Free solar, battery and generator calculators
How many panels, how big a battery, what size inverter or generator — for a van, RV, cabin or a house in an outage.
What are you powering?
Pick your project — each one opens its own calculator, then the next steps.
- Off-grid cabin or RVSize a whole off-grid system at once: battery bank, solar array, charge controller and inverter from your daily watt-hours, sun hours and biggest loads.
- How long will my battery last?How long a battery runs a load: amp-hours, voltage and chemistry against watts, with inverter loss and duty cycle.
- How many solar panels?How many solar panels you need: array watts and panel count from your daily watt-hours, peak sun hours, system losses and battery losses.
- Choosing an inverterWhat size inverter you need: continuous watts for everything running at once, surge watts for motor starts, and the DC amps it pulls from a 12, 24 or 48 V battery.
- Generator for a camperWhat size generator for a 30 amp camper or 50 amp RV: running and starting watts for the air conditioners plus everything else, against the rig’s service.
- Power outage backupHow many kWh of battery to keep a house (or just the essentials) running through an outage, and how many battery units or 100 Ah batteries that is.
- Portable power stationHow long a portable power station runs a fridge, heater, CPAP, Starlink or fan, from its watt-hours, the load and the inverter loss.
- Adding up my loadsAdd up appliance watts × hours into watt-hours a day, amp-hours from the battery and peak watts — the number every solar and battery size starts from.
Every calculator, by project
Loads & energy use 3
Batteries 5
Solar panels 8
Home solar & EVs 3
Inverters & generators 5
Popular sizes, already worked out
- How long a 100Ah battery runs a fridge41 hours — a 12 V compressor fridge (50 W, on half the time) on 100Ah LiFePO4
- How many watt-hours in a 100Ah battery1,280 Wh (12.8 V LiFePO4) or 1,200 Wh (12 V lead-acid)
- What size inverter to run a microwave1,500 W pure sine (for a 1,200 W microwave)
- What size generator for a 30 amp camper3,250 W running / 4,100 W starting
- How much power a 100W solar panel produces per day387 Wh a day at 4.5 sun hours
- How many solar panels to charge a 100Ah battery3 × 100 W panels (288 W)
- What size charge controller for a 400 watt solar panel50 A MPPT on 12 V (33.3 A minimum)
- How many amps is 1000 watts at 12 volts83.3 A (92.6 A through an inverter)
- How many amps a 1000 watt inverter draws92.6 A
- How many solar panels to run a refrigerator2 × 400 W panels (479 W) plus about 87 Ah of battery
- How long a 20 lb propane tank lasts for heating24 hours at 18,000 Btu/h
- How long a generator runs on 5 gallons of gasAbout 14.9 hours (3500 W generator at half load)
- How many solar panels for 1000 kWh per month7.93 kW — 20 × 400 W panels (Dallas)
- How many kWh to power a house for a day28.4 kWh a day — about 31.6 kWh of battery
- What size generator to run a refrigerator1,200 W starting — a 2,000 W-class generator
- What size battery bank for a 3000W inverter326 Ah of 12 V LiFePO4 for an hour — 4 × 100 Ah
Guides
- Appliance wattage chart: running and starting wattsHow many watts does a refrigerator, microwave, AC, sump pump or TV use? Running and starting watts for home and RV appliances, with daily energy.
- LiFePO4 vs lead-acid (AGM and flooded) for solarLiFePO4 vs AGM and flooded lead-acid for solar, RV and boat banks: usable capacity, cycle life, weight, efficiency, cold charging and safety.
- 30 amp vs 50 amp RV: the watt math30 amp vs 50 amp RV service explained with the watt math: 3,600 W on one 120 V leg vs 12,000 W on two, what each runs and what generator it needs.
- Generator safety: carbon monoxide, backfeeding and transfer switchesPortable generator safety from CPSC and OSHA: carbon monoxide, where to put it, why never to backfeed through an outlet, transfer switches and refueling.
- Battery safety for solar, RV and van systemsSafe handling, charging and installation of LiFePO4 and lead-acid batteries: short circuits, BMS, charging temperatures, hydrogen venting and disposal.
- Peak sun hours by city (and what they mean)Peak sun hours for US cities from NREL’s PVWatts — yearly average, worst and best month — plus what a peak sun hour is and how to look up your own.
All 12 guides → — including the battery group size chart, 12V battery voltage chart, propane tank size chart, starting vs running watts and solar words explained.
Frequently asked questions
How do I size a solar system?
Start with the watt-hours you use per day (watts × hours for each appliance). The battery stores a day or two of that, the panels refill it on your sun hours, and the charge controller and inverter are sized from the panels and your biggest loads. The off-grid solar calculator does all of it at once; how to size an off-grid solar system explains each step.
How long does a 100Ah battery last on solar-system loads?
A 12.8 V 100 Ah LiFePO4 battery has about 1,024 Wh usable at 80% depth of discharge, so about 10 hours at 100 W on DC (about 9 hours through an inverter). A 12 V lead-acid battery used to 50% gives about 600 Wh. The battery runtime calculator works it out for your load.
What does “peak sun hours” mean?
The day’s sunlight expressed as hours of full 1,000 W/m² sun. NREL’s PVWatts gives a tilted, south-facing panel about 3.8 a day in Seattle and 6.6 in Phoenix, averaged over the year — and far less in winter. See peak sun hours by city.
Do the calculators work in metric?
Yes. Electrical units — watts, watt-hours, volts and amps — are the same everywhere; fuel, propane, temperature and distance switch between US and metric units with the toggle.
Do you size wire, fuses and breakers?
No, deliberately. Those come from NEC or ABYC ampacity tables and the equipment manuals, and depend on details a calculator can’t see. Have a qualified installer check them.
Can I plug a generator or inverter into a wall outlet to power my house?
No. That is “backfeeding”, which CPSC calls extremely dangerous: it can electrocute utility workers and neighbors. Use a transfer switch installed by a licensed electrician, or run cords to individual appliances. See generator safety.