Electric dehumidifier running costs vs moisture absorber and silica gel

Up until recently, single use moisture absorbers were mainly marketed at keeping small spaces such as wardrobes dry.  This makes sense given that most wardrobes don’t have a power socket.  Now there are many larger moisture absorbers such as the UniBond Aero 360 marketed at keeping rooms or even the whole home dry.

In this article, I roughly calculate the running costs of various types of dehumidifiers and suggest when to use an electric dehumidifier, moisture absorber or silica gel.

Update 27th May 2017: Added a video to show water dripping inside an electric dehumidifier while in operation.

Non-electric and rechargeable dehumidifiers

I’ll start by looking at various single use and rechargeable dehumidifiers.  These are cordless and operate in places such as storage boxes, wardrobes and motorhomes depending on the collection capacity.

Moisture Absorber

Most moisture absorbers use salt crystals such as calcium chloride to absorb moisture from the air.  As moisture is absorbed, the crystals start dissolving to form a brine solution which drips into the container below.  Once the salt crystals have dissolved, the brine solution can be discarded and a fresh salt crystal refill is required to absorb further moisture.

For the running cost, I’ll pick the popular and regularly advertised UniBond Aero 360, where the cheapest refill pack I could find is a 4 pack of refills for £9 on Amazon when I posted this.  Going by the description, each refill lasts up to 3 months, although it’s not clear how much water each refill collects from the official product details.

After checking various user product reviews on Amazon, I came across a handful mentioning that each refill collects roughly 1 litre.  Many users reported getting roughly 3 weeks per refill compared to the advertised ‘up to 3 months’.  I’ll base my calculations on these figures.

With Amazon’s 4 pack offer, this works out at £9 for 4 litres or £2.25 for 1 litre.  In Euro, this currently converts to about €2.50 per litre.  If a refill lasts 3 weeks, then this works out about €3.57 per month.  It may seem like a cheap running cost, but even at this more rapid than advertised usage rate, this only works out about 58ml per day collected.

Rechargeable Dehumidifier (silica gel based)

Most reusable dehumidifiers use silica gel that adsorbs moisture from the air and the regeneration (recharging) process varies depending on the type of product.  Large silica gel bags generally have the highest capacity to cost ratio, but some can only be recharged by baking them at 120C for several hours in the oven.  Some smaller silica gel bags such as the Pingi can be recharged in the microwave.  There are also hard-cased models that can be plugged in overnight to recharge.

Silica gel can adsorb up to around a quarter of its weight in water before it needs to be regenerated.  Some silica gel packs and containers contain a colour indicator to show when it needs to be recharged.  This is especially important with microwave regeneration as attempting to microwave a dry pack can cause it to overheat.

I’ll base the running cost with the popular 300g Pingi Dehumidifier pack that states it holds 60ml of water and takes 6 minutes to regenerate in a 600W microwave.  This may not seem like a lot of moisture, but it’s adequate for small semi-sealed enclosures such as plastic storage boxes and small wardrobes.  For a large wardrobe, one can easily use a handful of desiccant packs.  For the electric cost, I’ll use Electric Ireland’s standard tariff of 17.2c/kWh at the time of posting.

Most microwave ovens consume about double their output rating in electricity, so a 600 watt microwave would use about 1200 watts of electricity.  For 6 minutes to regenerate the 300g Pingi, this works out at 0.12kWh or 2.064c per 60ml, assuming it was fully saturated each time.  For a litre, this works out at 34.4c per litre.

It is worth noting that regenerating any silica gel pick in the microwave or oven will result in the humidity being released into the room. To avoid steaming up the kitchen, operate the extractor fan to help vent most of the released water vapour.  For oven baking, allow the silica gel packs to cool down before removing them from the baking tray.  Some packs are heat sealed which can easily break open if handled hot.

Electric dehumidifiers

There are three main types of electric dehumidifiers available, which include Peltier, compressor and desiccant types.  I will look at these starting from the smallest type and use Electric Ireland’s standard tariff of 17.2/kWh for calculating the running costs.

