Off-grid solar system builder

List what you run, with watts and hours a day, then pick your voltage, days of autonomy and sun hours. You get the battery bank, the solar panels, the inverter and the main cable, an energy-flow diagram and the battery's charge through a typical day and a bad winter one.

Inputs

Your loads

Watts from the nameplate, hours it actually runs each day (a fridge runs about a third of the time). Startup watts only for motors and compressors. AC runs through the inverter; DC runs straight from the battery.

12 V suits most vans and small RVs. Above about 2,000 W of inverter, 24 V or 48 V keeps the current and the cable down.

Days the batteries alone should run everything with no sun or charging.

For the months you use the system. 4–5 is a typical baseline (Battle Born); a local solar map gives your own figure.

About 2.5–3 in winter (Skyenergi). Used for the winter chart, not for the sizing.

Ah

The amp-hour rating of the 12 V battery you plan to buy.

W

The main cable carries the most current in the system. Keep it short.

Battery bank

542Ah at 12.8 V

For 2,290 Wh a day from 4 loads and 2 days without sun: 6 × 100 Ah batteries, all in parallel.

Sized for 4 sun hours. On 2.5-hour winter days the panels make 1,600 Wh of the 2,522 Wh the bank gives out, so it lasts about 5 days before reaching its floor. For all-winter use, plan on 1,261 W of panels or a generator.

Daily energy
2,290 Wh
Solar panels
800 W
Inverter, continuous
1,550 W
Inverter, surge
2,100 W
Main cable
2 AWG
Winter
5 days
Your system, and its battery over a day
Solar panels800 W4 × 200 WBattery bank542 Ah6 × 100 Ah, 12.8 VInverter1,550 WSurge 2,100 WAC loads2,090 Wha day, 3 loadsDC loads200 Wha day, 1 load2,560 Wh2,322 Wh2 AWG2,090 Wh200 Wh

Line widths follow the watt-hours a day that flow along them.

Battery charge over 24 hours

02040608010005101520Time of day (hour, 0 = midnight)State of charge (%)Low 82%
  • Typical day, 4 sun hours
  • Lowest charge to reach (20%)

On a typical day the panels make 2,560 Wh and the bank gives out 2,522 Wh. It falls to 82% before sunrise and climbs back to 100% while the sun is up.

4 loads use 2,290 Wh a day. With 2 days of autonomy that needs a 542 Ah bank at 12.8 V (6 × 100 Ah) and 800 W of panels for 4 sun hours. The inverter should be rated 1,550 W continuous with 2,100 W of surge. The main cable from the battery carries 143 A and needs 2 AWG copper over 5 ft. The diagram shows the energy flowing each day: 2,560 Wh from the panels into the battery, 2,322 Wh through the inverter to 2,090 Wh of AC loads, and 200 Wh straight to the DC loads. The chart follows the battery's charge over 24 hours: on a typical day it falls to 82% before sunrise; on a 2.5-hour winter day it falls to 79%. Sized for 4 sun hours. On 2.5-hour winter days the panels make 1,600 Wh of the 2,522 Wh the bank gives out, so it lasts about 5 days before reaching its floor. For all-winter use, plan on 1,261 W of panels or a generator.

Safety first

A battery bank can deliver hundreds of amps into a short circuit, enough to melt cable, start a fire or cause serious burns. The main cable to the inverter carries the most current in the whole system.

  • Fuse every circuit, with the main fuse as close to the battery as you can. The fuse protects the wire, so it must not be bigger than the wire’s rating.
  • Follow the makers’ instructions for the batteries, charge controller and inverter, including their limits on current, voltage and series and parallel connections.
  • Follow your local electrical code and any marine or RV standard that applies to your vehicle or boat.
  • Have a qualified person check the installation before you switch it on. This page sizes the parts; it is not a wiring plan.

