Gas generator
Carbon monoxide: Never run a gas generator indoors, in a garage or near windows. Its exhaust contains carbon monoxide, which you cannot see or smell and which can kill.
When a gas generator is the cheaper choice, and when solar is
A gas generator costs less to buy and more to run. A solar generator, meaning a battery power station with solar panels, costs more to buy and very little to run. Which one ends up cheaper depends on how long you own it and how much you actually use it.
The gas generator’s cost climbs every year: fuel for each outage, fuel for the short runs that keep the engine healthy, and routine maintenance. The solar generator’s cost is mostly paid on day one. After that, sunlight is free, and the main later cost is a replacement battery if you use it heavily enough to wear the first one out.
Plot both as running totals and you get two lines. The gas line starts lower and rises. The solar line starts higher and stays flat, stepping up only if a battery replacement falls inside the period. If the lines meet before you would replace the equipment anyway, solar ends up cheaper. If they never meet, or meet decades later, gas does. The calculator shows the year they cross, or says plainly that they do not cross within the years you entered.
Cost is not the only difference. A gas generator can keep running for as long as you can refuel it, and a typical portable unit puts out more power than most power stations. A solar generator is limited by its battery size and by how much the panels can put back each day. The coverage and peak-load checks in the results show where each one falls short for your numbers.
What fuel actually costs over the years
The price at the pump is only part of the cost of running a gas generator. The calculator counts three fuel costs a year: fuel burned during outages, fuel burned on exercise runs, and maintenance.
Exercise runs matter more than most people expect. A generator that sits for months may not start when you need it, so owners are usually told to run it regularly under a light load. Twelve runs of 20 minutes a year is a modest schedule, and every one burns fuel whether or not the power goes out.
Fuel use per kilowatt-hour also depends on load. Makers usually quote runtime at a quarter or half load, and a generator running lightly burns more fuel for each kilowatt-hour it delivers than one working near its rating. The calculator derives fuel use from the maker’s own runtime figure and shows the arithmetic, so you can see which load it assumes.
Gasoline does not keep well. Untreated, it starts to go stale within months, and ethanol blends can absorb water. A fuel stabilizer slows that down, which is why it sits in the maintenance line along with oil and a spark plug. Even with stabilizer, stored fuel has to be used and replaced on a rotation, and many local fire codes limit how much gasoline you can keep at home. Propane keeps much longer in a sealed tank, but a 20 lb cylinder costs far more per gallon than the bulk delivery price that government averages track, and an exchange costs more again because the swapped tank holds only about 15 lb. The calculator defaults to a typical cylinder refill price and lets you switch to the bulk price if you have a large tank.
The default gasoline and bulk propane prices come from the U.S. Energy Information Administration’s weekly retail price surveys, and the cylinder price from national retailers, with the source and date shown next to the field. Fuel prices move, so treat the default as a starting point and change it to your local price.
Carbon monoxide: the risk that comes with gas
A gas generator’s exhaust contains carbon monoxide, a gas you cannot see or smell. In an enclosed or partly enclosed space it can build up to dangerous levels within minutes, and portable generators are a well-documented cause of carbon monoxide deaths during power outages.
Never run a gas generator indoors, in a garage, in a basement or crawl space, or near windows, doors or vents, even with them open. Federal safety guidance says to keep it at least 20 feet from the house, with the exhaust pointed away from any opening. Install battery-powered or battery-backed carbon monoxide alarms, and if anyone feels dizzy, sick or weak while a generator is running, get to fresh air at once.
A battery power station produces no exhaust, so it can sit indoors next to the equipment it powers. That is a safety difference, not a cost one, and it does not show up in the numbers above.
Noise, and where each one can be used
Portable inverter generators are the quieter kind of gas generator, but they are still engines. Expect a steady sound that neighbours will hear, especially at night and at higher loads. Larger open-frame generators are louder. Some homeowners associations and cities restrict when and where a generator may run, so check before you rely on one.
A power station makes almost no sound apart from a cooling fan under heavy load. Because it has no exhaust, it can run inside, in an apartment, or in a bedroom for medical equipment. A gas generator has to stay outside, at a distance, protected from rain, and that is often the deciding factor for people without a yard.
What solar recharge really delivers in winter and summer
The solar line in this calculator assumes a fixed number of peak sun hours a day and an 80% derate: the panels deliver about 80% of their nameplate rating times those hours. That allows for heat, dust, wiring and imperfect angles. It is an average, and real days swing widely around it.
Winter is the hard case. Days are shorter and the sun sits lower, so across much of the country a December day yields well under half of what a June day does, and a cloudy or snowy spell can bring recharge close to zero for several days. Outages from winter storms tend to arrive at exactly that time. If your outages come mostly in winter, try the calculator with fewer sun hours and see whether the coverage check still passes.
The coverage check compares the daily recharge with the energy you use each day. On the first day of an outage, a full battery carries you. After that, solar has to refill what you used. If recharge falls short, the calculator says how much of your daily need it covers. In a long outage, that is the part you would have to do without, or make up some other way.
Why the answer depends on how often your power goes out
The comparison turns on one number more than any other: outage days per year. A gas generator that runs only a few days a year costs little beyond its purchase price and upkeep, and its lower price usually keeps it ahead for many years. As outage days rise, its fuel bill rises with them, while the solar generator’s cost barely moves. Somewhere in between the two lines meet, and the calculator finds where.
Heavy use cuts the other way for solar. A battery is rated for a number of full charge cycles before its capacity drops to about 80%. Most people using a power station for backup will not reach that figure in a decade, but someone cycling it daily for long outages might. If the cycles you enter pass the rated life inside the ownership period, the calculator adds a replacement battery in that year.
Your own records are the best guide. Count the days your power was out over the last few years, set the years to how long you would realistically keep the equipment, and change the fuel price to what you pay. Then look at both the crossover year and the coverage checks before deciding which trade-off suits your home.
This calculator is an estimate for planning. It does not include financing, tax credits, the value of your time spent refuelling, or the cost of storing fuel safely.