When you first start researching robotic lawnmowers, you will see models highlighting a range of features, from GPS tracking and smart features, to cutting width and battery charging time. Having gained a lot of knowledge about the various features over the last few years, I will try to summarize the main things to look for, highlighting the must have features of robotic lawn mowers and what to avoid.
1. Cutting Height (Min & Max)
Being from the UK, I have always had lawns that generally look better cut short and often, so I run my current robot lawnmower at the lowest setting of 20 mm. However, depending on your grass variety, climate and preference, you may wish to mow your lawn at a considerably higher length.
A lot of robot lawnmowers top out at approximately 60 mm maximum cutting height, but some more premium models have maximum cutting heights of up to 102 mm. This is definitely a feature to be aware of to ensure that your choice of mower can fulfill your needs, although, for the majority of users, most models will provide a suitable cutting height.
2. Cutting Width
As a robotic lawnmower moves across your lawn, it will cut a strip of grass that equates to the width of the cutting blades. Models at the more economical end of the market, and those designed for smaller gardens, typically have a narrower cutting width. Models that use multiple razor blades on a rotating disc typically have a narrower cutting width than models that use a solitary spinning blade.
A narrower cutting width is not necessarily a bad thing, as this will often mean quieter operation. Most robot lawnmowers are designed to take considerably longer to cut your lawn than when being cut with a conventional lawnmower, so I would advise looking more closely at the noise output of the mower while in operation, rather than the cutting width of the blades. Both machines will get the job done, but a near silent model wont disturb the peace of your garden.
3. Battery Type
This is an important area to consider when selecting a robotic lawn mower, as not all models use modern lithium ion batteries. The type of model to avoid is those that still use lead acid batteries. Lead acid batteries are cheaper than lithium ion batteries and are still found in some models at the lower end of the price spectrum. However, the advantages over lithium ion batteries stop there.
They hold much less charge than a lithium ion battery of the same size and weight, last for fewer charge cycles and take a lot longer to charge than lithium ion batteries. A typical model using a lead acid battery will take 16-20 hours to charge versus 1-2 hours for a lithium ion battery.
Another important point is that the capacity of a lead acid battery tends to reduce from one year to the next, and the degradation in charge capacity accelerates at temperatures about 25 degrees celsius.
Lithium-Ion batteries, have all of the advantages that lead acid batteries do not. They have higher energy density, longer life, faster charging and lighter weight. The cost of lithium ion batteries has decreased markedly over the last few years. I currently run the Flymo 1200R in my own garden and replacement batteries currently cost £35 and last for 3-5 years. I think it is essential to pick a model that uses lithium ion battery technology, as the advantages far outweigh the higher cost.
4. Operational Time
The run time of a robot lawn mower is determined by a number of factors. Larger, heavier mowers, with single blade cutting systems will usually have greater power requirements, which needs to be offset by higher battery capacity to maintain run time between charges. Obstacles, such as steep hills, long grass and multiple obstacles in your garden requiring lots of turning and maneuvering will increase power consumption.
Ultimately, run times can range anywhere between approximately 60 minutes up to several hours, such as the Husqvarna 450X, which manages 270 minutes cutting time after only 60 minutes charging.
5. Charging Time
Like your robot lawn mower’s running time, charging duration will vary from one model to the next. The type of battery has the greatest impact on the charging time, but the capacity and charging rate will also play a significant role. Most consumer orientated mowers that use lithium ion batteries normally take around an hour to charge fully and you should get longer than this of mowing time.
As mentioned previously, the models to avoid are those that still use lead acid batteries, as these can take 16-20 hours to charge, so significantly reduces the flexibility of when you can schedule your mower to cut your lawn.
All robot lawn mowers are mulching mowers, which means that they are designed to cut your lawn frequently, removing a small portion of the length of the grass during each cutting, and leaving these tiny clippings to be integrated into the soil of your lawn, returning the nutrients to be used by the growing grass.
Ask anyone who is serious about maintaining a good looking, healthy lawn and they will tell you that the essential elements are frequent cutting, watering and provision of nutrients to your soil.
Prior to owning a robot lawnmower, I used to fertilize my lawn 3-4 times per year. I would get a fantastically healthy green lawn for 2-3 weeks, but each time I cut the grass and removed the clippings, my grass would get less and less green and vibrant. I had tried leaving the clippings on the grass, but didn’t have time to cut the grass frequently enough, so the clippings would clump up and make the grass look unsightly.
I now fertilize my lawn at the start of the growing season, and my mower cuts a little every day, returning all the nutrients to the soil, ensuring that my lawn stays green and healthy all the way through the growing season.
7. Blade Types
There is a significant distinction to be made between the two main cutting systems that will be found on the vast majority of robot lawn mowers.
Firstly, there are mowers which have a number of razor blades attached to a spinning disc. These are very effective and use little power. The downside of this system is that as the blades are located quite far under the machine, it is very difficult for the mower to cut right to the edge of your lawn. The razor blades are also fragile and will blunt quite quickly, needing replaced every few months.
