What self-consumption rate is and how to calculate it
Self-consumption rate measures how much of the electricity your solar panels generate you actually consume within your home, rather than sending it back to the grid via the Smart Export Guarantee.
The formula is straightforward:
Self-consumption % = (Solar kWh used at home / Total solar kWh generated) x 100
Example: your solar monitoring app shows 3,000 kWh generated last year. Your home consumed 1,200 kWh of that directly. Your self-consumption rate is 40%.
You can find these figures from your solar inverter's monitoring app (SolarEdge, SolarLog, GivEnergy, or Fronius apps all provide this). Your smart meter or the Octopus Energy app can show your import and export totals, which allow you to calculate self-consumption indirectly if your inverter monitoring is limited.
If you do not have these figures yet, 35-40% is a reasonable default assumption for a working household where people are out during daytime hours in spring and summer, when solar peaks.
Typical UK self-consumption rates
| Household type | Typical self-consumption rate |
|---|---|
| Standard household, mostly out during the day | 30-40% |
| Household with someone at home during the day | 45-55% |
| Home working household, active appliance use | 55-65% |
| EV charged from solar during the day | 60-70% |
| Solar + home battery (any household type) | 70-85% |
| Solar + battery + EV charging | 80-90% |
Without a battery, self-consumption is governed almost entirely by when your household consumes electricity. The grid only draws from the solar when your appliances are running. Midday peaks in generation, the typical UK solar output curve, frequently outrun home consumption, sending surplus to the grid at low SEG rates.
A battery fundamentally changes this by absorbing midday surplus and releasing it during evening demand peaks, when solar generation has dropped to zero and the household is consuming its most electricity of the day.
Why self-consumption rate changes your tariff decision
The choice between Octopus Agile and Octopus Flux is one of the most common questions solar households face. Self-consumption rate is the single most important factor in making that decision.
Agile's value comes from buying cheap electricity at off-peak times, particularly overnight at 2-8p per kWh. Flux's value comes from a higher export rate (typically 15-20p versus SEG's 4-6p) during peak export windows, rewarding households that send a lot back to the grid.
| Self-consumption rate | Better tariff | Reason |
|---|---|---|
| Below 40% | Likely Flux | High export volume; Flux's export premium matters more |
| 40-55% | Borderline | Run both calculations with your actual usage numbers |
| Above 55% | Likely Agile | Lower export volume; Agile's cheap overnight import delivers more value |
The threshold is approximately 55% self-consumption. Below that, Flux often wins. Above that, Agile often wins. The crossover point shifts slightly depending on your total consumption, your overnight usage patterns, and whether you have an EV. Run the numbers for your specific household using your actual generation and consumption data before switching.
How to improve self-consumption without a battery
Every percentage point of improved self-consumption is money that previously went to the grid at 4-6p now staying in your home at 26p value. Four practical steps that cost little or nothing:
1. Run high-consumption appliances during solar peak hours. The dishwasher, washing machine, tumble dryer, and oven all consume significant electricity. Running them between 10am and 2pm on sunny days directly increases self-consumption. Each appliance moved from evening to midday adds 2-5 percentage points to your self-consumption rate.
2. Install a solar diverter for hot water. An immersion diverter, such as a myenergi eddi, monitors surplus solar generation and automatically diverts excess to your immersion heater. This absorbs 1-3 kWh of midday surplus daily in summer. At around £400 installed, payback is typically 2-3 years. Self-consumption improvement: 8-12 percentage points for most systems.
3. Charge your EV during the day. If your car is at home during the day, scheduling EV charging from 10am to 3pm on solar days captures large volumes of surplus generation. A 7 kW charger running for two hours absorbs 14 kWh, more than most household appliances combined. If you work from home occasionally, this is the single most impactful change available without a battery.
4. Use smart plugs to time smaller appliances. Smart plugs on devices without built-in delay start, such as air purifiers, dehumidifiers, and slow cookers, can be set to run during peak solar hours. Each device adds incremental self-consumption with minimal cost.
Combined, these four steps can push self-consumption from 35-40% to 55-65% without any battery purchase. That is enough to shift a household above the Agile/Flux threshold and begin capturing significant overnight savings on Agile tariffs.
The sweet spot: what rate justifies a battery purchase
A battery investment makes the most financial sense when it can deliver a meaningful improvement in self-consumption. The calculation:
- Current self-consumption: 40% of 3,000 kWh = 1,200 kWh self-consumed
- With battery, self-consumption rises to 78%: 2,340 kWh self-consumed
- Improvement: 1,140 kWh per year kept at home instead of exported
- Value of keeping that electricity: 1,140 kWh x 26p (avoided import) = £296/year
- Minus SEG income lost on exported units: approximately £50-70/year
- Net self-consumption benefit: roughly £230-250/year
Self-consumption improvement alone does not justify a £10,000 battery. The payback on that basis alone is over 40 years. But it is not the only source of value. Add Agile arbitrage (£500-700/year on a 10 kWh battery), and the combined payback drops to 10-12 years. Self-consumption improvement is a bonus, not the primary investment case.
If your self-consumption rate is already above 65%, the self-consumption improvement from a battery is smaller. In that case, the Agile arbitrage value becomes the dominant factor in the battery investment decision. Check AgileAlert's live prices to see the overnight-to-peak spread available in your region today.