Off-Grid Solar Calculator

Size the array from daily kWh ÷ (sun hours × efficiency), and the battery from daily kWh × autonomy days ÷ depth of discharge. A cabin using 5 kWh per day at 4.5 peak sun hours and 75% efficiency needs a 1.48 kW array — about four 400W panels. For 3 days of autonomy on a 48V bank at 80% depth of discharge, it needs 5 × 3 ÷ 0.80 = 18.75 kWh of battery, roughly 391 Ah at 48V. The calculator above sizes the array, battery bank in kWh and Ah, and panel count together.

An off-grid system has two halves that must be balanced: generation (solar panels) to replace each day's energy, and storage (batteries) to carry you through nights and cloudy stretches. Undersize the panels and the battery never fully recharges; undersize the battery and you run out overnight. This tool sizes both from the same daily-consumption figure so they stay matched.

Two formulas work together:

The variables:

• Daily kWh — the total energy your loads consume in a day. This single number drives both the array and the battery.
• Peak Sun Hours — equivalent hours of full sun; size to the worst month for year-round off-grid living.
• Efficiency — a conservative 0.70–0.75 off-grid, because energy passes through the charge controller, battery (round-trip loss), and inverter before reaching the load.
• Days of Autonomy — cloudy days the battery must cover with no solar. 2–3 is typical; more in cloudy climates.
• Depth of Discharge — usable fraction of the battery; 80% lithium, 50% lead-acid.

This is the same math split across our dedicated tools: the array half mirrors the solar panel wattage calculator, and the storage half mirrors the solar battery bank calculator and battery bank size calculator. The off-grid calculator combines them so array and battery stay in balance.

Example 1 — Off-grid cabin, 5 kWh/day, 4.5 sun, 3 days, 48V, 80% DoD, 75% eff: Array = 5 ÷ (4.5 × 0.75) = 5 ÷ 3.375 = 1.48 kW (four 400W panels). Battery = 5 × 3 ÷ 0.80 = 18.75 kWh = 391 Ah at 48V.

Example 2 — Efficient home, 10 kWh/day, 5.0 sun, 2 days, 48V, 80% DoD, 80% eff: Array = 10 ÷ (5.0 × 0.80) = 10 ÷ 4.0 = 2.5 kW (seven 400W panels). Battery = 10 × 2 ÷ 0.80 = 25 kWh = 521 Ah at 48V.

Example 3 — RV/van, 1.5 kWh/day, 5.5 sun, 1 day, 12V, 80% DoD, 75% eff: Array = 1.5 ÷ (5.5 × 0.75) = 1.5 ÷ 4.125 = 0.36 kW (one 400W panel). Battery = 1.5 × 1 ÷ 0.80 = 1.875 kWh = 156 Ah at 12V.

Example 4 — Cloudy off-grid homestead, 8 kWh/day, 3.0 sun, 4 days, 48V, 80% DoD, 72% eff: Array = 8 ÷ (3.0 × 0.72) = 8 ÷ 2.16 = 3.70 kW (10 panels). Battery = 8 × 4 ÷ 0.80 = 40 kWh = 833 Ah at 48V. The cloudy climate and long autonomy make this system far larger than the sunny cabin despite a similar load.

A well-designed off-grid system balances three components so no single part carries an unfair burden. Oversizing one to compensate for another is expensive and inefficient.

The array must replace each day's energy plus recharge whatever the battery gave up overnight — on an average day, in your worst month. If the array is too small, the battery slowly discharges over a cloudy week and eventually cuts out. Off-grid arrays are typically oversized 25–50% versus a grid-tie array of the same load, which is why this calculator uses a conservative 75% efficiency.

The battery carries the load through nights and cloudy stretches. Days of autonomy set its size, and every extra day adds proportional cost. Beyond about 3 days, a backup generator is usually cheaper than more battery — it covers the rare multi-day storm while the solar-plus-battery handles the other 95% of the year.

The generator (or grid, in a hybrid system) is the insurance policy. Sizing autonomy to 2–3 days and adding a generator is almost always more economical than sizing the battery for the worst week of the year. Confirm your controller with the charge controller size calculator and revisit runtime under specific loads with the battery runtime calculator.

