How long will a power station last, and which station, UPS, solar panels or generator to choose for blackouts

Six tabs, one list

Everything starts from Appliances: a list with ready templates (fridge, router and ONT, laptop, phone, TV, LED lamps, a gas boiler’s electronics, a circulation pump, a CPAP machine, a freezer, a kettle, a microwave, a water heater and more). Change any number or add your own appliance. The other tabs use the same list: How long it lasts for your battery, Which to buy for a given outage, Charging with your outage schedule, Solar panels by month and Generator with fuel and money.

The summary on the right (on a phone, the bar at the bottom) always shows how long the battery lasts, the average and peak load and the energy a day, with warnings when something does not fit. Under every tab, How it is calculated shows the formulas with your numbers, so any result can be checked on paper. The list and the settings stay in this browser; Save to history keeps a calculation to compare later.

How much appliances really draw

Every appliance has four numbers. Power is what it takes from the socket while it works (the label says the maximum; a plug-in power meter tells the truth). Start × is how many times more a motor or a compressor takes for a fraction of a second when it starts. Runs, % is the share of the time it really draws power: a fridge compressor switches on and off. Hours a day is how long you keep it on.

From these, PowerBench works out the energy a day (the sum of power × pieces × share of time × hours), the average load (that energy over 24 hours: it decides how long a battery lasts), all on at once and with a motor starting (it adds the biggest start of one appliance, because motors seldom start at the same instant). The last two must fit the station’s rated and peak power. Appliances marked needs a pure sine (motors, compressors, boiler electronics, CPAP) need an inverter with a pure sine wave.

How long a battery lasts

A battery labelled 1024 Wh does not give 1024 Wh to your fridge. On the tab How long it lasts the capacity is reduced step by step, and every step can be changed:

• Depth of discharge: the part of the battery you allow to use. 90 % for LiFePO4, 85 % for NMC, but only 50 % for lead-acid if it has to live more than a couple of hundred cycles.
• Reserve for aging: after a few hundred cycles a battery holds less; 10 % is a fair reserve for a station a year or two old.
• Temperature: a cold battery gives less, about 0.7 % per degree below 20 °C for lithium and 1 % per degree below 25 °C for lead-acid. A station on a cold balcony at 0 °C loses about a seventh.
• Peukert effect (lead-acid only): the capacity of a lead battery is rated for a 20-hour discharge. A larger current gives less: at twice the rated current an AGM battery gives about 90 %.

Then the inverter: it turns the battery’s direct current into 230 V and loses 8–15 % on the way (efficiency 85–92 %), and it burns its own consumption of 5–30 W all the time it is on, even with nothing plugged in. So a router of 15 W on a big station spends more on the inverter than on itself; for such loads the station’s USB or 12 V outputs last much longer.

usable energy = capacity × DoD × (1 − aging) × k(cold) × k(Peukert)
draw on the battery = average load / efficiency + own consumption
hours = usable energy / draw on the battery

Example: 1024 Wh LiFePO4, 90 %, 10 % aging, 20 °C → 829 Wh usable. A fridge, a router and four lamps average 65 W; with a 90 % inverter and 10 W of its own the battery gives 82.6 W, so 829 / 82.6 ≈ 10 hours.

The warnings say when the station cannot take everything at once (its rated power), the start of a motor (its peak power; twice the rated is assumed when you leave it empty), when a modified sine meets appliances that need a pure one, and when a lithium battery is too cold to charge.

LiFePO4, NMC or lead-acid

Most new stations use LiFePO4, and for daily blackouts it is the right choice: thousands of cycles make each kWh cheap. NMC is lighter and fine for a station that travels. Lead-acid batteries with a UPS are cheaper at first, but each one lives a few hundred cycles at half depth: choose the type in the calculator and it switches to amp-hours, 50 % depth and the Peukert correction.

Which station to buy

The tab Which to buy runs the calculation backwards: for 4, 8 and 12 hours and the outage length you set, it shows the nominal capacity you need (with the usable part and the margin), the continuous and peak power, the same capacity in amp-hours for LiFePO4 at 12.8 V and for AGM at 12 V, and a class:

When you compare stations, look at more than capacity:

• Pure sine output, and a continuous power above “all on at once” with the margin.
• Peak power above the start of your fridge or pump.
• Charging power from the mains: with outages of 4 hours on and 4 off, a station that needs 5 hours to charge never fills up.
• UPS (EPS) mode and its switchover time if a computer or a boiler must not notice the outage: under 10–20 ms is good.
• Battery chemistry and cycles, the warranty, and whether extra batteries can be added later.

