An eminently sensible and simple question…
…that is surprisingly complicated to answer. Hang on, here goes.
The heat pump keeps track of the electricity it consumes – that’s a good start. Here are the numbers for December.
So in December the heat pump used 692 kWh of electricity. There’s two problems – firstly there’s no way of knowing how accurate these numbers are – they are certainly inaccurate when the heat pump spends a lot of time in standby – Mitsubishi admitted there was a bug. In December the heat pump spends most of its time heating so, fingers crossed, the bug shouldn’t make a significant difference.
The second problem is that the heat pump uses electricity from three different sources – the grid, our solar panels and our battery. Grid electricity is charged at two different rates and the energy from the battery might have originated from solar or charging overnight at cheap rate.
Our electricity bill tells us how much peak and cheap rate electricity we use but the bills don’t line up with calendar months so they don’t help here. The Smart meter knows nothing about the Octopus Go tariff so it is no help either. Both our solar and battery systems track grid import but only in total not split by cheap/peak. Enter Home Assistant (HA) to save the day. In HA it is possible to set up utility meters to track import pretty much anyway you like. The meters use the Solar PV system grid import sensor and HA runs the peak and cheap rate meters at the correct times. There’s a guide to setting up utility meters for Octopus Go here and below is the Energy Dashboard in HA for December.
The key numbers are 785 kWh cheap rate, 479 kWh peak rate and 105 kWh from Solar. So in total we consumed 1363 kWh at a cost of £212 (ignoring the standing charge – life is too short). So our average cost per kWh was 15.5p. The heat pump used 692 kWh and therefore the cost was £107.63 give or take.
We can do a little better by calculating the EV charging cost which we know exactly – it’s always cheap rate or solar. In December the EV used 115 kWh all at cheap rate – no solar (hardly a surprise in Dec). Modifying the average calculation to exclude the EV charging increases the average unit cost slightly to 15.7p and the heat pump cost to £108.55.
The Tesla Powerwall keeps track of house consumption – comparing house consumption with grid import shows a discrepancy of around 37 kWh – it’s the round trip battery loss. In December the battery supplied 351 kWh to the house so the loss is about 10.6% – pretty happy with that.
December was an unusual month in that the builders were here running dehumidifiers and heaters trying to get plaster to dry – explains why the heat pump only accounted for slightly over half of our bill in mid-winter. Here’s the numbers from November to February.
| Nov | Dec | Jan | Feb | |
| Total Grid Import kWh | 745 | 1366 | 1107 | 739 |
| Heat pump use kWh (% of import) | 300 (40%) | 691 (51%) | 700 (63%) | 428 (58%) |
| Heat pump cost | £29.11 | £108.55 | £94.03 | £47.10 |
Our Octopus Go Faster fix is ending just before the end of Feb – new Octopus Go rates are around 30% higher and have only four hours cheap rate rather than five so next winter will see a significant cost rise if energy costs don’t fall before then. That said the heat pump is proving way more economical than expected – good news!
Very tempted to add a power meter to measure the actual heat pump consumption to eliminate the buggy internal meters but trying to hold on to a sense of perspective! Would be nice to know the facts – the power meter costs around £70 and a couple of hours to wire in the current clamps. It integrates with HA and could be added into the energy dashboard – birthday project!