1. Enter daily consumption in kWh. Sum your loads or use a metered figure. Be honest — off-grid punishes underestimates.
2. Enter peak sun hours. Use your worst-month value for year-round autonomy, or the annual average with a generator as backup.
3. Set days of autonomy. 1 with a generator, 2–3 for full off-grid, 4–5 for cloudy climates with no backup.
4. Choose bank voltage. 12V for small RV systems, 24V for cabins, 48V for homes.
5. Set depth of discharge and efficiency. 80% DoD and 75% efficiency are good lithium defaults; use 50% DoD for lead-acid.
6. Read the design. Array kW, battery bank kWh and Ah, and 400W panel count all update live.

Every number this calculator produces flows from one input — daily kWh — and off-grid systems punish underestimates harder than any other design. A load audit is the single most valuable step before sizing. List every device, its power draw in watts, and how many hours per day it runs; multiply and sum to get daily watt-hours, then divide by 1,000 for kWh. A 60W fridge running an effective 8 hours of compressor time uses 480 Wh; ten 10W LED bulbs for 4 hours use 400 Wh; a 1,500W well pump for 20 minutes uses 500 Wh.

The loads people forget are the ones that sink off-grid systems. Inverters draw idle power around the clock even with nothing plugged in — often 10–30W, which is 240–720 Wh per day before any real use. Phantom loads from chargers, routers, and standby electronics add up. Well pumps, water heaters, and anything with a heating element or motor are surprisingly hungry. Seasonal loads like heating, cooling, and lighting swing consumption dramatically between summer and winter.

The gold standard is to measure rather than estimate: a plug-in energy meter or a whole-system battery monitor recorded over a week captures real behavior, including duty cycles and phantom draws, far better than a spreadsheet. Add a 10–20% contingency on top for growth and bad-weather margin. Once your daily kWh is solid, this calculator sizes the array and battery together; cross-check the storage half with the solar battery bank calculator.

This calculator sizes generation and storage, but an off-grid system also needs an inverter big enough to run your loads at once, and it is sized by a different rule: peak simultaneous power, not daily energy. Add up the wattage of everything that could run at the same moment — the microwave while the well pump kicks on while the lights are on — and choose a continuous inverter rating above that peak, with headroom.

Surge is the catch. Motors, compressors, pumps, and anything with a startup inrush draw 3–7 times their running wattage for a fraction of a second when they start. A well pump rated 1,000W running might surge to 3,500W at startup, so the inverter's surge rating (not just its continuous rating) must cover it. A quality pure sine-wave inverter is worth the cost off-grid: it runs motors, electronics, and sensitive equipment cleanly, whereas modified sine-wave inverters can buzz, run hot, or damage some devices.

Match the inverter's DC input voltage to your bank — a 48V bank needs a 48V inverter — because higher voltage means lower current for the same power, allowing smaller cables and better efficiency, exactly as with the rest of the system. For whole-home off-grid, a hybrid inverter/charger that also manages the battery and an optional generator simplifies the build. With array, battery, controller, and inverter all sized, confirm runtime under specific loads using the battery runtime calculator and the controller amps with the charge controller size calculator.

• Underestimating consumption. Phantom loads, inverter idle draw, and forgotten devices add up. Meter for a week before trusting a paper estimate.
• Sizing to the annual average instead of winter. Off-grid systems must survive the worst month, not the yearly mean.
• Skimping on efficiency. Round-trip battery and inverter losses are real; 75% is a safer off-grid derate than 80%+.
• Buying too little battery for the array (or vice versa). An unbalanced system wastes solar or runs out overnight.
• Choosing 12V for a large system. High current forces huge cables and controllers; 48V is far more practical above ~2 kW.
• No generator plan. Sizing the battery for a 7-day storm is far costlier than a small backup generator.

RV and van (0.5–2 kWh/day). Small systems: one or two 400W panels, a 100–300Ah 12V lithium bank, and a 40–60A MPPT controller. Mobility means you can chase sun, so 1 day of autonomy is usually enough.

Tiny house (2–5 kWh/day). With efficient DC appliances and a mini-split, a 1.5–3 kW array and 10–20 kWh battery on 24V or 48V cover most tiny homes. Use the panels-to-run-a-house calculator to translate array kW into panel count.

Off-grid cabin (3–8 kWh/day). Weekend cabins can run lean; full-time cabins need a 2–5 kW array, 15–30 kWh battery, and a generator for winter.

Homestead / full off-grid home (10–30 kWh/day). These are 48V systems with 5–10 kW of solar, 30–75 kWh of battery, and almost always a backup generator. Confirm storage with the solar battery bank calculator and the array with the solar panel wattage calculator.