Peltier Dehumidifier (thermoelectric / solid state)

This type of dehumidifier uses a solid state thermoelectric device that pumps heat from one side to the other when power is applied.  As air is drawn over the cold side, moisture condenses and drips into the container below.  This chilled dry air is then drawn over the warm side before being vented out by a fan.

Peltier based dehumidifiers are typically rated up to 500ml per day have a container capacity up to 2 litres depending on the model.  While their power consumption may seem low at between 20 and 60 watts typical, they generally run non-stop until the water container has filled up.  The only noise generated is from the small fan inside that draw the air through the unit, so they are generally quiet to run.

Based on the user reviews I checked of various devices rated at 500ml per day, the extraction rate is about half this value when operated around 20C 60%.  So, I’ll base this calculation on the Pro Breeze 1500ml, drawing 60 watts and reported to extract about 250ml per day.

At 250ml per day, it will take 4 days of non-stop operation to collect 1 litre, assuming ample humidity to sustain this collection rate.  60 watts continuous over 4 days works out at 5.76kWh.  At 17.2/kWh, this works out at 99.1c per litre collected.

Desiccant Dehumidifier

A desiccant dehumidifier contains a desiccant wheel usually made of zeolite to adsorb moisture as air is drawn through it.  As the wheel desiccant wheel rotates, a heater blows hot air through a slice of the wheel to regenerate the zeolite.  This moist hot air goes through a heat exchanger where it cools and the moisture condenses, which then drips into the water collection bucket below.

Zeolite retains most of its adsorption capacity regardless of the room temperature and humidity level.  Thus, desiccant dehumidifiers operate close to their rated capacity even at very low room temperatures and at low relative humidity levels.

Based on what I’ve read in user reviews and my experience with the Trotec 55 E, most 7 litre rated models use around 630 watts on high extracting 7 litres per day and 330 watts on low extracting just over 3 litres per day.  With the Trotec 55 E based on watt meter measurements, it extracts roughly 500ml per kWh on high and 400ml per kWh on low.  The 500ml per kWh is roughly what I’ve heard other desiccant machines collect, so I’ll base my collection on this.

This works out at 2kWh per litre on high and 2.5kWh per litre on the lower setting.  At 17.2/kWh, the running cost works out at 37.4c per litre on high and 43c per litre on low.

Compressor dehumidifier

A compressor dehumidifier works as follows:  A compressor compresses refrigerant gas into a condenser coil where it liquefies and releases latent heat, heating the condenser coil.  As this refrigerant liquid passes through an expansion valve into the evaporator coil, it evaporates back to a gas drawing in latent heat, cooling down the evaporator coil.  As room air is drawn in over the cold evaporator coil, moisture condenses and drips into the water collection bucket below.  This cold air is then drawn over the hot condenser coil and the dry air is vented back into the room.

The extraction rate varies a lot depending on the room temperature and even the relative humidity.  The higher the room temperature and relative humidity, the higher the extraction rate.  So unsurprisingly, the extraction rate promoted on most compressor dehumidifiers is based on the highest operating temperature and relative humidity the machine is designed to operate at, usually 30C 80% RH, more typical of a greenhouse than a damp room.

In colder rooms, particularly below 15C, the condensed moisture will freeze on the condenser coil, building up a layer of frost.  When this happens, the dehumidifier needs to run a defrost cycle.  Most dehumidifiers switch off the compressor periodically for several minutes to allow the frost to melt.  Higher end dehumidifiers contain a heater or a refrigerant reversing valve (known as hot gas defrost) to rapidly defrost the coil, improving the extraction rate at low room temperatures.

Based on my experience using various compressor dehumidifiers, they extract around a third of their rated capacity with a typical room temperature of 20C and relative humidity of 60%.  This rate falls in half in colder rooms of 10C to 15C (depending on the defrost method) with the same relative humidity level of 60%.  The power consumption is typically ¾ the rated power consumption at these temperatures.

For a typical 12 litre dehumidifier rated at 240 watts, this will collect about 4 litres a day at a room temperature 20C 60RH or about 2 litres per day in a cold room about 15C.  Power consumption will be about 180 watts average under these operating conditions.

