Off-grid solar system calculator
Enter your daily energy use, your worst-month sun hours and your biggest loads. The calculator sizes the battery bank, the array, the charge controller and the inverter together, so the parts match.
Enter your measurements
More options: DoD · Headroom · Losses …
- Battery bank
- 220 Ah · 24 V
- 12 V 100 Ah batteries
- 6 batteries · LiFePO4
- Solar array (minimum)
- 1,083 W
- Panels
- 3 panels · 400 W
- MPPT controller output current
- 70 A
- Inverter continuous rating
- 2,000 W
- Inverter surge rating
- 2,500 W
- Battery current at peak AC load
- 65 A
Battery: 3,000 Wh × 1.5 days ÷ 80% usable ÷ 25.6 V = 220 Ah (6 batteries)
Panels: 3,000 Wh ÷ (3.5 sun h × 86% × 92% battery round trip) = 1,083 W → 3 panels
Controller: 1,200 W ÷ 24 V × 1.25 = 63 A → 70 A
Inverter: 1,500 W × 1.25 = 1,875 W → 2,000 W; surge: 1,500 W + (1,200 − 200) W = 2,500 W
Quick answers for common sizes
| Example | Answer |
|---|---|
| An off-grid solar system for a house | 26 × 400 W panels, about 71 kWh of battery, 8,000 W inverter |
| How much solar for an RV | 600 W of solar, 200 Ah LiFePO4, 2,000 W inverter (for 2 kWh a day) |
The same system at other sun hours
| Peak sun hours per day | Solar array (minimum) | Panels | MPPT controller output current |
|---|---|---|---|
| 2 | 1,896 W | 5 panels | 104 A |
| 3 | 1,264 W | 4 panels | 100 A |
| 4 | 948 W | 3 panels | 70 A |
| 5 | 758 W | 2 panels | 50 A |
| 6 | 632 W | 2 panels | 50 A |
Worked examples: an off-grid solar system for a house and how much solar for an RV. Want one part at a time? Use the battery bank, solar panel, charge controller and inverter calculators.
How the parts are sized
battery Ah = Wh per day × days of autonomy ÷ usable depth of discharge ÷ battery volts array W = Wh per day ÷ (peak sun hours × (1 − losses) × battery round-trip efficiency) MPPT output amps = array W ÷ system volts × (1 + headroom) inverter W ≥ peak AC watts × (1 + headroom), and surge ≥ the biggest start
- Battery. Sized for your days of autonomy at a usable 80% for LiFePO4 (Victron rates cycle life at 80% DoD) or 50% for lead-acid (Trojan recommends 20–50% for long life).
- Panels. Sized to refill a day’s use on your sun hours after PVWatts’ default 14% losses and the battery round trip (92% LiFePO4, per Victron; 80% assumed for lead-acid).
- Charge controller. For a worked example see what size charge controller for 1000 watts of solar. An MPPT controller’s output current is roughly array watts ÷ battery volts (Victron’s example: 100 W ÷ 13 V = 7.7 A), plus headroom. Its maximum PV input voltage must also exceed your array’s cold open-circuit voltage — check that on the series and parallel calculator.
- Inverter. Continuous rating above everything that can run together, and a surge rating above the biggest motor start.
Choosing 12, 24 or 48 V
| System size | Usual voltage | Why |
|---|---|---|
| Up to ~1,500 W inverter, small van or boat | 12 V | Runs 12 V appliances directly; currents still manageable |
| ~1,500–4,000 W inverter, RV or cabin | 24 V | Half the current of 12 V for the same power |
| Above ~4,000 W, homes and large cabins | 48 V | A quarter of the 12 V current; many home inverters are 48 V |
What this calculator leaves to you
It sizes the main parts. It does not size wire, fuses, breakers or disconnects — those come from NEC/ABYC ampacity tables and each product’s manual, and a qualified installer should check them. Connecting an off-grid system to a building’s wiring also needs to follow the electrical code; a home system tied to the grid needs a permit and utility approval.