Thankfully, replacement blades are cheap and it only take 2-3 minutes to change all of the blades.
Secondly, there are mowers, such as those made by Robomow, which use a solitary cutting blade, which looks not dissimilar to the blades found on conventional lawnmowers. Due to their size and sturdier build construction, they typically need replaced less frequently, and are able to cut a wider strip of grass as the mower moves. They also cut closer to the edges of the mower undercarriage, so edging is more effective.
The main downsides of this type of cutting mechanism is that it uses more power, creates more noise, and can cause more damage to items inadvertently left on the grass.
8. Noise Level
Robot lawn mowers are much quieter than conventional electric or petrol powered lawnmowers. This is due to a variety of factors. However, is is more important for robotic lawn mowers to be quiet, as they work much more slowly than conventional lawn mowers and will be in use for much greater periods of time. If I had to contend with a noisy mower cutting my lawn for hours at a time, I wouldn’t even consider buying a robotic lawnmower. Thankfully, at 58 decibels, my own robot lawn mower can’t even be heard when it is working away on the grass 10-15ft away and I am relaxing on the patio.
It should be noted, that robots that use a single blade cutting system typically create more noise than those which use multiple mobile blades on a cutting disc. A typical multiple blade robot lawnmower will emit 58 decibels under use, whereas a single blade robot lawnmower will emit 72 decibels.
To put this in perspective, a difference of 10 decibels equates to a doubling in perceived noise. Going by equivalent sound levels, multi-blade machines are roughly equivalent to the noise of normal conversation. Single blade robot lawnmowers make as much noise as a vacuum cleaner.
As a result of this, I would strongly recommend a model that uses a multi-blade system, particularly if you have a small garden and will spend a lot of time in your garden while the mower is in action.
9. Safety Features
Robot lawn mowers are all equipped with lift and tilt sensors, which will cause the blades to immediately stop spinning if the mower is unexpectedly lifted or tipped. The blades of most mowers are well under the unit, so inadvertent contact with a human or pet will result in no injury, and the mower will simply turn and go in a different direction.
Models which use a cutting disc and razors have much less cutting power in the blades, and even if the mower goes over the top of a small item left on the grass, it will cause very little damage, as the blades have sufficient power to cut blades of grass, but insufficient power to cut much else.
Models with a single cutting blade can cause significant damage to items run over, but again, the chance of injury to a human or pet is minimal as you must lift or tilt the mower to access the blades, causing the machine to immediately stop the blades.
Overall, it is hard to even compare the safety of conventional lawnmowers with robotic lawnmowers, as I feel that the difference in safety is so great. Personally, I am happy to let my children enjoy the garden under supervision while my robotic lawnmower is in operation, but would never leave them unattended in the garden while the mower is working, as we all know what mischief children can get up to.
Generally, I schedule my mower not to operate during the day at the weekends, so this helps to mitigate any risk that may be present. I’ve written a detailed article about the safety aspects of robot lawn mowers, which you may be interested to read.
10. Collision Sensors
All robot lawnmowers have basic collision detection systems, which allow them to detect and move away from obstacles, to continue their work unimpeded. Basic models rely on contact with obstacles to trigger the unit to back up, turn and move in a different direction.
More advanced models have proximity sensors which allow the robot to slow down before impacting the obstacle, so that it can cut as close as possible to the object, without undue trauma to the mower or the object. Further developments will be integrated into new models, such as visual obstacle detection, much the same technology which is being developed for autonomous vehicles.
11. Maximum Slope Tolerance
Robot lawn mowers are generally designed for efficiency rather than power, so most models are limited in their capacity to handle very steep slopes. Greater slope ability requires more powerful motors, greater battery capacity and heavier duty wheels. This is definitely an area that you should check before making a purchase of a robotic lawnmower.
If your garden is reasonably flat, you will have no problems at all going for any model, but for steeper or more irregular lawns, you will need to ensure that the model you are purchasing can handle the steepest slopes in your garden.
An important point to note is that some manufacturers report maximum slope ability in degrees, while others report in grade percentage. These are not the same.
Grade is normally expressed as a percentage of the rise/run of a measured slope.
The slope angle is a measure of the angle of inclination of the slope to the horizontal.
The majority of models will be able to manage slopes of at least 14° (25%), but higher end models can manage up to 25° (45%). If you are not sure how to calculate the slope of your lawn before deciding which model to purchase, try this quick guide.
One of the major reasons why you may hold off purchasing a robotic lawn mower is the cost associated with purchasing one. For me, this was what put me off until the last few years, as prices have fallen significantly. As the technology matures and volumes sold increase, the price of robot lawnmowers will continue to fall, making them more comparable to conventional lawnmowers.
Many of the more affordable models can be purchased for £450-£700, and when you compare this to the cost of a conventional lawnmower and factor in the free time that you gain from not having to mow the lawn, the economics start to make more sense. There are also fewer mechanical parts on an electric robot lawnmower, compared to a petrol powered conventional lawnmower, so servicing and maintenance costs are cheaper.