Based on actual watt meter measurements of various compressor dehumidifiers, they typically extract about 1 litre per kWh at 20C 60% RH and about 500ml per kWh between 10C and 15C at 60% depending on the dehumidifier.  At 1 litre per kWh, this works out at around 17.2c per litre running at room temperature and around 34.4c per litre at 10C to 15C.

As these figures are based on typical compressor dehumidifiers I’ve used over the years, the running costs are likely to be much lower with the newer eco models that have more efficient compressors and air flow handling.  Unsurprisingly, the eco models also have a much higher upfront cost, so the lower running cost would only benefit users that run them over extended periods such as for drying laundry.

Summary of estimated running costs

• Moisture absorber (UniBond Aero 360) – €2.50 per litre
• Rechargeable / silica gel (microwave based) – 34c per litre
• Compact Peltier based (Pro Breeze 1500ml) – 99c per litre
• Desiccant model (auto/low/quiet setting) – 43c per litre
• Desiccant model (continuous/high setting) – 34c per litre
• Compressor model (warm room, 60% RH) – 17c per litre
• Compressor model (cold room, 60% RH) – 34c per litre

Which type to use

Moisture absorber (e.g. UniBond Aero 360) – Enclosed spaces, particularly without an electricity supply, such as a walk-in wardrobe or an unoccupied caravan/motorhome.  The high extraction capacity compared to silica gel makes it ideal for those who do not want to frequently regenerate silica gel packs, particularly in a motorhome or caravan over the winter period.  As a rough guide for calcium chloride crystals, these collect roughly 1.5 their weight in water at 60% relative humidity, so a 1kg pack will typically absorb about 1.5 litres of water.

Rechargeable / Silica gel – These are ideal for small enclosed spaces such as wardrobes and plastic storage containers.  There is no risk of a leak or spill either as the moisture is adsorbed into the silica gel.  My recommendation would be to go for the type that can be regenerated in the microwave.  The cheaper silica gel that needs to be baked can take several hours to regenerate or even several days on top of a radiator.  As a rough guide, these hold roughly ¼ their weight in water, so a 400g pack will typically adsorb 100ml of water before it is saturated.

Peltier / thermoelectric dehumidifier – Best suited for larger wardrobes with a power socket inside or a caravan/motorhome that is in regular use.  To keep the running cost down, I suggest using a timer when used in a wardrobe and placing a hygrometer inside to keep an eye on the humidity.

Compressor dehumidifier– Ideal for heated homes and especially for drying laundry, these are the most energy efficient at extracting moisture.  Wet laundry quickly raises the relative humidity of the air as the water evaporates and the higher the extraction rate gets with a compressor dehumidifier.

Desiccant dehumidifier – Best suited for unheated rooms such as a loft or garage where the temperature typically falls below 15C in the winter.  These maintain an extraction rate near their rated capacity even at very low temperatures, whereas many compressor models fall below 1/6^th their rated capacity around 10C to 15C.  This means that a bulky 30 litre rated compressor dehumidifier may only extract as much as a 7 litre rated desiccant dehumidifier below 15C, even if the running cost per litre is similar.

A desiccant dehumidifier can also be used in place of an electric heater as it will product similar heat to a low power electric heater while doing its job at collecting moisture.  When operated on low (auto / eco setting), it can help maintain the temperature in the room after the central heating has switched off.  Finally, a desiccant dehumidifier runs quietly like a fan heater without the low fridge-like drone of a compressor model.

Electric vs moisture absorber example

To give an idea of how quickly an electric dehumidifier gets to work compared to a non-electric moisture absorber, the following image on the left is one of the main selling points of the UniBond Aero 360 moisture absorber and the right image is what a desiccant dehumidifier (Trotec 55 E) collected in just 2 hours on high poured into a pint glass.

In fact, had I tried running the desiccant dehumidifier for 12 hours, it would have filled its 3 litre container before the 12 hours were up, assuming there was ample humidity in the room to collect.

If you are still not convinced how rapidly a dehumidifier collects water, have a look at the following short video: