Saturday 30 September 2023

How much can you save from solar PV and battery with a heat pump?

You can substantially reduce your electricity bills if you have solar panels and a battery. What about if you have a heat pump? It certainly does make a difference but most of your heating demand is in the winter and most of your PV generation is in the summer. You would need an enormous battery to save enough in summer to use in winter. Suppose you put in enough solar panels to match your annual use so you are net zero for electricity consumption. What would this mean for your bills and for carbon emissions? This chart shows example savings from an energy use model.

Impact on the electricity bill for the model house heated with an air source heat pump, firstly with no panels, then with solar panels (6,700 kWh/year), then adding a medium or large size battery. Assumes a flat tariff with 30p/day fixed charge, 25p/kWh for electricity supplied and 12p/kWh for electricity export.

I have used a model of a fairly typical semi-detached home. Using a combi boiler, the gas bill for this model is about 10,500 kWh/year, electricity 2,800 kWh/year. Switching to an air source heat pump for heating, electricity demand increases to 6,700 kWh/year and I have given the house solar panels to match, so net zero electricity over the year. The battery charge-discharge efficiency is 90% and the solar inverter efficiency is 95%.The weather is typical for Cambridge, with hourly resolution. 

NB. Few people have more than 4 kWp, partly for lack of suitable roof area, and partly as you need special permission from the DNO (Distribution Network Operator) to exceed this. If these were all south facing you might get 3700 kWh from them. I have also modelled the impact of an array that size. The savings are smaller but definitely worth having. Also you can increase the savings with a tariff that has lower rates for off-peak electricity - I have used the Octopus Cosy tariff as an example.

With net zero electricity - bills reduce up to 70%.

The chart above shows annual bills, first without the solar panels, then adding solar panels, then a medium battery and then a very large battery. I have assumed electricity costs are a fixed cost of 30p/day, 25p/kWh for import and 12p/kWh gained from export, which is typical from October but your tariff may be very different. The green bars show the final cost, including all components. The addition of solar panels alone cuts the cost by 60%. Adding in the battery reduces costs further but only up to 70%. These figures are heavily dependent on the tariff for importing and exporting power. With a higher export tariff, the bill reduces more but the extra benefit of the larger battery is less.

Solar power is a poor match for heating demand.

This chart shows the energy demand (without solar panels) alongside solar generation. In December, demand is eight times generation while in June and July generation exceeds demand by a factor of three.


Total heat demand (red) and total solar generation (green ) are the same (well almost).

The seasonal variation will vary depending on how much electricity you use generally and your ratio of space heating demand to hot water demand. This model house uses 8 kWh a day for appliances etc in summer, and water heating demand is about a quarter of the space heating demand. If you use more for appliances or hot water this will increase your summer time heating demand so you will get more benefit from your solar panels.

The larger battery makes little difference except in spring and autumn.

Does it help to have a large battery? This chart shows how much electricity is imported by month, adding solar panels and two sizes of battery: 6 kWh and 12 kWh. The larger battery makes very little difference in winter as there is not enough solar generation and nothing to speak of in high summer as demand is so low. It does make a difference in the shoulder months. 


Without  a battery you can use your solar power only if you use it at the time.  Households vary as to when they use electricity. In this model the household is in during the day and using power at low levels but no effort is made to shift electricity use to match solar generation Your own household may be very different so savings without a battery are very hard to predict.

Grid emissions are lower in summer than winter.

Given you are net zero for electricity, you might think that you are net zero for carbon emissions too. However this is not the case, at least, not according to SAP, the model used to calculate your home EPC (Energy Performance Certificate) rating. The SAP approach is to assume that your PV export displaces electricity from another source which has average carbon emissions for the grid at that time. For individual projects it is common to take a different approach, assuming that you are displacing electricity from a particular type of generator such as a gas power station. However, the SAP model considers the building stock as a whole so mean grid electricity emissions is a fair assumption of the overall value of your installation. 