CalculatorSolar Load Calculator →Haven’t got a daily watt-hour figure yet? Start with your appliance list.What you’ll need
- 12 V 100 Ah LiFePO4 batteryPremium price
About 1.28 kWh rated in a box roughly the size of a group 24–31 lead-acid battery at around half the weight. Check the BMS discharge limit and whether it can charge below freezing.
See options on Amazon (opens in a new tab) - 400 W residential-size solar panelPremium price
Fewer, larger panels for a cabin, shed or ground mount. Their higher voltage usually means an MPPT controller.
See options on Amazon (opens in a new tab) - MPPT solar charge controllerMid-range price
Converts a higher panel voltage into charging current, so you can wire panels in series and lose less in the cables. Size it on output amps and maximum PV voltage.
See options on Amazon (opens in a new tab) - Pure sine wave inverter, 2000–3000 WPremium price
For microwaves, coffee makers and power tools. At 12 V these draw hundreds of amps, so the battery and cabling matter as much as the inverter.
See options on Amazon (opens in a new tab) - Battery monitor with shuntMid-range price
Counts amp-hours in and out, so you see true state of charge instead of guessing from voltage — which barely moves on a LiFePO4 battery until it is nearly empty.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How big an off-grid solar system do I need?
Start from your watt-hours per day. Each day of autonomy needs that much usable battery; the array needs to refill it on your worst-month sun hours. A 3 kWh-a-day cabin at 3.5 sun hours is roughly 1,100 W of panels and 220 Ah at 24 V of LiFePO4 for 1.5 days.
How much solar do I need for my RV?
It depends on what you run. An RV using 2 kWh a day with a 12 V fridge, lights, fans and a little microwave use needs about 560 W of panels at 4.5 sun hours. See how much solar for an RV.
Can I run a whole house off-grid on solar?
Yes, but it is a big system: the average US home’s 28.4 kWh a day (EIA, 2024) needs roughly 10.3 kW of panels at 3.5 sun hours and about 71 kWh of LiFePO4 for two days. See an off-grid solar system for a house.
Why does the calculator ask for my worst month?
Because an off-grid system has to work on the darkest days you use it. Sizing on the yearly average leaves you short every winter; PVWatts shows Chicago’s lowest month at about 2.7 peak sun hours against 4.5 for the year.
Related guides
Sources
- National Laboratory of the Rockies (formerly NREL) — PVWatts® Calculator (opens in a new tab) — Look up the solar resource (peak sun hours) and yearly output for your own address; default system losses 14%.
- NLR (formerly NREL) — PVWatts® v8 API, NSRDB PSM V3 TMY data (opens in a new tab) — City figures on this site: fixed open-rack array facing due south, tilted at the site’s latitude, 14% system losses, queried October 2026.
- Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.
- Trojan Battery — Technical support FAQs (opens in a new tab) — “We generally recommend a discharge of 20 to 50% of the battery’s rated capacity even though the battery is capable of being cycled to 80%”; 20-hour and 100-hour ratings; freezing points by state of charge.
- Trojan Battery — 27TMX data sheet (opens in a new tab) — Group 27 flooded: 85 Ah (5-hr), 97 Ah (10-hr), 105 Ah (20-hr), 117 Ah (100-hr); state of charge vs open-circuit voltage table; charger settings.
- Victron Energy — Which solar charge controller: PWM or MPPT? (2020) (opens in a new tab) — MPPT output current = panel watts ÷ battery voltage (100 W ÷ 13 V = 7.7 A); a PWM controller pulls the panel down to battery voltage (81 W from a 100 W panel at 25 °C, about the same as MPPT at 75 °C); temperature-coefficient method; example panel Voc 21.6 V, −0.35 %/°C.
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
- Champion Power Equipment — Generator Wattage Chart (opens in a new tab) — Typical running and starting watts for household, RV and job-site appliances (a manufacturer’s planning chart — your nameplate wins).
- U.S. Energy Information Administration — Electric Sales, Revenue, and Average Price, Table 5A (2024 data, released October 2025) (opens in a new tab) — 2024 average residential customer: 863 kWh a month (about 10,359 kWh a year, 28.4 kWh a day); Louisiana 1,202 kWh a month, Hawaii 495.