OffGrid Sizer: backup power, solar & DC wiring calculators

25 free calculators with the formulas shown and every assumption editable. No sign-up, and your inputs never leave your browser.

Off-grid solar sizing, step by step

Planning a cabin, van, RV or off-grid home? Start with the all-in-one estimate, then check each part in order.

1. Rough out the whole system · Off-grid solar system calculator
2. Size the inverter from your loads · Inverter size calculator
3. Plan the battery bank · Battery bank series & parallel calculator
4. Check battery runtime · Battery bank runtime calculator
5. Size the solar array · Solar panel size calculator
6. Check panel string voltage · Solar string voltage calculator (series & parallel)
7. Choose the charge controller · Solar charge controller size calculator
8. Size the DC cables · DC wire size calculator (12 V, 24 V, 48 V)
9. Pick the fuses · DC fuse & breaker size calculator

Power stations & batteries

• Amp-hours to watt-hours converter

  Convert battery amp-hours (Ah) to watt-hours (Wh) and back for 12 V, 12.8 V LiFePO4, 24 V, 48 V and single lithium cells, with nominal voltage guidance.

• Battery charging time calculator

  How long will it take to charge your battery? Estimates bulk and absorption time for LiFePO4, AGM, gel and flooded batteries from charger amps, plus wall power.

• Battery bank runtime calculator

  How long will a 100Ah (or any) battery run your load? Accounts for chemistry, usable depth of discharge, inverter losses and the Peukert effect.

• Battery voltage chart and state-of-charge calculator

  Convert a resting battery voltage into state of charge for LiFePO4, AGM, gel and flooded lead-acid at 12, 24 or 48 V, with sourced voltage charts for each.

• CPAP battery runtime calculator

  Estimate how many nights a power station or battery will run a CPAP machine, with or without the heated humidifier, on DC or AC power.

• Portable power station runtime calculator

  Estimate how long a portable power station will run a fridge, CPAP, router, lights or any mix of devices, including inverter losses, idle draw and output limits.

• What size power station do I need?

  List your devices and how many hours a day you run them to get the battery capacity (Wh), continuous output and surge rating your power station needs.

• Battery bank series & parallel calculator

  Work out the voltage, amp-hours and total energy of a battery bank wired in series, parallel or series-parallel, with wiring tips for matched batteries.

Solar

• Solar charge controller size calculator

  Find the amp rating your MPPT or PWM solar charge controller needs from array watts, battery voltage or panel Isc, with the standard 25% safety margin.

• Off-grid solar system calculator

  Size a complete off-grid solar system from your appliance list: battery bank, solar panels, charge controller, inverter, main fuse and battery cable in one place.

• Solar charging time calculator

  How long will a solar panel take to charge your power station or battery? Includes real-world panel output, solar input limits and peak sun hours.

• Solar panel size calculator

  Turn daily energy use into the solar array watts and number of panels you need, using peak sun hours and realistic losses. For RVs, vans, cabins and power stations.

• Home solar payback calculator

  Estimate home solar payback years, 25-year savings and ROI with production, rate increases, degradation and incentives. Updated for the 2026 tax-credit changes.

• Solar string voltage calculator (series & parallel)

  Check that your solar panels wired in series and parallel stay under your charge controller's max PV voltage on the coldest day, and see array voltage, current and watts.

Wiring & DC electrical

• DC fuse & breaker size calculator

  Pick a DC fuse or breaker size: at least 125% of continuous current, no larger than what your wire can safely carry. Covers 12 V, 24 V and 48 V circuits.

• DC wire size calculator (12 V, 24 V, 48 V)

  Find the right copper wire gauge (AWG) for a 12 V, 24 V or 48 V DC circuit from current, cable length and allowed voltage drop, checked against ampacity.

• Inverter size calculator

  Size a power inverter from your appliance list: continuous watts, motor start-up surge, the next standard size and DC current from a 12, 24 or 48 V battery.

• DC voltage drop calculator

  Calculate voltage drop, percent drop, voltage at the load and watts wasted as heat for a copper DC cable of any AWG, length and current.

Generators & home backup

• Generator vs power station cost calculator

  Compare the multi-year cost of a gas generator against a battery power station for your outage or camping use, including fuel, electricity, maintenance and cost per kWh.

• Generator fuel & runtime calculator

  Estimate how long a generator runs on a tank, how much gasoline or propane a multi-day outage takes and what it costs, from your generator's fuel-use spec.

• Generator size calculator

  Add up running and starting watts for your fridge, sump pump, furnace, well pump and more to find the generator size you need for a power outage.

• Home backup battery size calculator

  Size a whole-home or critical-loads backup battery: kWh needed for your outage length, how many battery units, continuous kW and surge, with optional solar recharge.

Energy costs & conversions

• Appliance energy cost calculator

  Find out how many kWh an appliance uses and what it costs to run per day, month and year from its watts, daily hours and your electricity rate.

• EV charging cost & time calculator

  Calculate how much it costs and how long it takes to charge an electric car at home, including charging losses, and compare cost per mile with a gas car.

• Watts ↔ amps calculator

  Convert watts to amps or amps to watts for DC, single-phase AC and three-phase AC circuits at any voltage and power factor.