cost-analysisheatingsolar

The True Cost of Cosiness: Comparing Running Costs of Hot-Water Alternatives When Your Home Has Solar

UUnknown

2026-02-14

9 min read

Compare pennies‑per‑use costs of hot‑water bottles, microwavables and rechargeable pads when charged from grid vs solar‑battery — and how to save more.

Why this matters now: cosiness vs soaring bills

Winter 2026 feels different: energy bills are still volatile, rooftop PV plus home batteries are far more common, and many homeowners are asking a practical question — what is the real running cost of staying cosy without heating the whole house? This guide compares the true running cost of the most popular personal heating options — traditional hot‑water bottles, microwavable grain bags, rechargeable electric pads and mains heating pads — when charged from the grid versus charged from your solar array and battery.

The headline: tiny per-use costs, big behavioural wins

Short story: the energy required to heat a single hot‑water bottle or charge a wearable pad is measured in hundredths of a kilowatt‑hour. That means per‑use costs are only pennies. But scaled up across a whole household and a whole winter, choosing the right product and charging it smartly from your rooftop PV or home battery can reduce your heating bill and your exposure to price volatility.

What you'll get in this article

Clear, reproducible energy and cost calculations for four product types
Grid vs solar‑battery cost scenarios using 2026 assumptions
Practical tips to maximise solar self‑consumption and comfort
Real‑world examples for a typical UK home with rooftop PV and a small battery

How I calculated costs (transparent assumptions)

To keep this useful and repeatable I define one representative “use” for each product and convert the heat or electricity used into kWh. I then apply three simple cost scenarios relevant to UK 2026 consumers:

Grid (typical): 30p/kWh — a mid‑2026 typical headline rate for many households (varies with supplier and tariff)
Grid (off‑peak): 15p/kWh — achievable if you have a time‑of‑use tariff or night rate
Solar + battery: 6p/kWh (effective)

Notes on the solar+battery figure: this is a conservative, operational marginal cost that accounts for rooftop PV LCOE and the round‑trip efficiency/lifetime cost of a small home battery. In late 2025 industry reports and installer data showed falling battery LCOE and improved payback for self‑consumed solar, so using 6p/kWh is purposefully realistic rather than optimistic.

Representative energy per use (methodology)

Here are the representative energy numbers used in the comparisons below. I selected values that reflect common product sizes and user behaviour.

Traditional hot‑water bottle (kettle heat): 2 litres heated from 15°C to 80°C. Heat energy ≈ 0.15 kWh. Allowing kettle inefficiency → grid input ≈ 0.17 kWh.
Microwavable wheat/grain bag: typical microwave cycle 1–2 minutes. Grid input ≈ 0.04 kWh (heat a wheat bag in the microwave during a sunny spell).
Rechargeable electric pad (wearable): battery capacity ~15–20 Wh; charging losses → grid input ≈ 0.025 kWh (rechargeable pads are also common in wearable recovery kits).
Mains plugged‑in heating pad: continuous draw ~20 W for 4 hours → 0.08 kWh.

Per‑use running costs (pence and pounds) — quick reference

Applying the three cost scenarios to the energy per use above gives the following per‑use costs.

1) Traditional hot‑water bottle (kettle) — 0.17 kWh per fill

Grid (30p/kWh): 0.17 × £0.30 = £0.051 (5.1p) per fill
Grid off‑peak (15p/kWh): £0.0255 (2.6p)
Solar + battery (6p/kWh): £0.0102 (1.0p)

2) Microwavable wheat bag — 0.04 kWh per heat

Grid: 0.04 × £0.30 = £0.012 (1.2p)
Off‑peak: £0.006 (0.6p)
Solar + battery: £0.0024 (0.24p)

3) Rechargeable pad (wearable) — 0.025 kWh per charge

Grid: 0.025 × £0.30 = £0.0075 (0.75p)
Off‑peak: £0.0038 (0.38p)
Solar + battery: £0.0015 (0.15p)

4) Plugged‑in heating pad — 0.08 kWh per evening

Grid: 0.08 × £0.30 = £0.024 (2.4p)
Off‑peak: £0.012 (1.2p)
Solar + battery: £0.0048 (0.48p)

Per‑use costs are tiny — pennies. The real value is avoiding whole‑house heating and charging smart from solar when possible.

Scale it up: winter season cost example

Personal heating is most useful during the core winter months. Let’s assume you use one device per evening for 120 winter nights (typical November–February behaviour for many households). Multiply the per‑use cost by 120:

Traditional hot‑water bottle on grid: 5.1p × 120 = £6.12 for the winter
Microwavable bag on grid: 1.2p × 120 = £1.44
Rechargeable pad on grid: 0.75p × 120 = £0.90
Plugged pad on grid: 2.4p × 120 = £2.88

Charge the same devices from solar+battery (using our conservative 6p/kWh) and winter costs fall even further — sub‑£2 for many options.

Why the numbers look so small — and why they still matter

Heating a small mass of water or a fabric bag only requires a tiny amount of energy. But the choice matters because:

Replacing even part of your space heating with localised personal heating can reduce thermostat settings and whole‑house fuel use.
Charging personal devices from solar or using off‑peak grid power reduces exposure to peak prices — and it’s simple to implement.
Combined across household members, the savings add up (and reduce carbon emissions).

