Backup power planner

How we size a backup pack

A backup pack is described by three different quantities, and confusing them is the most common reason a purchase misses the mark. Energy in watt-hours says how long the pack will last. Continuous power in watts says how much can run at the same time. The starting surge says whether the inverter can get the fridge compressor turning at all. A pack with plenty of capacity and a weak inverter shuts down in the first second, with all that energy still sitting unused.

Appliance energy = sum of (power x units x hours x duty cycle)

Drawn from the battery = appliance energy / inverter efficiency

Nominal capacity = (drawn from the battery / depth of discharge) x (1 + reserve)

Peak at start-up = largest single surge + continuous power of everything else

We give the answer as a range rather than one number, because some appliances do not run continuously. A fridge cools in cycles, and over a day the compressor really runs somewhere between a third and two thirds of the time, depending on the room temperature, the efficiency class and how often the door is opened. One number here looks more confident but is not honest: the lower figure describes mild conditions, the upper one a heatwave and full load. Size the pack against the upper figure, because an outage rarely arrives when conditions are comfortable.

The peak at start-up assumes one start at a time: we take the largest single surge and add the continuous power of everything else. Several motors starting in the same instant is not modelled, and we say so rather than pretending to a precision we do not have. If the fridge and the pump can start in the same second, treat the figure shown as a lower bound and leave the inverter a clear margin.

Examples

A house through a full day without power

A fridge, a router, four LED lights for five hours and two phone chargers. Continuous power comes to 240 W, and the peak when the compressor starts is 720 W. Appliance energy lands between 1868 and 2588 Wh, so at 90 percent inverter efficiency, discharge to 80 percent and a ten percent reserve you need a pack of roughly 2850 to 3950 Wh. The whole spread comes from how hard the fridge works that particular day.

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Will a 1 kWh power station last the night

A station of 1024 Wh, a 1000 W inverter with a 2000 W surge rating, and a fridge plus a router to run for twelve hours. Power is not the problem: 140 W continuous and 620 W at the peak. Energy is less certain. In mild conditions the station covers the demand with room to spare; in a heatwave it drops to about 83 percent, which means running out roughly two hours early. This is exactly the case where a single averaged number would have said everything was fine.

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Keeping the heating alive in winter

A circulation pump and boiler controls for a full day, a little lighting and a router. The boiler itself draws very little, but it has to run around the clock and it has a motor, so it starts with a surge. That works out at 170 W continuous, 450 W at the peak and 1236 to 1668 Wh of energy. With a fifteen percent reserve, justified by the cold, you need a pack of roughly 2000 to 2700 Wh. This example shows best that pack size is decided by running time, not by how powerful the appliance is.

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Frequently asked questions

Where do I find the power of an appliance if the rating plate does not state it?

If the plate gives current and voltage, multiply them: 1.5 A at 230 V is about 345 W. Where the appliance has an external power supply, the figure is printed on it. The profiles in the calculator suggest values typical for the category, but an individual unit can differ noticeably, so a reading from your own appliance always beats a suggestion.

What is the difference between continuous power and starting surge?

Continuous power is what the appliance draws in normal operation. The starting surge is the brief draw at the moment a motor or compressor starts, usually several times higher and lasting a fraction of a second. The inverter has to survive both. The most common failure of a backup setup is not running out of energy; it is the inverter shutting down when the fridge starts.

Why do I get a range instead of one figure?

Because appliances with a duty cycle, such as a fridge, a freezer or a pump, do not draw power continuously, and the share of running time depends on temperature and load. The range shows mild and difficult conditions. One number would be more comfortable, but it would suggest a precision that is not there. Size the pack against the upper end of the range.

What depth of discharge should I set?

Lithium iron phosphate batteries take 80 percent, and often 100 percent of the usable capacity the manufacturer states. A lead-acid battery should not go below half if it is to survive hundreds of cycles. A deeper discharge works once, but shortens the life of the battery. Ready-made power stations usually quote capacity that is already usable, and then you enter 100 percent.

Does inverter efficiency really matter?

At a typical 90 percent, every tenth watt-hour turns into heat, so the pack has to be correspondingly larger. An inverter also draws current simply to stay awake, even with nothing plugged in, which becomes noticeable over a long outage. If you can run a load directly on direct current, you avoid these losses entirely.

Why add a reserve when the answer is already a range?

The range describes uncertainty about how the appliances behave. The reserve covers something else: ageing of the battery, cold weather that lowers the available capacity, and the fact that an outage can last longer than you assumed. Ten percent is a sensible minimum, and fifteen or more is justified when the heating depends on it in winter.

Can I share my appliance list with someone?

Yes. The copy link button stores the whole list together with the assumptions in the page address, so the other person opens exactly the same setup. With very long lists the address passes the point where some messengers truncate links, and the calculator warns you when that happens. Saving locally on your device has no such limit.

Does this calculator replace having an electrician size the installation?

No. It gives an honest pack size and shows the assumptions behind it, but it does not cover cable sizing, protection devices, earthing or the transfer switch between the mains and the backup supply. Wiring a backup supply permanently into a house installation requires a qualified electrician, and in many countries a notification as well.

How much does a desktop computer draw, and why does the graphics card change everything?

The figure printed on the power supply says what the supply can deliver, not what the computer draws. An office desktop without a separate graphics card usually sits between 60 and 120 W, while a machine with a strong card can reach 400 to 600 W, but only while the card is loaded by a game or a render. So enter the peak figure in the power field, and use the duty cycle to say what share of the time the computer really works that hard. Count the monitor separately.

Sources

• The rating plate and manual of the specific appliance: continuous power, starting current and supply requirements.
• The datasheet of the battery, inverter or power station: usable capacity, efficiency, continuous power and surge rating.

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