For people who currently employ a gardener, the economics make even more sense. In my area, for me to employ a gardener to cut my grass once per week would cost £25. If this is done from April to September, it would cost £575 per year to maintain my lawn. This was less than the price I paid for my current robotic lawnmower. Now obviously, you could reduce the frequency you were cutting your lawn, or try to get a keener price from a gardener, but this would still equates to less than a 2 year payback for people who currently have a gardener cutting their lawn.
Robot lawnmowers typically start at approximately £450, but premium models for large gardens, such as the Husqvarna 450X, can cost up to £3000.
13. Multi Zone Capability
For people who have a garden with more than one lawn, it is important to pick a mower that has multi-zone functionality. Robot lawn mowers with multi-zone capability can travel independently and directly to and from several cutting areas, allowing them to transition between areas of lawn without needing to manually reset its location. Often, this may involve running the perimeter wire along, beside or under paths. T
The settings of most robot lawn mowers can be operated via a control panel on the mower to adjust cutting times, days and cutting blade height. More advanced models have wifi and bluetooth connectivity, allowing you to use a smartphone or tablet app to log in and adjust the settings remotely. Many of these features are down to personal preference.
15. Apps For Mobile Devices
So you want to check on your lawnmower’s progress while at work or on holiday. Yep, there’s an app for that. Many of the more premium and newer models now have apps that can be used to monitor your mowers work schedule, cutting pattern and performance. You can adjust settings to make the mower focus attention on certain areas, decrease or increase the cutting intensity and to check on the vital stats of your mower’s performance.
16. Artificial Intelligence?
Most robotic lawnmowers on the market today use the same approach that was initially championed by Roomba from iRobot, for vacuum cleaning. This is basically a random walk where the robot uses a behavior based architecture. The robot changes it’s pathing behaviour regularly, both randomly and based on its sensor input.
Why random you ask? Simply, because doing more complex tasks would require the robot to build and maintain a map of the lawn, while localizing within this incomplete map, static and non-static obstacles which must be avoided. This would be very computationally heavy, and therefore expensive to implement, and would not lead to a significantly better result, when the task of mowing the lawn can be done sufficiently with a random walk technique.
As technology develops, and the cost of computational power falls, we are starting to see models coming to the market that are incorporating basic AI capabilities, similar to the features which are being developed for autonomous driving. This should lead to the robot lawnmowers of the future needing less initial set up and being able to perform their job more efficiently and with less errors.
17. GPS Tracking
GPS tracking is available on a number of the more expensive models on the market today. Whilst this feature can allow you to view the precise location of your mower in real time on a connected app and adjust the mowers behavior and mowing pattern, it is also a very significant safety feature.
Although most models have safety features which will render them useless should they be stolen, it would also be nice to get it back if a thief does wander off with it. With GPS features, you could could report the theft and provide the exact location of the unit to your local police, who could then retrieve it and apprehend the thief.
18. Weather Resistance
The vast majority of models are rated for use in most weather conditions, although most manufacturers recommend lifting them in from the garden in severe storms or in freezing conditions, due to the risk of damage. Lithium Ion batteries do not respond well to extreme cold temperatures, so I would always recommend lifting your lawnmower into a dry store or garage at the end of the growing season. Typically, when the temperature is under 10ºC, your grass will stop growing, so you should have plenty of time between this and the onset of any snow and frost.
Some models have moisture sensors and will return to their docking station when it is raining and only cut the grass when it is dry. I don’t view this as being a particularly important feature, as most models are sufficiently waterproof rated to prevent any issues in the wet. Also, being from a rather damp region of the UK, my poor mower may not get any work done for weeks on end if it could only work in dry conditions.
19. Theft Concerns
As alluded to above, robot lawn mowers can be a target for thieves, although to be honest, I really don’t know why, as the security features of even the most basic models render stolen devices useless. All models that I am aware of use pin locks and alarms, so that any time you want to start the mower after it has been interrupted or lifted, you must insert the pin code.
Some models are also intrinsically linked to the specific base station that comes with the mower, so that theft of the mower will also render it useless, if someone tries to use it with a different base. GPS features allow you to know the exact geographic location of the unit, as long as there is charge in the battery. While not actively mowing, the battery should hold it’s charge for months at a time, so this should not be an issue.
All robotic lawnmowers are electrically powered and use very little power in operation. I was astounded to learn that the annual electricity cost of my unit is less that £10. This is for a lawnmower, operating 7 days per week in a garden of approximately 400m². My lawnmower has a rated power requirement or 30 watts in operation, which is about the same as half a standard lightbulb, or four standard LED lightbulbs.
If you have any questions about robot lawn mowers, please leave a question in the comments section, or use the contact form to get in touch. I would be delighted to help. I have written a comprehensive guide to robot lawn mowers here. This discusses all of the main features in detail and I have recommendations for the best robot lawn mowers for all lawn sizes.