How it works

The builder adds up your loads, then hands the result to the four starter calculators in turn. Each answer here is the one you would get by typing the same figures into that calculator.

daily energy       = Σ watts × hours a day
energy drawn       = DC loads + AC loads ÷ 0.90 (inverter)
battery bank (Ah)  = energy drawn × days ÷ usable share × 1.10 ÷ bank voltage
solar panels (W)   = energy drawn ÷ (peak sun hours × 0.80)
inverter (W)       = AC running watts × 1.20; surge = the rest running + the biggest startup
main cable         = smallest copper size that keeps the drop under 3% and carries 125% of the current
  • Loads use the watts on the nameplate and the hours they really run. A refrigerator cycles, so Virginia Tech’s guide counts a third of the hours it is plugged in. Startup watts matter only for motors and compressors.
  • The battery bank uses the battery bank calculator with a 10% margin: 80% usable for lithium, 50% for AGM, 12.8 V for a “12 V” lithium battery.
  • The panels use the solar panel calculator, sized for the energy the battery gives out, so they refill it on a day with your sun hours.
  • The inverter uses the inverter calculator with every AC load on at once, the cautious case, plus Battle Born’s 20% headroom.
  • The main cable uses the DC wire calculator for the run from the battery to the inverter: the inverter’s battery current at its full rating plus any DC loads, sized as continuous, marine 105 °C cable, 3% drop.
  • The charts run the battery through repeated days (load spread evenly, panels producing between 6 am and 6 pm) and draw the last one. The winter day starts where a typical day leaves the bank.

Worked example

The default list is a small cabin or a large van: a refrigerator (200 W, 8 hours, 1,000 W to start), four LED lights on DC (40 W, 5 hours), laptop and phone charging (60 W, 4 hours) and a microwave (1,000 W, 15 minutes).

  • Daily energy: 1,600 + 200 + 240 + 250 = 2,290 Wh. The 2,090 Wh of AC loads cost 2,322 Wh at the battery, so it gives out 2,522 Wh.
  • Battery: 2,522 × 2 days ÷ 0.80 × 1.10 = 6,936 Wh, which is 542 Ah at 12.8 V, or six 100 Ah batteries.
  • Panels: 2,522 ÷ (4 × 0.80) = 788 W, so four 200 W panels (800 W).
  • Inverter: 1,260 W running × 1.2 = 1,512 W, so look for 1,550 W continuous. When the fridge starts, 1,060 W is already running, so the surge is 2,100 W.
  • Main cable: 1,512 W ÷ 0.90 ÷ 12 V = 140 A, plus 3 A of lights, is 143 A. Sized at 125% that needs 2 AWG marine cable, which drops 2.3% over 5 ft.
  • Winter: on 2.5 sun hours the panels make 1,600 Wh against 2,522 Wh, so the bank lasts about 5 such days. Keeping up all winter would take 1,261 W of panels.

Tips from the road

  • Measure before you buy. A plug-in watt meter or a battery monitor over a few real days beats any table, especially for a fridge, whose hours depend on the weather and how often you open it.
  • Go up a voltage before you go up a cable. If the main cable comes out at 2/0 or bigger, a 24 V system halves the current and usually drops two or three sizes.
  • Plan winter separately. If you live off-grid through the winter, either size the panels for the winter figure or budget for a generator; extra batteries only delay the shortfall.

FAQ

How big an off-grid solar system do I need?

Add up your loads as watts × hours a day. That daily energy, divided by the share of the battery you will use and multiplied by your days of autonomy, sets the battery bank. The same energy, divided by your peak sun hours and an 80% system efficiency, sets the panels. The four default loads use 2,290 Wh a day and need a 542 Ah lithium bank at 12 V and 800 W of panels on 4 sun hours.

Why are the panels sized for more than my daily use?

AC loads go through the inverter, which loses about 10%, so the battery gives out more than the loads use. The panels are sized to put that back, so the bank refills on a normal day. On the default loads the battery gives out 2,522 Wh for 2,290 Wh of use.

