Does solar reduce your bill, or just your consumption?
Both, but not equally — and the gap between them is where people feel let down. Solar cuts the units you buy. It does nothing at all to the standing charge, and the units it cuts are the cheap daytime ones rather than the expensive winter evenings.
Solar reduces units bought. It does not reduce the bill by the same proportion.
Three structural reasons:
- The standing charge does not move. It is a daily fee for being connected, and generating your own power does not touch it.
- Only self-consumed units reduce your import. Everything else is exported, and shows up as a separate payment rather than as a smaller bill.
- Generation and demand are out of phase, both across the day and across the year. The units solar replaces are daytime summer ones; the units that dominate your bill are winter evening ones.
None of this means solar is not working. It means the bill is the wrong instrument to measure it with.
The standing charge, and why it matters more than people expect
Every day you are connected, you pay a fixed amount before a single unit is used. Solar does nothing to it.
The arithmetic consequence is uncomfortable for small systems and low-consumption households: as your unit consumption falls, the fixed charge becomes a larger share of what remains. A household that halves its imported units does not halve its bill.
This is also why the break-even question is sensitive to how the industry splits fixed and variable charges. A rebalancing towards standing charges and away from unit rates would leave total bills roughly unchanged while making every solar installation in the country slightly less valuable.
The daily mismatch
Generation peaks in the middle of the day. Household demand peaks in the morning and, especially, the evening.
Without storage, MCS’s own guidance puts self-consumption at 14% to 23% for a typical household — so the large majority of what a system generates is exported, at a rate several times below what you pay to import.
That is the single most important number in domestic solar, and it is covered in full at self-consumption.
A household that generates 3,400 kWh and uses 23% of it has:
- avoided buying about 790 kWh — a real reduction in the bill
- exported about 2,630 kWh — which produces a payment somewhere else entirely
Look only at the electricity bill and the system appears to be underperforming by a factor of four. Look at both documents and it is doing exactly what the model said.
Check your export statement before concluding anything. This is the most common reason people believe their solar is broken when it is not.
The seasonal mismatch
The other half of the phase problem, and the more visible one.
On our own modelling, December generation is a small fraction of June’s at every UK location — the irradiance page has the monthly figures. Meanwhile UK household electricity demand rises in winter, as lighting hours grow and, in electrically heated homes, as heating demand arrives.
So the months where solar contributes least are the months where the bill is largest. A household judging solar by its January statement will always be disappointed, and a household judging it by July will always be delighted. Both are looking at the same system.
A battery does not fix this, because it is a seasonal gap rather than a daily one. It shifts hours, not months — which is exactly what our battery calculator shows when December’s spare generation column collapses.
How to actually measure what solar is doing
Four steps, in order:
1. Compare units, not pounds. Prices change; units do not lie. Look at kWh imported per month, this year against last.
2. Compare like months. June against June. A month-on-month comparison across a season tells you about the sun, not about the system.
3. Add the export payment. It is a separate document, often paid quarterly, sometimes by a different supplier from your import.
4. Then compare against generation. Your inverter reports what was produced. Production minus import reduction is roughly what you exported — and if those three numbers are wildly inconsistent, something is worth investigating.
When it really is underperforming
The structural reasons above explain most disappointment. They do not explain everything. Genuine causes worth ruling out:
- A tripped or failed inverter, which can go unnoticed for months without monitoring
- Shading that was not modelled, or has grown — see shading
- A string offline — half the array producing nothing while the other half looks normal
- An array smaller or differently oriented than the quote described
- Soiling, on shallow-pitched panels especially
If the numbers still do not reconcile, proving underperformance sets out how to evidence it and what your rights are.
Sources
- Energy price cap unit rates and standing charges
- MGD 003: Solar PV Self-Consumption, issue 2.0 (1 April 2022) Read locally. MCS restricts reproduction, so we cite clauses and quote sparingly.
- PVGIS 5.3 Seasonal generation from our own PVGIS modelling. Contains modified Joint Research Centre data.
Contains public sector information licensed under the Open Government Licence v3.0.
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