Sunday, 11 July 2021

Do people with PV panels consume more electricity?

Calculations of the benefits of renewable energy usually make the assumption that people will continue to use electricity in the same way they did before. However, a lot of studies show this is not the case. Typically electricity use does increase by up up 20% of the renewable energy generated - a rebound effect of 20% However other studies find that on average electricity use is unchanged. A lot depends on financial incentives, and some on attitudes and some on the technology used. This suggests it is possible to 'nudge' this behaviour to gain the most benefit - how is the UK doing in this respect? It may be that we have, by good luck or judgement, avoided the worst outcomes.

The rebound effect reduces savings for energy efficiency up to 50%

The rebound effect is usually discussed in the context of energy efficiency. Suppose I buy a more efficient car, which uses 10% less fuel. If I now travel 5.5% further, that means I have used up half my energy savings in rebound. Alternatively, I could spend the money I have saved on fuel in another way - for example on more new clothes. This is an indirect rebound and the carbon implications rapidly become complicated. Overall, rebound effects can be very significant - at a global level, energy efficiency only makes perhaps half the savings you would expect [1]

Rebound effects for solar energy have been observed up to 17% to 21%

A rebound effect for solar energy brings an increase in consumption (of electricity or of other services) due to having the solar panels. The overall effect is not as bad as for energy efficiency but still substantial. In Phoenix, Arizona, a rebound effect of 18% has been observed [2] and in Sydney, Australia up to 21%[3]. However that was for early installers who had a very high Feed in Tariff rate (60c/kWh). Households with a lower rate (20c/kWh) had a slightly lower rebound effect: only 17%. This suggests that money has quite a lot to do with it.

Rebound to improve comfort

Rebound is not always a bad thing, if you consider more than just carbon emissions. In Phoenix, Arizona, the effect was highest in the summer when people are using air conditioning, so presumably the lower bills allowed some extra comfort [2]. A German interviewee reported getting an e-bike to replace a normal push-bike 'Last year I had a stroke, and that set me back a bit. And now I need the support to get up the hills'. That is a bit of a rebound, as it uses more electricity than before. However, having got the bike, he used it in place of the car quite often which brings savings [5]. 

Rebound for luxury - sometimes 'justifed' by 'moral licence'

Sometimes the extra use can only be classed as luxury. For example one family in Sweden started heating their greenhouse: 'I would love to grow [more plants]. We have a greenhouse but we have never heated the greenhouse before. But now I feel a little freer to do that, since we have free electricity. Although we need to heat it mainly at night. [4]

Another family in Germany built a new outhouse: The modules had become so much cheaper and the feed-in tariff was still attractive, and I have to admit, we were able to build this lovely outbuilding and the carport and firewood shed' [6]

Some people justify an increase in electricity use by saying they have already reduced their emissions by installing the panels so it does not matter. For example one family, having moved into a very efficient (PassivHaus standard) home and having installed PV panels felt it was reasonable to use a lot more heating in winter. 'In earlier years we were OK with 21 degrees in winter. But now, since we've lived in this house, anything under 22 degrees is, ahm, uncomfortable ... so now we heat to between 24 and 26 degrees.' [5]. Being a very efficient home, their energy consumption was still lower than average, but it was twice what it should have been.

Rebound due to switching from other fuels to electricity

People with PV panels are more minded to get an electric car, which reduces emissions - but in some cases, with an electric car powered by free electricity there comes an incentive to travel further than before. [4]

Adapting behaviour to increase or decrease self consumption

Most people with solar panels like to increase use of their own electricity. In the UK, for many years export was deemed to be half of generation, and there was no incentive to export at all. Many people used their 'spare' power to heat hot water for their taps - hence new products to allow this such as Solar Cache and iBoost solar immersion controller. Usually this was displacing use of gas which meant increasing carbon emissions, at least in the early days when there was little renewable energy on the grid.

When export is actually measured the incentive to self consume depends on the relative price of imported and exported power. In Queensland, Australia, the Feed in Tariff for exported power started at 44c/kWh but from 2012 was dramatically reduced, initially to 8c/kWh and later lower still. Not surprisingly many households getting the high rate do all they can to avoid using their own electricity. 'We have obviously tried to reduce our use during the solar generation hours because of the 44 cent feed-in-tariff'; 'I put my dishwasher on at night, if I think about it'; “Well, I wash first thing in the morning, 5 o′clock or so in the morning; and I do the ironing at that time of the morning, too”. [7]

The households on the low rate do the opposite. We are slightly more diligent in the fact that the pool pump is only on during the day. We use the dishwasher, washing machine and dryer as much as we can just during the day”. [7]

Some people will go to considerable lengths to ensure that they only use their own power for charging their EV. One commuter, having to travel 40 miles to and from work, and not getting enough from their panels in winter to provide the necessary charge, switched to car-sharing with a friend so that he could charge the car one day and use it the next. [5] 

Capped export can mean rebound to avoid 'waste'

In Germany, the installation rate of PV panels has been high enough to put strains on the distribution grid, to the extent that many installations are only allowed to export up to 70% of their nominal maximum power. This means that when the sun is high the system generates more power than can be exported. If this is not used it is perceived as a waste. One household resolved the problem by installing a solar powered fountain to run over the middle of the day - but it ran all year, whether or not there actually was any spare power.

