Key takeaways
- Wood pellets hold about 4.8 kWh per kg, so a tonne is roughly 4,800 kWh of fuel
- Bulk blown pellets at £235–£360 a tonne deliver heat for about 5.8p–8.8p/kWh through an 85% efficient boiler
- Bagged pellets cost roughly 10p/kWh of delivered heat — the delivery method matters more than the boiler
- An average 12,000 kWh heat demand needs about 2.9 tonnes a year and somewhere dry to store it
- Mains gas is 8.1p/kWh delivered and a heat pump about 6.5p, so biomass wins on fuel only at the cheap end of the pellet market
- No Boiler Upgrade Scheme grant for most households, which is where the total-cost case usually collapses
The short answer
A biomass boiler costs whatever a kWh of pellet costs, divided by how efficiently the boiler burns it. Wood pellets carry about 4.8 kWh per kg, so a tonne is around 4,800 kWh of raw fuel. Divide the delivered price by that and you have your pence per kWh before losses; divide again by the boiler's seasonal efficiency and you have what the heat actually costs.
At bulk blown prices of £235 to £360 a tonne, that is 4.9p to 7.5p per kWh of fuel, or 5.8p to 8.8p per kWh of delivered heat through a boiler running at 85%. Buy the same pellets in 15kg bags at around £389 a tonne and you are at 8.1p of fuel and roughly 10p of heat.
What it costs, worked through
Every figure below is pellet price divided by 4,800 kWh a tonne, then divided by boiler efficiency. Nothing else is in it.
| How you buy it | Price/tonne | Fuel cost | Heat cost at 85% | 12,000 kWh a year |
|---|---|---|---|---|
| Bulk blown, cheap end | £235 | 4.9p/kWh | 5.8p/kWh | ≈ £690 |
| Bulk blown, typical | £310 | 6.5p/kWh | 7.6p/kWh | ≈ £910 |
| Bulk blown, dear end | £360 | 7.5p/kWh | 8.8p/kWh | ≈ £1,060 |
| 15kg bags | £389 | 8.1p/kWh | 10.1p/kWh (at 80%) | ≈ £1,215 |
Bagged pellets are shown at 80% efficiency because they tend to feed smaller, less well modulated appliances. The 12,000 kWh column is a mid-sized UK home's annual space and water heat demand; scale it to your own.
How much does a house get through?
Work backwards from the heat you need. For 12,000 kWh of delivered heat at 85% efficiency you must burn about 14,100 kWh of pellet, and at 4.8 kWh per kg that is roughly 2.9 tonnes a year.
That is the number people underestimate, because 2.9 tonnes is a physical problem before it is a financial one. It wants a dry store or a hopper, vehicle access for a blower lorry, and a delivery booked before the cold arrives rather than during it. A large or poorly insulated house on 20,000 kWh is closer to five tonnes, which is a shed's worth.
Against gas, a heat pump and storage heaters
Comparing fuels only works at the point of delivered heat, so every row below has its efficiency already applied.
| Heating | Fuel price | Efficiency | Delivered heat |
|---|---|---|---|
| Heat pump | 26.11p/kWh electricity | COP 4.0 | 6.5p/kWh |
| Biomass, bulk pellets | £235–£360/tonne | 85% | 5.8–8.8p/kWh |
| Mains gas | 7.33p/kWh | 90% | 8.1p/kWh |
| Biomass, bagged pellets | £389/tonne | 80% | 10.1p/kWh |
| Storage heaters, Economy 7 | 14.53p/kWh night | 100% | 14.53p/kWh |
Read that honestly and biomass is not the cheap option people expect. It only beats mains gas at the lower half of the pellet market, and it never quite catches a heat pump. Where it is decisively better is against storage heaters, which cost about 14.53p a kWh on the Economy 7 night rate — a pellet boiler roughly halves that on fuel.
Where the sums fall apart
Fuel is the part that flatters biomass. Everything around it does not.
- No grant for most people. The Boiler Upgrade Scheme pays £7,500 towards a heat pump and covers biomass only in limited rural off-gas-grid cases. The old Renewable Heat Incentive has closed. On a £10,000–£18,000 install that gap is worth more than any plausible fuel saving over the life of the boiler.
- Maintenance is real work. Ash removal, an annual service and a flue to sweep. Budget £200–£400 a year against £100–£200 for a heat pump.
- Pellet prices move more than capped fuels do. Gas and electricity unit rates are capped and published quarterly. Pellets are a commodity, and the range in the table above is the market's own spread, not a forecast.
- Efficiency is seasonal, not nameplate. A boiler quoted at 93% will not average that across a British heating season with part-load running and frequent cycling. 85% is the honest planning figure.
The case that does work: you have your own woodland or a cheap local pellet supply, you are off the gas grid, and you have the space. Free fuel changes the arithmetic completely, and nothing above applies. Our heat pump vs biomass comparison goes through the whole-life cost side of that decision.
Cutting the bill
- Buy blown, not bagged. This is the single biggest lever on the page — roughly 4p a kWh between the two, which on 12,000 kWh is about £500 a year.
- Fill the store in summer. Pellet prices rise into the heating season, and a full store in August is bought at the year's cheapest rate.
- Insulate before you size the boiler. Every kWh you do not need is a kWh you do not buy, and an oversized biomass boiler cycles badly. The insulation payback calculator shows what pays back first.
- Check the grant position before committing. If a heat pump is viable on your property, the £7,500 usually settles the argument. The grant eligibility checker takes about a minute.
Sources
Pellet energy content of 4.8 kWh/kg and the bagged price of £389 a tonne are from Stovesonline's wood pellet cost guide; the bulk blown range of £235–£360 a tonne is from SDL Pellets and other UK suppliers, and is a market spread rather than a quote. Gas and electricity unit rates are the Ofgem default tariff cap for 1 July to 30 September 2026, direct debit, averaged across England, Scotland and Wales and including VAT: Ofgem price cap, July 2026. All pence-per-kWh and annual figures are our own arithmetic from those inputs, shown in full above so you can check them. Correct as of 31 July 2026; the cap changes quarterly and pellet prices move with the season.