PowerBench names no brands on purpose: check the numbers on the label or in the official specifications of the model you are looking at.

A UPS for a gas boiler

A wall-mounted gas boiler with a built-in pump takes 60–150 W; without electricity it stops, and the heating with it. The usual answer is a UPS with a pure sine and an external battery, or a station in UPS mode. Size it the same way: add the boiler and the pump (and the router, if you like) and see the capacity on Which to buy. For 85 W through a 12-hour night (with the default 10 % reserve for aging and 15 W of the inverter’s own consumption) a lead-acid AGM battery at 50 % depth needs about 3 kWh nominal, 256 Ah at 12 V, so three 12 V 100 Ah batteries; LiFePO4 needs about 1.6 kWh.

Three things matter more than capacity: the UPS must give a pure sine, many boilers need the right phase and earth (plug in the same way as before, use a socket with earthing), and the UPS’s charger must be able to fill the battery between outages: compare its charging current with the schedule on the Charging tab.

Charging and the outage schedule

Charging time is the energy to put back (the usable part plus the charging losses) divided by the charging power, with a “tail” for the last 10–20 %, which goes slower: ×1.1 for lithium, ×1.4–1.5 for lead-acid. From a generator a station charges through the same mains input, so it is limited both by the station and by 80 % of the generator’s power. From the sun it depends on the panels, the station’s solar input (the smaller of the two counts) and the month.

The outage schedule (for example 4 hours without power, 4 with) is simulated minute by minute for three days from a full battery: during an outage the battery feeds the appliances, when the power is back they run from the mains and the battery charges, slower near the top. The chart shows the charge; the verdict says whether it lasts every outage, whether it charges back in time, how long the refill takes and how low it gets. If it does not fill up, it gets lower with every cycle until a day comes when it runs out.

Solar panels by month

A panel’s watts (Wp) are measured at 1000 W/m² of sun. A peak sun hour is one hour of such sun, and the monthly irradiation in kWh/m² a day is exactly that number of hours. A panel then gives about Wp × sun hours × (1 − losses) watt-hours a day; losses of 14 % (heat, dust, wires, the controller) are the default of PVGIS.

The sun hours are a static table, averaged over 2005–2020, for all regional centres of Ukraine and a few European cities, at tilts of 0°, 15°, 30°, 45°, 60° and 90° facing south; values in between are interpolated. They were taken once from PVGIS (Photovoltaic Geographical Information System, Joint Research Centre of the European Commission, database PVGIS-SARAH2), so the tool sends no requests. You can also type your own sun hours, for example from a PVGIS calculation for your address.

That is the honest picture: the same household needs six times more panels in December than in June. In winter panels help, but rarely carry a house alone, and snow or shade from a tree or a neighbouring building (which is not in the data) cuts the output several times. On a balcony rail (90°) a panel gets more of the low winter sun than lying flat, but less in summer; choose the tilt with the buttons and compare the months.

Generator or station

A generator burns part of its fuel just to keep turning and part in proportion to the load: litres an hour = kW × (a + b × load share). The typical coefficients for a petrol, an inverter petrol, a diesel and an LPG generator are filled in; if the manual gives the consumption at 50 % and 100 % load, enter your own. For a 3 kW petrol generator this gives about 0.4 l/h idling and 1.7 l/h at full power.

The cost of an hour is fuel × price plus oil and service; the cost of a kWh is that divided by the load. That is why a generator is expensive for small loads: running a fridge and a router (200 W) a 3 kW generator burns most of its fuel just to turn, and a kWh costs several times more than at a decent load. A station’s kWh costs the mains tariff with charging and inverter losses plus wear: the station’s price spread over all the energy it stores in its life (cycles × usable capacity).

The chart adds both up day by day, from their prices, for the hours without power a day you set, and shows on which day the station pays off. It also says when the station alone cannot cover the whole outage: then the best pair is often a station for the night and the quiet hours, and a generator to charge it a few hours a day at a good load.

