy
Battery ratings mislead until depth of discharge is applied: a 100 Ah AGM battery at 50% DoD delivers 50 Ah; the same size lithium at 90% delivers 90 — and survives 3–5× more cycles. The runtime calculator shows what a given load actually extracts; the charge time calculator shows how long replenishing takes at a given charger current.
The C-rate is the guardrail: lead-acid prefers ≤ 0.2C, lithium tolerates 0.5–1C. Charging too fast causes gassing (flooded lead-acid) or lithium plating (cold Li-ion) — permanent damage.
String voltage: the silent killer
Panels rated at 25 °C produce higher voltage when cold — about 0.29% per °C for silicon. A string that measures 400 V at noon can hit 450 V at dawn on a −10 °C day; if the inverter's maximum is 500 V, the margin is thinner than it looks. Exceeding it destroys the inverter, usually instantly and out of warranty.
The string sizing calculator applies the temperature correction against the inverter's absolute maximum and MPPT window. It's the one solar check with catastrophic consequences for getting it wrong.
Economics: production times price
Production is geometry and weather: array watts × peak sun hours × derate. Economics multiply that by tariffs: self-consumed energy displaces the retail rate (valuable), exports earn the feed-in rate (often a fraction of it). The savings calculator runs the full payback math with honest caveats — degradation, inverter replacement and rate changes excluded.
Tilt is the last refinement: latitude is the annual optimum, ±15° biases winter or summer. The tilt calculator gives the angles and azimuth for your hemisphere. Within ±10° of optimum the yield penalty is only a few percent — orientation and shading matter more.
Questions about solar, energy & batteries calculations
What derate factor should I use for solar?
0.75–0.85 for well-installed fixed systems: inverter ~96%, wiring ~2%, soiling ~3%, temperature ~4–8%. Dusty sites or poor ventilation push it lower — PVWatts' default 0.86 is optimistic for real rooftops.
Lithium or lead-acid for off-grid?
Lithium (LiFePO4) wins on cycle life (3,000–5,000+ vs 500–1,200), usable capacity (80–90% vs 50% DoD) and weight — the premium pays back within the first battery replacement cycle for most usage patterns.
How many days of autonomy do I need?
2–3 days suits most climates. Critical loads in cloudy regions justify 4–5; sunny climates with a generator backup can use 1–2.
All Solar, Energy & Batteries calculators
- Solar Panel Output CalculatorEnter panel wattage and your location's peak sun hours to get realistic daily, monthly and yearly energy — including the standard loss derate.
- Battery Runtime CalculatorEnter battery Ah and voltage, your load in watts, plus DoD and inverter efficiency, to get realistic runtime — not the naive Ah ÷ A figure.
- Off-Grid System CalculatorEnter your daily energy consumption and get the battery bank, solar array and inverter sizes needed — with every sizing factor (DoD, autonomy, derate) explicit.
- Charge Controller CalculatorPick the controller type and get the minimum amp rating — MPPT from array watts ÷ battery voltage, PWM from total Isc — with the NEC margin applied.
- Solar String Size CalculatorEnter panel electrical specs and string layout to check cold-temperature Voc against the inverter maximum — the check that protects your equipment.
- Battery Bank Size CalculatorEnter your daily energy need and design choices (DoD, autonomy) to get the bank size in Ah and kWh, plus how many battery blocks to arrange.
- Battery Charge Time CalculatorEnter battery Ah, charger amps and the charge window to get realistic charge time — including the C-rate flag that protects battery life.
- Solar Savings CalculatorEnter system size, sun hours, rates and cost per watt to get annual savings, simple payback and the 25-year figure — with the honest caveats listed.
- Solar Panel Tilt CalculatorEnter your latitude to get the optimal tilt for year-round, winter-biased or summer-biased mounting, with the correct azimuth direction.
- Solar Panel Count CalculatorEnter your energy target and local sun hours to get the required array Wp, the number of panels and the rough roof area needed.