Why is the inverter so big when I only run one thing at a time?

The builder assumes every AC load could run at once, which is the safe case. If you never run the microwave and the coffee maker together, open the inverter calculator from the link under the result and enter only what really runs together.

What does the winter switch show?

The battery's charge on a day with the winter sun hours you enter, starting where a typical day leaves it. If the panels make less than the bank gives out, the chip under the result says how many such days in a row the bank lasts before reaching its lowest safe charge, and how many watts of panels would keep up.

Should a load be AC or DC?

DC loads run straight from the battery, such as 12 V lights, fans, pumps and 12 V fridges. They skip the inverter's losses and do not count toward its size. Anything with a household plug is AC.

Sources

  1. ENERGY SERIES: Estimating Appliance and Home Electronic Energy Use, Virginia Cooperative Extension (Virginia Tech), from the US DOE Energy Saver guide. Wattage × hours used per day ÷ 1,000 = daily kWh; a refrigerator runs about a third of the time it is plugged in; typical wattages (coffee maker 900–1,200 W, microwave 750–1,100 W, hair dryer 1,200–1,875 W, toaster 800–1,400 W, 19-inch TV 65–110 W).
  2. Generator Wattage Chart: Common Appliances & Tools, Champion Power Equipment. Running and starting watts, for example a refrigerator 150–400 W running and 800–1,200 W starting, a 5,000 BTU window air conditioner 450–600 and 900–1,200 W, a 13,500 BTU RV air conditioner 1,500–2,000 and 2,800–3,500 W; a microwave, lights and chargers need no extra starting watts.
  3. How to Size a Deep Cycle Battery Bank, Battle Born Batteries. Watts × hours = watt-hours; watt-hours ÷ system voltage = amp-hours; two days off-grid doubles the capacity.
  4. How to Size Solar Panels for a Deep Cycle Battery System, Battle Born Batteries. Daily watt-hours ÷ peak sun hours = solar watts, then add 20–30% for real-world losses.
  5. How to Size an Inverter (And When Surge Limiting Is Needed), Battle Born Batteries. The continuous rating covers the running watts of everything on at once; the surge rating covers the biggest startup load.
  6. How to Size a Victron Off-Grid Power System, Alchemy Industrial. The same chain worked end to end, from daily consumption to battery capacity and array watts.
  7. Sizing Solar Panels, a Campervan and Off-Grid Guide, Skyenergi. Peak sun hours are roughly 2.5–3 in winter and 5–6 in summer.
  8. ABYC E-11 marine DC wiring guide, ToolGrit. Voltage drop of 3% for critical circuits such as inverters; ampacity and voltage drop are separate tests and the larger wire wins.

Formula and sources last checked September 30, 2026. How we test formulas.

  • Battery bank size calculatorHow many amp-hours of battery you need for an RV, van, boat or cabin, from your daily watt-hours and days of autonomy, with a 24-hour state-of-charge chart.
  • Solar panel size calculatorHow many watts of solar panels you need for an RV, van, boat or cabin from daily watt-hours and peak sun hours, with a chart of panel output against load.
  • Inverter size calculatorWhat size inverter you need for a van, RV or cabin. Continuous watts for what runs together, surge watts for the biggest motor starting, and battery current.
  • DC wire size and voltage drop calculatorFind the AWG wire size for a 12, 24 or 48 V DC circuit from current and distance. Checks voltage drop and ampacity with NEC and Coast Guard tables.
  • Amp-hour and battery runtime calculatorHow long a battery runs a load. Enter amp-hours, voltage, battery type and watts. Covers LiFePO4 and AGM, inverter losses and lead-acid discharge-rate loss.
  • Solar charge controller size calculatorWhat size solar charge controller you need, MPPT or PWM: its amp rating, the voltage it must take on a cold morning and the PV fuse, from your panels.

Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.