Safety

• A generator only outdoors. Its exhaust carries carbon monoxide, which has no smell or colour and kills within minutes. Never in a flat, on a balcony or a loggia, in a garage, a basement or a shed, and not under windows: at least several metres from doors, windows and vents (US safety agencies say 6 m), with the exhaust pointing away. Put a CO detector inside and refuel only a generator that is off and cool.
• Gas boilers need a pure sine. A cheap UPS or inverter with a “modified sine” can burn the boiler’s board or the pump, and makes motors hum and heat up. Many boilers also depend on phase and earth.
• No charging LiFePO4 below 0 °C. Lithium batteries may be used in the cold, but charging them below freezing plates lithium inside and ruins the cells. Let a station from a cold car or balcony warm up indoors for a few hours, unless it heats its battery itself.
• Overload and extension leads. A kettle, a water heater or a heater takes 1.5–2 kW alone. Do not run several at once from a station or a generator, use a fully unwound extension lead of at least 1.5 mm², never chain them, and do not cover the station.
• Never back-feed the house wiring. A “plug-to-plug” cable into a wall socket sends voltage back into the grid: it can kill an electrician on the line and burn appliances when the power returns. Connect a house only through a changeover switch installed by an electrician.
• Batteries. Keep them away from heaters and sun, do not charge a swollen or damaged battery, and charge lead-acid batteries only where there is air: they give off hydrogen.
• Medical equipment. For a CPAP machine or an oxygen concentrator plan for at least twice the outage, keep a second source and agree with your doctor what to do if the power is off longer.

All the formulas

energy a day         E = Σ W × n × k × t                  (k = share of time it runs, t = hours a day)
average load         P̄ = E / 24
all on at once       P = Σ W × n
with a start         P(start) = P + max W × (start × − 1)
usable energy        E(b) = C × DoD × (1 − aging) × k(t) × k(P)
cold                 k(t) = 1 − 0.007 × (20 − t) lithium,  1 − 0.01 × (25 − t) lead-acid
Peukert (lead-acid)  k(P) = (I₂₀ / I)^(k − 1),  I₂₀ = C / U / 20,  k = 1.12–1.25
draw on the battery  P(b) = P̄ / efficiency + own consumption
runtime              t = E(b) / P(b)
size for an outage   C = P(b) × hours / (DoD × (1 − aging) × k(t) × k(P)) × (1 + margin)
charging time        t = C × DoD / charging efficiency / P(charge) × tail
solar, a day         E = Wp × sun hours × (1 − losses)
panels needed        Wp = (E / inverter efficiency + own consumption × 24) / (sun hours × (1 − losses))
generator fuel       l/h = kW × (a + b × P / P(rated))
generator kWh        (l/h × price + service) / load in kW
station kWh          tariff / (charging eff. × inverter eff.) + price / (cycles × C × DoD) / inverter eff.

The calculator checks itself against worked examples computed by hand: open PowerBench at #/selftest to see every case and its answer.

Privacy and offline use

Everything is calculated in your browser. The list and the settings are kept in this browser’s storage so they are still there tomorrow, and saved calculations stay in its history until you delete them. A share link carries only the numbers of the calculation (the appliances, the battery, the settings) after the # in the address, which is never sent to a server. The sun table is part of the page, so PowerBench needs no internet at all once opened, and it can be installed as an app: exactly what you want when the power is off.

Limitations

• Every number is an estimate. Real appliances draw differently from their labels, batteries age differently, and a fridge works harder in a warm kitchen. Plan with a margin and measure the important appliances with a plug-in power meter.
• The average load decides the runtime; the order in which appliances run does not. A kettle in the evening and a kettle in the morning take the same energy.
• The sun table is a long-term average for a south-facing panel without shade; a cloudy week can give much less, and east or west panels give less than south ones.
• Generator coefficients are typical; old or overloaded generators burn more.
• No brands. Classes are by capacity and power; check the specifications of a particular model on the maker’s site.

Frequently asked questions

How long will a 1 kWh power station run a fridge?

A typical fridge uses about 1 kWh a day, so a 1024 Wh LiFePO4 station runs it alone for about 13 hours with the default reserve for aging and 15 W of the inverter’s own consumption, and up to about 19 hours when new and with a frugal inverter. Put your fridge into PowerBench to see the exact number.

Will a station run a gas boiler?

Yes, if it has a pure sine output: a boiler with its pump takes 60–150 W, so a 1 kWh station keeps it on for roughly 5–10 hours. Check that the boiler is plugged in with the right phase and earth.

What size station do I need for a 4-hour outage?

Add your appliances and open Which to buy: for a fridge, a router, a laptop and lights it is usually 500–800 Wh; with a gas boiler and a pump, 1–1.5 kWh.

Why does the station last less than its Wh divided by the load?

Because only 85–90 % of the battery is used, the inverter loses 8–15 % and burns its own 5–30 W, and cold and age take their share. The formulas under How it is calculated show each step.

Can I charge a station on a frosty balcony?

Not a lithium one below 0 °C. Bring it in and let it warm up first.

Is a generator cheaper than a station?

For big loads and long outages often yes; for a fridge and a router a generator burns most of its fuel idling. The Generator tab compares both day by day with your prices.