This text is by Thomas, co-founder at HASHLYNK. He looks back on the planning of his own PV system and therefore writes in the first person from here on.
When I look at the top view of my own PV system today, I see two things at once: a system that has achieved its original goal – and roof space whose potential I gave away. The south side is densely covered. On the west and north-east sides, however, there are visible gaps. Around 36 more modules would have fitted there. Depending on the choice of module, that would have made around 32 kW possible instead of the 18 kW installed. In short: I planned my PV system too small.
That was the biggest mistake in my planning. The system works and achieves the goal it was built for. I had simply set that goal too narrowly. Today I take this experience with me into every HASHLYNK project.
What you will learn in this article
- how you can size a PV system correctly and avoid designing it too small
- why additional modules are often cheaper in an initial installation than in a later expansion
- how a larger module area affects battery storage, heat pump and grid imports, especially in winter
- how a flexibly controllable load creates additional ways to use surplus electricity
| Scenario | Installed capacity | Context |
|---|---|---|
| System as built | 18 kW | Designed for own demand at the time |
| Possible full coverage | approx. 32 kW | Around 36 additional modules on the west and north-east sides |
Key insight
Taking today’s self-consumption as the only yardstick was my mistake. I should have calculated full coverage of the roof, the weak winter months and the use of the additional surplus together.
Who is this relevant for?
For anyone who is planning or expanding a PV system and wondering whether to cover the roof fully with PV.
The goal was right, the yardstick was too narrow
My guiding principle when planning was “sized to demand”. I had set myself a clear goal: at least 80 per cent lower grid imports. I achieved that goal. On paper, the system is therefore a success.
Nevertheless, in hindsight, the design was too small. I had thought almost exclusively in terms of self-consumption: household, heat pump and part of the consumption via the battery storage. What I had not considered was an additional, controllable load behind the meter – that is, a load that draws power exactly when there is actually surplus at the site.
So the mistake was not in the installation. I had simply assumed that the right system size could be derived from my electricity demand at the time alone.
Covering the roof fully with PV: 18 kW installed, around 32 kW would have been possible
Around 14 kW of unused capacity is no small remainder. Compared with the 18 kW installed, almost the same capacity again would have fitted on the roof.
The missing 14 kW or so are not only noticeable on sunny summer days. I miss them above all when irradiation is weak and every bit of additional module area counts. And expanding today takes more effort and costs more than installing the modules at the same time back then would have.
Oversizing a PV system: why I don’t think the larger system is riskier today
My existing system is expected to pay for itself, partly thanks to the subsidy at the time. By today’s calculation, that would very probably also have applied to full coverage.
The reason lies in the cost structure of a PV system. Scaffolding, travel, planning, the sub-distribution board and a large part of the electrician hours are incurred regardless of whether 18 or 32 kW is installed in the end. Additional modules and mounting structure increase the investment, but many fixed costs are spread over more capacity. As a result, the specific costs per installed kW often fall.
My set-up including storage cost around €1,400 per kW after subsidies; the subsidy was €400 per kW. These are my figures, not a general price assumption. The effect is still easy to understand: with a later expansion, some of the planning, installation and connection work has to be done a second time.
Good to know
Full coverage is not a rule of thumb. Roof, structural capacity, grid connection, budget and consumption need to be checked. My advice: at least run the numbers on full coverage before you decide against it.
The battery would have benefited, especially on weak days
With my current system, there are around 75 days a year on which production is not enough and electricity has to be drawn from the grid. These days are mainly in the winter half of the year, when it is cloudy or foggy.
More module area on several orientations would have improved the situation. In diffuse light, a perfect south orientation becomes less important. What counts then is above all how much active area can convert light into electrical energy at all. Additional modules on the west and north-east areas would have charged the battery more often on weak days and further reduced grid imports.
I had underestimated this effect. A larger system does bring more surplus in summer, but it also raises generation in the transitional seasons and in winter. That is where the additional area would have helped me most.
Making good use of PV surplus: what Bitcoin mining changes
When I planned the system, Bitcoin mining as a way of using the surplus was not on my radar. With what I know today, it would be part of the calculation.
A miner can be installed in a small space and its output can be controlled. So Bitcoin mining with PV surplus does not mean that the device runs at full power around the clock. A controller follows the available surplus: lots of PV electricity, more output; less PV electricity, less output or a pause.
For my system, that would have meant another consumer behind the meter. Household and battery would still have been supplied first; the remaining electricity would have powered the miner. For this task, we at HASHLYNK developed the HashRack for putting surplus electricity to economic use.