This is perhaps one reason why Germany has been a leader in developing power to gas technology, using 'spare' power to make either hydrogen or methane. One of the pioneering plants is at Hassfurt and nearby residents have been able to visit. 'We were amazed to see how it's possible, using computer technology, to calculate within seconds, when it's profitable, moment by moment, to buy electricity from the grid to make gas, and at what moments it's not profitable. We were really enthused to see what's possible'. This is a very popular installation, even though the efficiency is only 30%.

One would have thought that the cap would lead to more people installing battery systems to save the extra power. However, use of the battery needs to be carefully optimised. 'So, for example, [X] had installed a 9.88 kilowatt-peak (kWp) PV system in 2016, which produces an average of 8000kWh per year. On a bright summer's day at and around midday it can easily produce at the rate of 8–10kW, but the household is only allowed to feed electricity into the grid at a maximum rate of 7kW. Further, unless the household are all at home and using appliances generously (which seldom happens on a working day), the battery is fully charged by about 11a.m. and cannot take any more.' [5]

Negative rebound happens too - especially when the metering is good.

Not everyone increases consumption after getting panels. For some, especially when the main motivation is environmental, there can be an overall decrease in emissions. This is mainly due to increased awareness 'Yes, I have changed, but not my wife or my son. I try to keep the energy-intensive appliances to a minimum, and lighting and so. It is the little things – we have not changed the temperature or anything. But we are more aware now since we talk about how much we produce and so on.' [4]

A good metering system, showing self generation and import/export is an important factor in raising awareness. However location is also important. Having it in a basement is very much a case of out of site out of mind. But one household who put the meters on a landing half way up the stairs found them checking it frequently - and reacting: 'This enabled the household to attempt to optimise consumption patterns to minimise consumption at times when renewable electricity was scarce and carbon-intensive electricity was predominant in the grid.' [6]

My thoughts:

Why do people think of their own power as free?

I find the rebound effect somewhat surprising, given the high cost of installing the panels to start with. However, unless you took out a loan and are still paying this off, there is no continuing reminder of all that cash you have already paid out. Hence there is a strong tendency to think of the electricity you generate and use yourself as 'free'. It seems some degree of rebound is almost inevitable, except in the most environmentally conscious people, or possibly the most frugal.

Export needs to be monitored

The UK policy of not monitoring export at all was probably a mistake, as it did lead to people using electricity to heat water as well as gas. However, at least we avoided the decrease in consumption when solar power is available, as we saw with the high feed in tariff in Australia. Fortunately, smart meters measure export as well as import and we do not need extra equipment once we have one.

Dynamic export and import prices can help to stabilise the grid.

The Octopus Agile tariff is the only one I know that varies based on wholesale prices and they have a variable export tariff as well as import. I believe the export price is always lower than the import price. That avoids the 'incentive to export' leading to decreasing consumption when there is a power glut. However, if the prices were allowed to go the other way it could help to stabilise the grid when demand is high.

Capping export means extra intelligence is needed to avoid waste

It is not unreasonable to cap export when the grid is under strain as is common now in Germany. However we are probably going to need to improve the grid for electric heating in winter anyway, and this will help. Batteries are the obvious solution to this, but they need to be configured so that there is capacity to store power when export is curtailed. That means not charging it up with the first sunshine over the horizon, but waiting until the mid-day period when curtailment is likely. Accurately optimising use of the battery requires a forecast of demand as well as weather, but in practice we know both pretty well.  This should be possible but I do not know of any products you can buy to do it.

[1] Guest post: Why ‘rebound effects’ may cut energy savings in half (Carbon Brief) 2021

[2] Y. Qiu, M. Kahn, B. Xing, Quantifying the rebound effects of residential solar panel adoption, J. Environ. Econ. Manag. 96 (2019) 310e341.

[3] Deng, P. Newton, Assessing the impact of solar PV on domestic electricity consumption: exploring the prospect of rebound effects, Energy Pol. 110 (2017)

[4] Jenny Palm, Jenny Palma, Maria Eidenskog, Rasmus Luthander, Sufficiency, change, and flexibility: Critically examining the energy consumption profiles of solar PV prosumers in Sweden. Energy Research and Social Science. 21 (2018)

[5] Ray Galvin, I'll follow the sun: Geo-sociotechnical constraints on prosumer households in Germany, Energy Research and Social Science (2020)

[6] Ray Galvin, Identifying possible drivers of rebound effects and reverse rebounds among households with rooftop photovoltaics, Renewable Energy Focus (2021)

[7] Jeff Sommerfield, Laurie Buys, Desley Vine, Residential consumers’ experiences in the adoption and use of solar PV, Energy Policy (2017)

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