The chart shows how carbon emissions vary through the year – lower in summer than in winter – and how solar PV export power has different emissions from the overall average. In summer PV export emissions are lower than average, because they tend to happen in the middle of the day when demand is fairly low and there is a lot of solar power available. However, in the winter there is a big difference between night time and daytime emissions and solar power exports happen during the day when demand and emissions are high.


Carbon factors from SAP 10.2

Our model house does not do any export in the winter because it uses all the power it generates. It imports electricity in the winter, when emissions are high, and exports in the summer, when demand is low. Hence there are significant net emissions as shown in the next table.

Carbon savings are lower than electricity savings and bill savings are still lower.

This table shows the overall impact of the solar panels and batteries on net electricity use,  carbon emissions and energy bill, for the whole year. It also shows the savings from 6 kWp solar panels, again split East/West giving overall generation 3,600 kWh/year. 

CaseGrid electricity savings (%) Carbon savings (%) Bill savings (%)
6,700 kWh/year solar generation (11 kWp, East-West)
solar panels only 9985 60
solar plus 6 kWh battery 98 84 69
solar plus 12 kWh 98 83 70
3,600 kWh/year solar panels (6 kWp, East-West)
solar panels only 54 47 37
solar plus 6 kWh battery 53 46 44
solar plus 12kWh battery 53 46 44

Carbon savings are lower than electricity savings because the summer-time export displaces grid power with lower emissions than the power you draw from the grid in winter.

Bill savings are lower still, because exported power (lots in the summer) earns 12 p/kWh while electricity taken from the grid costs 25 p/kWh. Still there is a very substantial saving.

Bill savings increase by adding a battery, so that the house uses more of its own generated electricity. However, carbon savings and electricity savings decrease slightly, because there are extra losses from using the battery. The larger battery makes little difference because most of the year the benefit is limited, either by low demand or low solar generation.

Using a tariff with cheaper off-peak rates can increase your savings.

So far I have only considered a flat tariff for import and export. You can do better in winter with a tariff that has lower cost at off peak times. This would allow you to charge your battery at low cost off-peak, lowering both bills and emissions. However, you have to optimise your charging schedule carefully to avoid running out of battery capacity when you could have stored your own solar power. Having a bigger battery brings more benefits in winter.

 For example, Octopus currently have a Cosy tariff which has six hours of off-peak price (40% less than standard) and three hours of peak-time price (60% more than standard). They also offer a very good export rate - 15p/kWh. Using this tariff my model [1] with the 6 kWp solar panels and a 6 kWh battery gives bill savings of 52% while the larger battery saves you 55% - as shown in the table below. Savings are relative to the no-solar, no-battery case.

CaseBill savings - flat tariff(%) Bill savings - Cosy tariff(%)
3,600 kWh/year ( 6 kWp East-West)
solar plus 6 kWh battery 4452
solar plus 12 kWh battery44 55

Different tariffs will give you you different savings.

Summary and final thoughts.

Solar panels are a great way to reduce your electricity bills and this is so with a heat pump too. However you make a lot more savings with a battery as well as the panels. 

With a flat tariff your savings are limited by solar generation and demand. There is little point in having a battery larger than necessary for summer time demand. You need to allow 2-3 kWh for heating hot water with your heat pump through the summer (unless you use solar power for this already). 

If you go for a tariff with cheap off-peak periods for you to charge your battery, then you can meet more of your demand in the winter through the battery at a cheap rate. This way the larger battery brings more bill savings. Tariffs vary but with the Octopus Cosy tariff it looks like an extra 3% saving a year from the larger battery, in this model with 3,600 kWh/year solar generation

Heat pumps use a lot of electricity but you may have enough roof space to get net zero electricity use with solar panels. You need even more panels to achieve net zero carbon, because grid electricity is less intensive in the summer. In this model, you would need 13 kWp (7,800 kWh generated) for 99% carbon savings. For zero bills you need even more, because the export value for electricity is so much less than the import price. With the Cosy tariff,  8,260 kWh gets you 90% saving - and there was no room on the roof for more solar panels.

[1] My model allows off-peak charging to 80% of battery capacity in December, January and February; 50% in March, April, September and October. It does not allow any charging room from the grid during May through August.

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