For my calculation, I assume roughly 5 to 15 cents per kilowatt hour for the mining revenue. This value fluctuates with hardware, efficiency, the Bitcoin network, market price and the timing of the sale. It is not a fixed remuneration and has to be recalculated with current data.
PV grid export limits: why using electricity locally becomes interesting
A common objection to larger PV systems is: what good is additional capacity if the grid operator limits feed-in anyway?
A grid export limit determines how much power may flow into the public grid. But it does not say that I cannot use the rest of the electricity at my own site. If a controllable load takes up the surplus behind the meter, more of my own generation stays in-house.
Grid connection, metering concept, protection equipment, electrical design and any notification or approval requirements must be clarified beforehand. Local consumption does not remove these requirements. But the grid export limit alone should not determine how large the PV system will be.
Using miner waste heat: electricity also becomes heat for heating
In mining, almost all of the electrical energy used is converted into heat. Depending on the installation, this miner waste heat can be used for heating, hot water or a suitable low-temperature process.
In my case, a miner with around 4 kW of electrical power could have provided a relevant share of the space heating in the transitional seasons. At a flow temperature of around 50 °C, this output can – depending on insulation standard, outside temperature and heating system – roughly be enough for about 100 m² of heated floor area.
Every kilowatt hour of usable heat would at the same time have reduced fuel costs. In my personal calculation, the comparison value for the pellet heating is around 12 cents per kilowatt hour of heat. The PV surplus would thus have generated two potential contributions to value: computing power and avoided heating costs.
Whether this pays off depends on the temperature level, the heat demand at the same time and the hydraulic integration. Without a heat consumer, it remains waste heat. With a suitable heating system, it replaces part of the heat that would otherwise have to be generated in another way.
What to do with PV surplus when the battery is full?
What to do with PV surplus when the battery is full? On sunny days, my battery is charged while the system keeps producing electricity. And demand does not stay the same over the years: an electric car, a heat pump or other electrical loads can change the load profile significantly.
Several uses come into question for the surplus: feed-in, an energy community, direct marketing, an electric car, storage or a flexibly controlled load. Which of them fits depends on the site and the time. When planning, I would therefore not rule out any option too early.
Mining does not cover every situation. But it can run when there is still electricity left over after self-consumption and storage. At HASHLYNK, HashLink handles the automatic control. It measures the surplus and releases more or less power to the connected system accordingly.
Sizing a PV system correctly: what I would do differently today
If I were to plan the same system again, I would not start with current consumption but with the entire technically usable roof. Then I would compare several scenarios: the minimum design for today’s self-consumption, a larger option for more yield in the weak months, and full coverage with a concept for the additional surplus.
Specifically, I would check the following points in the very first planning phase:
- Which roof areas are technically and structurally usable?
- What capacity makes sense today and with later electrification?
- How do additional modules change generation in the transitional seasons and in winter?
- Which grid export limits apply at the site?
- How can storage and controllable loads work together?
- Is there a usable heat demand?
- Which sub-distribution, protection equipment and hydraulics should be prepared for later expansion stages?
Answering these questions early costs significantly less than replanning an undersized system years later.
How HASHLYNK puts PV surplus to economic use
That is why at HASHLYNK we do not start with the question of which miner to install. First we look at generation, consumption, storage, grid export limit and heat demand. From this data, we work out how much power the site can actually use. Then we plan the hardware, electrical integration, HashLink control and – if the heat is needed – the hydraulics.
We take the same approach at 20 kW as at 200 or 2,000 kW. The technology gets bigger, the questions stay the same. With the free HASHLYNK site assessment, we check which system size matches the measured surpluses and whether the project pays off.
Conclusion: I should have covered the roof fully
My 18 kW system fulfils the goal it was built for. Nevertheless, I would plan it bigger today. The free areas on the roof remind me that a PV system should not be measured only by current self-consumption.
A larger system can lower the costs per installed kW, charge the battery more often in weak months and supply more energy for an electric car, heat or mining. Whether full coverage pays off has to be calculated. Back then I did not even look at this option – that was my real mistake.
If you are currently planning a PV system, have full coverage calculated too. If you already have a system and still have space on the roof, we at HASHLYNK will check what an expansion would bring and how the additional PV surplus can be used on site.
You can find more on sizing in the HASHLYNK knowledge hub under Why PV oversizing makes sense. What happens to the revenue after mining and how the heat factors into the calculation is covered in the article How the revenue logic of HASHLYNK systems works.