Practical advice: maximise comfort and minimise cost

Here are actionable steps you can take today.

1. Charge small electric pads in daylight when your PV is generating

Solar production is often available for late morning to mid‑afternoon even in winter. Charge rechargeable pads or a power bank while generation is happening to capture near‑zero marginal cost energy.
Use a smart plug or the charger’s schedule where available. New smart home systems (2025–26) support 'charge when PV > X kW' rules.

2. Use microwavables when you're at home during solar production

If you work from home or are at home in daylight, heat a wheat bag in the microwave during a sunny spell. The per‑use energy is tiny and it’s a great fit for direct solar use.

3. Prefer rechargeable pads to continuous mains pads

Rechargeable pads store energy efficiently for personal use; they typically use one or two hundredth of a kWh per charge. Plugged‑in pads can draw more over long sessions — consider rechargeable, wearable options where possible.

4. Combine thermal retention with personal heating

A hot‑water bottle plus a duvet, hot‑water bottle inside a terry sleeve, or a microwavable bag tucked into a sleeping bag reduces the need for second charges.

5. Safety and product selection

Only use products as intended — do not microwave products not labelled as microwavable.
Look for battery safety certifications (CE/UKCA, overheat protection) on rechargeable pads.
Check physical durability; a cheap rechargeable pad that fails in a season wastes embodied energy and money.

Example household: 4 kWp PV + 5 kWh battery (real‑world winter math)

To put the per‑use numbers into household perspective, consider a typical suburban UK house with a 4 kWp rooftop system and a 5 kWh battery. Winter generation varies by location, but a conservative winter day average might be 1.5–2.5 kWh/day in December. Charging nightly rechargeable pads (0.025 kWh × 2 people × 120 nights) uses:

0.025 × 2 × 120 = 6 kWh total over winter

That’s a tiny slice of a 5 kWh battery capacity and a small share of seasonal PV generation — but by charging those pads during sunny hours you avoid drawing even a small amount from the grid at higher prices. If the household instead heated rooms continuously, the delta would be measured in hundreds or thousands of kWh.

Advanced strategies (2026 trends to use now)

Several technical and policy trends that crystallised in late 2025 let you reduce running costs further.

Smart home charging rules: modern inverters and home energy systems now offer simple rules (charge device when PV > X, or when battery SOC > Y) — use them to prioritise daytime charging for personal heating gear. See our field review of the HomeEdge Pro Hub for controller examples.
Time‑of‑use tariffs: Ofgem‑approved tariffs are more widely available in 2026; charge small devices overnight on cheap rates if daytime PV is limited.
Better battery economics: battery cost declines in 2024–25 mean the effective cost of stored solar is lower than it was in 2021–23 — your night‑use personal heating can be charged from the battery at a lower effective rate than peak grid power.
Integration with heat controls: some newer systems let you lower whole‑house thermostat and heat a person locally for a large net saving. This behavioural smartness is the biggest lever.

Where to invest first for the best returns

Buy a well‑rated rechargeable pad or microwaveable wheat bag — low up‑front cost, fast payback by avoiding a higher thermostat.
If you have rooftop PV, add a smart plug or use the inverter’s device control to charge daytime.
Consider a modest battery only if you want evening solar self‑consumption; batteries now pay off faster in households with EVs and e‑bikes and high evening demand.

Common questions

Q: Are hot‑water bottles more carbon‑efficient than electric pads?

Yes in many cases, because the energy to heat water can be small and, if your hot water comes from an efficient combi or an electric kettle powered by low‑carbon electricity (solar), the footprint is low. The difference becomes negligible if you charge electric pads using low‑carbon solar energy.

Q: Is the battery always worth it for charging pads?

No — for tiny daily loads like personal heating, the battery isn't necessary for cost savings if you can charge during daylight. The battery adds value when you need to move solar energy to the evening or want resilience.

Closing summary — quick takeaways

Per‑use energy is tiny. A hot‑water bottle fill is ~0.17 kWh, microwavables ~0.04 kWh, rechargeable pads ~0.025 kWh.
Per‑use cost is pennies. On a typical 30p/kWh grid tariff most uses cost <6p; on solar+battery they fall to well under 1–2p.
Big wins come from behaviour. Charge during the day, use thermal retention, and lower whole‑house thermostats to amplify savings.
2026 tech helps. Smart charging rules, lower battery costs and broader time‑of‑use tariffs make it easier to capture cheap solar for personal heating.

Action steps — what to do this week

Identify which personal heating items you use and estimate uses per week.
If you have PV, try charging a rechargeable pad during daylight and compare your usage patterns for a week.
Install a smart plug or use your inverter’s device control to schedule charging when your panels are producing.
Consider one small purchase (good rechargeable pad or quality microwavable bag) rather than multiple cheap items — longevity matters.

Ready to cut costs and keep cosy?

If you want help quantifying how much your specific setup (solar array size, battery capacity, occupant routines) can save, we can run the numbers for you. Use our quick solar+comfort calculator or get a free quote from accredited local installers — prioritise smart charging and thermal retention for the fastest wins.

Get started: try charging one rechargeable pad from your PV this week and compare your bill — small experiments scale into guaranteed savings.

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