This article explains how your energy use may increase under various weather conditions – to help you understand how this affects your car’s range and so your journey planning.
Weather conditions affect all types of car but for a Tesla driver there are two categories: ordinary factors increasing energy use (which we are now more concerned about because range is more important) and factors specifically affecting the electric drivetrain.
Tesla do not publish any of the details of how the car’s temperature management systems work, so the details presented in this article are based on experiment and hearsay.
Cold temperatures affect the car in a number of different ways. First is the effect of air density on drag – the biggest component of energy consumption at cruising speeds. Air is more dense at lower temperatures, and so takes more energy to push the car through it. This affects all types of car, and can cause a difference of up to 10% in fuel/energy consumption between summer and winter, depending on the severity of your climate, and assuming you drive at the same speed. The remainder of the issues are specific to the electric drivetrain.
Cabin heating is a significant winter cost: when operating at full blast, the various heating systems use over 7kW, so would run a fully charged battery flat in about 10 hours without going anywhere! However, full power will rapidly raise the temperature and maintaining it at a comfortable level will take much less – maybe 1kW (depending how warm you like it and the outside temperature). So ‘no-compromise’ winter heating typically incurs a cost of about 4kWh (12 miles range) to warm up the car at the start of the journey, and about a 5% penalty when cruising at 60mph (or 10% when averaging 30mph around town). If starting from a place with charging, you can avoid the start-up penalty (at least in terms of battery range) by pre-heating the car before unplugging from the charging point. The cruising cost can be reduced by turning down the temperature and using the seat heaters instead – seat heaters averaging about 50W per seat use much less power, especially for a lone driver.
The temperature also has a direct effect on the battery itself. At very low temperatures, the power that can safely be drawn from the battery without damaging it is reduced, and it can’t be charged at all. At slightly higher temperatures, full discharge power and slow charging is available; only above 10C is ‘normal’ performance available. These effects mean that the battery must be heated: fortunately, the simple act of using the battery (either charging or discharging) has a heating effect – normally a disadvantage, but working in our favour at low temperatures. There is also an electric battery heater that can draw power from the battery (or charger) to directly heat the battery cooling fluid and so the battery. The exact consumption of the battery heater is not known, but it is in excess of 2kW. There are a range of scenarios, based on ambient temperature:
- Above 10C: Normal operation, no heating required.
- Freezing to 10C: Regenerative braking is reduced, but the car does not run the battery heater. No direct impact on range, but if the driving pattern would normally take advantage of full regen (city driving, steep downhill) then energy will be wasted by using the normal brakes. The battery will eventually warm up in use to eliminate the regen limit, but this can take a long time, especially if being driven gently.
- -20C (approx) to 0C: Car runs the battery heater. Regen is severely limited or disabled altogether. Significant penalty on range due to the battery heater until the battery warms up and the heater turns off – again this depends on how hard the car is being driven: around town, the self-warming from discharging will be small and there’s no self-warming from regen since it is disabled; driving at higher speeds the battery will warm more quickly.
- Below -20C: The battery heater will run continuously while driving: the loss of heat to the outside through the bottom of the battery is sufficient that it never reaches comfortable operating temperature. Considerable loss of range – especially at low speeds: at higher speeds there is some self-warming and the heater makes up a smaller proportion of the total consumption. At extreme low temperatures the battery heater may even run while parked – hence the instruction in the manual not to leave the car below -30C for more than 24 hours.
Several of these effects have a ‘startup penalty’, and are worse when driving at low speeds around town. The combination therefore means that the worst possible case is a day with extreme low temperatures where you are making a succession of short trips around town and letting the car cool down while parked for a reasonable time at each stop. That scenario can give some really terrible consumption figures, but it’s only really a problem if you are doing it away from home with no overnight charging available: you can’t actually cover very much distance in a day with that sort of driving pattern, so the reduced range doesn’t matter. For longer distance driving, there’s the startup penalty and then maybe 20% loss of range for continuous driving in severe cold.
Charging in the cold can also give problems under some circumstances. If the temperature is below freezing, then charging will take longer than usual: the battery may need to be heated before any charging can start, and while charging the battery heater will consume some proportion of the charge power available. When charging from a 120V domestic supply (North America etc.) the total power available isn’t enough to run the battery heater at full power, and the already slow charging will reduce to a trickle. Less intuitively, this can also be a problem with supercharging: if you stop at a hotel close to a Supercharger and plan to charge in the morning before departure, your battery may cool down overnight and take a long time to warm up in the morning – the Supercharger can’t make the battery heater go any faster than usual.
