I have noticed many times that a lot of the discussions about relative costs of running heat pumps vs boilers rely on some form of extrapolation. Typically, a poster will have good historical data about their old way of doing things but will be posting a comparison with what they expect the heat pump costs to be based on extrapolating a month or two's data from a fairly newly installed unit.

Given a heat pump tends to be less efficient at times when it's most called for, the running costs of a heat pump are rather more seasonal than those of an oil or gas boiler. As a result, plenty of people are seeing a winter electricity bill for the heat pump and fainting into their sofa, without necessarily seeing the flip side - the lower costs during more moderate weather. In short, it makes it difficult for someone looking to make the heating switch to make a truly informed choice.

I've now passed a milestone. Although we've had our ASHP for well over a year, it took a while before UKPN could sort themselves out and provide us with the necessary details to allow us to export our excess solar energy to the grid. Nonetheless, 6th March saw the anniversary so I now have a full year's data of running our ASHP alongside a solar PV installation that can power our kit and export any excess under Octopus' agile export tariff. This means, of course, that I can now give a definitive answer of the effect of switching heating sources and of installing a solar PV system without having to resort to any extrapolation or guesstimates. Since I expected this would be useful to people, here are the key figures.

We used to have an oil boiler and consistently got through two tanks of oil each year; one delivery after 5 months and then one after 7 months, reflecting the greater usage during winter vs summer. Looking at the delivery dates and using the prices on those dates, I was able to work out that running our oil boiler over the past year would have cost £1,806.

Looking at our electricity bills for the same period, we have spent a net of £1,707.53 (importing £2,229.41 and being credited £521.88 for exported energy). However, this also includes non-heating related electrical usage that we would still have had to spend if we had our oil boiler. I'm happy to share my workings if people really want to see them, but suffice to say:

• The cost of running the ASHP purely from grid-supplied electricity would have cost us £1,775.19, so simply switching from oil boiler to ASHP saved us £30.81 over the year.
• The cost of running the ASHP using home-grown electricity and topping up from the grid cost us a nominal £1,274.87.
• The cost of running the ASHP after solar exports have been taken into account (i.e. our net savings overall from the ASHP/solar PV combination) are £976.43, which is a saving of £829.57 over the year.

Obviously, this doesn't take into account the upfront installation costs (£3,500 after RHI for ASHP, £7,500 for solar array and battery), so there is definitely a time yet before we hit our theoretical break-even point, and nor does it take into account any value we may have added to our house by installing either or both new systems. Similarly obviously, the reasons for the big savings are split pretty evenly between moving to a heating technology that uses energy we can generate at home and money paid to us from a decent rate of export tariff. The markedly better efficiency of the ASHP has been almost exactly offset by the price differential between oil and electricity, so in fact doesn't make a significant difference.

All that said, those bills were definitely unevenly split with our December, January and February bills being £468, £493 and £352 respectively. I'm well aware many people will struggle with summer months with negative outflows and then be hit with £1,300 outgoings in just three months. That's an important consideration to bear in mind.