Every household generating solar power eventually asks some version of the same question: should this spare energy be exported, stored in a battery, used to charge the car, or put toward heating water? Most advice answers this with a blanket rule — "always use your own solar" or "always charge the battery first." Neither is reliably true. The correct answer depends entirely on your own rates, and it can flip between two households with identical solar systems.
This guide gives you the actual method — the same one used to build the numbers behind every comparison on this site — so you can apply it to your own tariff, not just take someone else's default assumption.
The method, in one comparison
For any spare solar energy, and any use you're considering for it, compare two numbers:
- What you'd earn by exporting it — your export rate, in pence per kWh.
- What you'd otherwise pay for the thing you're considering powering with it instead — the displaced cost.
If the export rate is higher, export it. If the displaced cost is higher, use it. That's the whole rule. Everything below is this same comparison, applied to the four things people most commonly want to divert solar toward.
Step 1: Check your export rate first
Before evaluating any diversion option, know your actual export rate. UK export tariffs vary meaningfully between suppliers — commonly somewhere in the 4p–15p/kWh range, sometimes tiered by system size. This single number is the benchmark every other comparison in this guide gets measured against, so get it right before doing anything else. Check your supplier's current export tariff directly — don't assume a generic figure applies to you.
Step 2: Battery charging — export rate vs. your off-peak rate
If you're deciding whether spare solar should charge a home battery or be exported:
- Compare your export rate to your off-peak electricity rate, not your peak rate. Here's why: if you didn't charge the battery from solar, you'd most likely charge it overnight from the grid at the off-peak rate instead — that's the real alternative cost, not the higher peak rate.
- If export beats your off-peak rate: export the solar, and let the battery charge overnight from cheap grid electricity as normal. You come out ahead either way, and this route is simpler.
- If your off-peak rate beats export: charging the battery from solar directly is the better move.
- On a single-rate tariff there's no off-peak alternative: without solar, the battery would only ever be charged at your normal rate, so compare your export rate against that normal rate instead.
This is a common point of confusion: people intuitively compare export against their peak rate, since that's the rate the battery ultimately protects them from. But the battery's charging decision is a separate question from what it later protects you from — don't conflate the two.
Step 3: EV charging — the same comparison, one extra wrinkle
If you're deciding whether spare solar should charge your EV instead of being exported, the comparison is the same as Step 2: export rate vs. your EV's normal charging rate (usually an off-peak tariff rate, if you're charging overnight as most EV owners do).
The wrinkle: many EV tariffs offer genuinely very cheap overnight rates specifically to encourage off-peak charging. If your off-peak EV rate is already very low, it's entirely possible that exporting solar and keeping the car on its normal overnight schedule beats diverting solar to it directly — even though "using your own solar to charge your own car" feels like it should obviously be the better choice. Check the actual numbers; don't assume.
Step 4: Hot water — compare against your heating fuel's real cost, not its headline rate
If you're deciding whether to divert spare solar into an immersion heater instead of exporting:
- Don't compare export against your electricity rate — compare it against the effective cost of heating water your existing way, most commonly gas.
- For gas, that means adjusting the headline per-kWh gas rate for boiler efficiency (typically around 85%), since not all the gas energy becomes usable heat. A gas rate that looks cheap on paper can have a meaningfully higher effective cost once this is accounted for.
- If export beats gas's effective cost: export the solar and let gas continue heating water. This is a common outcome, and it surprises people — "wasting" solar by exporting it, while paying for gas separately, can genuinely be the better financial choice.
- If gas's effective cost beats export: diverting solar to the immersion heater is worth it.
Step 5: Apply the same method to anything new
Air conditioning, a home office setup, a pool pump — whatever you're considering powering from spare solar, the method doesn't change: compare your export rate against the real cost of powering that thing another way. If a new appliance or system enters the picture, run it through the same comparison rather than reaching for a generic rule of thumb.
Why this matters more than most solar advice admits
The financial gap between getting this right and getting it wrong is frequently larger than the gap between different brands of panel or battery. A household diverting solar to a use that's actually worth less than exporting is quietly leaving money on the table every single day the system runs — not a one-off mistake, but a compounding one.
Want this method applied automatically to your own tariff, across solar, battery, EV and hot water at once? Our free calculator runs this exact comparison for every combination you select.