Most of these problems can be avoided by charging straight away – so the battery is still warm from driving – rather than waiting to the next morning. If you use the charge timer to access off-peak rates, you may need to start earlier than usual to ensure a full charge. In the scenario of starting from a hotel near a Supercharger with a frozen car, it might actually save time to do a few circuits of local roads to get some warmth into the battery before connecting to the supercharger
Warm temperatures are in general less of a problem than cold: the A/C uses much less power to cool the cabin than the heater uses to warm it, and the battery can normally be cooled with ambient air, using only a tiny amount of energy for the fans and pumps. However, the A/C use can still be noticeable and you may want to minimise use when looking for the best possible range.
When driving at open-road speeds, it is almost always better to keep the windows closed and run the fans or A/C than suffer the increased aerodynamic drag from having the windows open, though this trade-off reverses at low speed – in slow moving traffic, keeping the windows open will use less energy.
If you have the panoramic roof, it can be useful to open it to the vent position (using the phone app if applicable) to let out hot air before you go to the car, reducing the work the A/C has to do when you turn it on.
In extreme hot conditions, the car may need to run the cooling systems while it is parked – consuming battery charge – so you need to factor this into your planning if for example you plan to leave the car parked at an airport while you are away.
Rain and Snow
Rain is one of the trickiest energy drains to deal with, as it can make a big impact on energy consumption yet is hard to predict in advance – it can easily upset your energy planning on a trip, at the last minute changing ‘just going to make it’ into ‘falling short’. In the worst case, the energy impact can be as much as 30%.
The problem is not so much the rain itself as standing water on the road after the rain has fallen – the tyres have to move that water out of the way as they roll. This gives a rather non-linear relationship: a light film of water on the road surface has little effect as the tyres just press through it, but if there is enough that the grooves of the tyres are pumping the water to the side in the form of spray, then that takes a huge amount of energy.
The only countermeasure is to slow down, and even then the gain is mostly from the usual aerodynamic gain of lower speed rather than reducing the effect of the water itself. Of course, when conditions are really bad you are probably slowing down anyhow for safety reasons, but you will probably still end up using more energy than usual.
Snow can have a similar effect: driving on hard-packed snow is little different from a normal road surface, but ploughing through freshly-fallen or drifting snow can have a big impact.
Not strictly a weather phenomenon as such, altitude has a number of effects that are worth considering.
High altitude per se is a benefit: the air is thinner and so gives less aerodynamic drage, and unlike an inernal-combustion engine, the electric drivetrain still gives full power regardless of altitude. If you are planning a drag race against your ICE-owning friends, host it on the top of the mountain! If you could find a mountain with a flat top, you would get your best ever range up there.
However, we are more often concerned with going up and down hills, and there is a fixed energy cost to raising a weight by a given height: driving along a slope that rises by 1000ft (300m) will use about 1.8kWh more energy than driving the same distance on a flat road, assuming you drive at the same speed in both cases. If you then go down the hill, that energy is released again, but how much benefit you gain depends on the circumstances: if you are going faster than the car would naturally roll down the hill, the energy simply contributes to driving the car and you get all of it back; if the hill is sufficiently steep that you need to use regenerative braking to control your speed, some energy is wasted in the process of charging the battery and then taking it out again; if the hill is so severe that you are using the ordinary brakes, then all of it is wasted. In normal cruising on fast roads, any regen is a small enough proportion that you can ignore it and so the energy taken to go over a series of ups and downs is near enough the same as on a flat road – you just need to account for the different elevation of your starting and finishing points. You also need to take care of the maximum elevation when going over a hill – you need to be sure you have enough charge to reach the top of the hill, even if regen would have put back plenty by the time you reach your destination at the bottom.
Nowadays, you can use the car’s navigation system to make this adjustment for you – look at the ‘trip’ tab of the energy app while a route is active in the navigation, and you will see peaks and troughs in the predicted energy graph corresponding to the hills on your route. However, if you want to make the adjustment manually, a good rule of thumb is about 6 miles of range per 1000ft of elevation difference (or 30km range per 1000m elevation).
The effect of a headwind is to increase the speed of the car through the air, as if you were driving faster. Since the impact on range is increases quickly at higher speeds, a relatively small headwind can have a significant effect. Driving at 60mph with a 30mph headwind gives an effective speed of 90mph and so 50% extra energy needed to travel the same distance. If you are making a round trip and the wind stays the same, you will get some of the energy back on the return trip – but the square-law effect means that you don’t get back as much as you lost on the way out. If you drive slower into a headwind and faster with a tailwind, you can do the trip in the same time and get closer to a neutral effect.
Drafting – driving close enough to a large vehicle in front that it diverts the airflow over the car – is always advantageous, but especially so with a headwind; driving close enough for maximum drafting effect is not safe, but a noticeable effect can still be achieved when driving at a safe distance.
A cross-wind can also impact energy consumption, if it is sufficient to disturb the air flow over the car (effectively making it a less aerodynamic shape). This is harder to counter – slowing down or drafting doesn’t help the cross-wind effect, although it still gives the usual improvement. However, cross-winds are rarely strong enough for this to be a noticeable problem.